diff --git a/explorer/BUILD b/explorer/BUILD
index 076a1f034740..441e368af351 100644
--- a/explorer/BUILD
+++ b/explorer/BUILD
@@ -51,6 +51,7 @@ cc_library(
         "//testing/file_test:file_test_base",
         "//testing/util:test_raw_ostream",
         "@com_google_absl//absl/flags:flag",
+        "@com_googlesource_code_re2//:re2",
     ],
 )
 
@@ -58,11 +59,11 @@ file_test(
     name = "file_test",
     size = "small",
     shard_count = 20,
-    tests = glob([
-        "testdata/**/*.carbon",
-        "trace_testdata/**/*.carbon",
-    ]),
-    deps = [":file_test_common"],
+    tests = glob(["testdata/**/*.carbon"]),
+    deps = [
+        ":file_test_common",
+        "@com_googlesource_code_re2//:re2",
+    ],
 )
 
 file_test(
@@ -71,14 +72,14 @@ file_test(
     args = ["--trace"],
     shard_count = 30,
     tests = glob(
-        # This omits trace_testdata because that's run with trace in the
-        # default mode.
         ["testdata/**/*.carbon"],
         exclude = [
             # `limits` tests check for various limit conditions (such as an
             # infinite loop). The tests collectively don't test tracing
             # because it creates substantial additional overhead.
             "testdata/limits/**",
+            # `trace` tests do tracing by default.
+            "testdata/trace/**",
             # Expensive tests to trace.
             "testdata/assoc_const/rewrite_large_type.carbon",
             "testdata/linked_list/typed_linked_list.carbon",
diff --git a/explorer/README.md b/explorer/README.md
index 15e2228779d1..5b777b0a9226 100644
--- a/explorer/README.md
+++ b/explorer/README.md
@@ -117,31 +117,6 @@ To explain this boilerplate:
         braces indicate a contained regular expression.
 -   The `package` is required in all test files, per normal Carbon syntax rules.
 
-### lit tests
-
-The [`lit_testdata/`](lit_testdata/) subdirectory includes other example
-programs.
-
-These tests make use of LLVM's
-[lit](https://llvm.org/docs/CommandGuide/lit.html) and
-[FileCheck](https://llvm.org/docs/CommandGuide/FileCheck.html).
-
-They share most of their header with those in `testdata`, with an additional
-`RUN` rule:
-
-```
-// RUN: %{explorer-run}
-// RUN: %{explorer-run-trace}
-```
-
-The `RUN` lines indicate two commands for `lit` to execute using the file: one
-without trace and debug output, one with.
-
--   `RUN:` will be followed by the `not` command when failure is expected. In
-    particular, `RUN: not %{explorer-run}`.
--   The full command is in `lit.cfg.py`; it will run explorer and pass results
-    to [`FileCheck`](https://llvm.org/docs/CommandGuide/FileCheck.html).
-
 ### Useful commands
 
 -   `./autoupdate_testdata.py` -- Updates expected output.
diff --git a/explorer/autoupdate_lit_testdata.py b/explorer/autoupdate_lit_testdata.py
deleted file mode 100755
index 127aa3922ce1..000000000000
--- a/explorer/autoupdate_lit_testdata.py
+++ /dev/null
@@ -1,35 +0,0 @@
-#!/usr/bin/env python3
-
-"""Updates the CHECK: lines in tests with an AUTOUPDATE line."""
-
-__copyright__ = """
-Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-Exceptions. See /LICENSE for license information.
-SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-"""
-
-import subprocess
-import sys
-from pathlib import Path
-
-
-def main() -> None:
-    # Subprocess to the main script in order to avoid Python import behaviors.
-    this_py = Path(__file__).resolve()
-    autoupdate_py = this_py.parent.parent.joinpath(
-        "testing", "scripts", "autoupdate_testdata_base.py"
-    )
-    args = [
-        str(autoupdate_py),
-        # Flags to configure for explorer testing.
-        "--tool=explorer",
-        "--testdata=explorer/lit_testdata",
-        "--lit_run=%{explorer} | %{FileCheck-strict}",
-        "--lit_run=%{explorer} --parser_debug --trace_file=- | "
-        "%{FileCheck-allow-unmatched}",
-    ] + sys.argv[1:]
-    exit(subprocess.call(args))
-
-
-if __name__ == "__main__":
-    main()
diff --git a/explorer/autoupdate_testdata.py b/explorer/autoupdate_testdata.py
deleted file mode 100755
index ea8ee6222dad..000000000000
--- a/explorer/autoupdate_testdata.py
+++ /dev/null
@@ -1,32 +0,0 @@
-#!/usr/bin/env python3
-
-"""Updates the CHECK: lines in tests with an AUTOUPDATE line."""
-
-__copyright__ = """
-Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-Exceptions. See /LICENSE for license information.
-SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-"""
-
-import subprocess
-import sys
-from pathlib import Path
-
-
-def main() -> None:
-    # Subprocess to the main script in order to avoid Python import behaviors.
-    this_py = Path(__file__).resolve()
-    autoupdate_py = this_py.parent.parent.joinpath(
-        "testing", "scripts", "autoupdate_testdata_base.py"
-    )
-    args = [
-        str(autoupdate_py),
-        # Flags to configure for explorer testing.
-        "--tool=explorer",
-        "--testdata=explorer/testdata",
-    ] + sys.argv[1:]
-    exit(subprocess.call(args))
-
-
-if __name__ == "__main__":
-    main()
diff --git a/testing/lit_test/BUILD b/explorer/autoupdate_testdata.sh
old mode 100644
new mode 100755
similarity index 54%
rename from testing/lit_test/BUILD
rename to explorer/autoupdate_testdata.sh
index 3b77481dbedb..994b15d7f3a2
--- a/testing/lit_test/BUILD
+++ b/explorer/autoupdate_testdata.sh
@@ -1,12 +1,8 @@
+#!/usr/bin/env bash
+
 # Part of the Carbon Language project, under the Apache License v2.0 with LLVM
 # Exceptions. See /LICENSE for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-package(default_visibility = ["//visibility:public"])
-
-exports_files(["lit_test.py"])
-
-py_binary(
-    name = "merge_output",
-    srcs = ["merge_output.py"],
-)
+bazel run -c opt --test_sharding_strategy=disabled //explorer:file_test \
+  -- --autoupdate
diff --git a/explorer/autoupdate_trace_testdata.py b/explorer/autoupdate_trace_testdata.py
deleted file mode 100755
index aa5a53ef27fc..000000000000
--- a/explorer/autoupdate_trace_testdata.py
+++ /dev/null
@@ -1,42 +0,0 @@
-#!/usr/bin/env python3
-
-"""Updates the CHECK: lines in tests with an AUTOUPDATE line."""
-
-__copyright__ = """
-Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-Exceptions. See /LICENSE for license information.
-SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-"""
-
-import subprocess
-import sys
-from pathlib import Path
-
-
-def main() -> None:
-    # Subprocess to the main script in order to avoid Python import behaviors.
-    this_py = Path(__file__).resolve()
-    autoupdate_py = this_py.parent.parent.joinpath(
-        "testing", "scripts", "autoupdate_testdata_base.py"
-    )
-
-    args = [
-        str(autoupdate_py),
-        # Flags to configure for explorer testing.
-        "--tool=explorer",
-        "--testdata=explorer/trace_testdata",
-        "--autoupdate_arg=--trace_file=-",
-        "--autoupdate_arg=-trace_phase=all",
-        "--extra_check_replacement",
-        r".+Time elapsed in (\S+): (\d+)ms",
-        r"Time elapsed in (\S+): (\d+)ms",
-        r"Time elapsed in \1: {{[0-9]+}}ms",
-        # Do not perform line number replacement.
-        "--line_number_pattern",
-        "(?!)",
-    ] + sys.argv[1:]
-    exit(subprocess.call(args))
-
-
-if __name__ == "__main__":
-    main()
diff --git a/explorer/file_test.cpp b/explorer/file_test.cpp
index 3e9785f974a3..19d83239dbe2 100644
--- a/explorer/file_test.cpp
+++ b/explorer/file_test.cpp
@@ -4,6 +4,7 @@
 
 #include "absl/flags/flag.h"
 #include "explorer/main.h"
+#include "re2/re2.h"
 #include "testing/file_test/file_test_base.h"
 #include "testing/util/test_raw_ostream.h"
 
@@ -17,7 +18,13 @@ namespace {
 
 class ExplorerFileTest : public FileTestBase {
  public:
-  using FileTestBase::FileTestBase;
+  explicit ExplorerFileTest(std::filesystem::path path)
+      : FileTestBase(std::move(path)),
+        prelude_line_re_(R"(prelude.carbon:(\d+))"),
+        timing_re_(R"((Time elapsed in \w+: )\d+(ms))") {
+    CARBON_CHECK(prelude_line_re_.ok()) << prelude_line_re_.error();
+    CARBON_CHECK(timing_re_.ok()) << timing_re_.error();
+  }
 
   auto Run(const llvm::SmallVector<llvm::StringRef>& test_args,
            const llvm::SmallVector<TestFile>& test_files,
@@ -52,13 +59,9 @@ class ExplorerFileTest : public FileTestBase {
       args.push_back(arg.data());
     }
 
-    // Trace output is only checked for a few tests.
-    bool check_trace_output =
-        path().string().find("trace_testdata/") != std::string::npos;
-
     int exit_code = ExplorerMain(
         args.size(), args.data(), /*install_path=*/"", PreludePath, stdout,
-        stderr, check_trace_output ? stdout : trace_stream_, fs);
+        stderr, check_trace_output() ? stdout : trace_stream_, fs);
 
     return exit_code == EXIT_SUCCESS;
   }
@@ -83,7 +86,37 @@ class ExplorerFileTest : public FileTestBase {
     return args;
   }
 
+  auto GetLineNumberReplacement(llvm::ArrayRef<llvm::StringRef> filenames)
+      -> LineNumberReplacement override {
+    if (check_trace_output()) {
+      return {.has_file = false,
+              .pattern = R"((DO NOT MATCH))",
+              // The `{{{{` becomes `{{`.
+              .line_formatv = "{{{{ *}}{0}"};
+    }
+    return FileTestBase::GetLineNumberReplacement(filenames);
+  }
+
+  auto DoExtraCheckReplacements(std::string& check_line) -> void override {
+    // Ignore the resulting column of EndOfFile because it's often the end of
+    // the CHECK comment.
+    RE2::GlobalReplace(&check_line, prelude_line_re_,
+                       R"(prelude.carbon:{{\\d+}})");
+    if (check_trace_output()) {
+      // Replace timings in trace output.
+      RE2::GlobalReplace(&check_line, timing_re_, R"(\1{{\\d+}}\2)");
+    }
+  }
+
+ private:
+  // Trace output is directly checked for a few tests.
+  auto check_trace_output() -> bool {
+    return path().string().find("/trace/") != std::string::npos;
+  }
+
   TestRawOstream trace_stream_;
+  RE2 prelude_line_re_;
+  RE2 timing_re_;
 };
 
 }  // namespace
diff --git a/explorer/lit_testdata/BUILD b/explorer/lit_testdata/BUILD
deleted file mode 100644
index 61957166d745..000000000000
--- a/explorer/lit_testdata/BUILD
+++ /dev/null
@@ -1,35 +0,0 @@
-# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-# Exceptions. See /LICENSE for license information.
-# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-
-load("//bazel/sh_run:rules.bzl", "glob_sh_run")
-load("//testing/lit_test:rules.bzl", "glob_lit_tests")
-
-glob_lit_tests(
-    name = "all_lit_tests",
-    data = [
-        "//explorer",
-        "//testing/lit_test:merge_output",
-        "@llvm-project//llvm:FileCheck",
-        "@llvm-project//llvm:not",
-    ],
-    driver = "lit.cfg.py",
-    test_file_exts = ["carbon"],
-)
-
-glob_sh_run(
-    args = ["$(location //explorer)"],
-    data = ["//explorer"],
-    file_exts = ["carbon"],
-)
-
-glob_sh_run(
-    args = [
-        "$(location //explorer)",
-        "--parser_debug",
-        "--trace_file=-",
-    ],
-    data = ["//explorer"],
-    file_exts = ["carbon"],
-    run_ext = "verbose",
-)
diff --git a/explorer/lit_testdata/fail_compile_error.carbon b/explorer/lit_testdata/fail_compile_error.carbon
deleted file mode 100644
index 928f6329d13d..000000000000
--- a/explorer/lit_testdata/fail_compile_error.carbon
+++ /dev/null
@@ -1,14 +0,0 @@
-// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-// Exceptions. See /LICENSE for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-// NOAUTOUPDATE
-// RUN: %{not} %{explorer} | %{FileCheck-strict}
-// RUN: %{not} %{explorer} --parser_debug --trace_file=- | %{FileCheck-allow-unmatched}
-
-package ExplorerTest api;
-
-// Just needs to produce an error to validate basic structure.
-fn Main() -> i32 {
-// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/lit_testdata/fail_compile_error.carbon:[[@LINE+1]]: {{.*}}
-}
diff --git a/explorer/lit_testdata/hello.carbon b/explorer/lit_testdata/hello.carbon
deleted file mode 100644
index 2d47f6940265..000000000000
--- a/explorer/lit_testdata/hello.carbon
+++ /dev/null
@@ -1,16 +0,0 @@
-// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-// Exceptions. See /LICENSE for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-// AUTOUPDATE
-// RUN: %{explorer} | %{FileCheck-strict}
-// RUN: %{explorer} --parser_debug --trace_file=- | %{FileCheck-allow-unmatched}
-// CHECK:STDOUT: Hello
-// CHECK:STDOUT: result: 0
-
-package ExplorerTest api;
-
-fn Main() -> i32 {
-  Print("Hello");
-  return 0;
-}
diff --git a/explorer/lit_testdata/lit.cfg.py b/explorer/lit_testdata/lit.cfg.py
deleted file mode 120000
index 232aeeaeaf42..000000000000
--- a/explorer/lit_testdata/lit.cfg.py
+++ /dev/null
@@ -1 +0,0 @@
-../../testing/lit_test/lit.cfg.py
\ No newline at end of file
diff --git a/explorer/lit_testdata/trace.carbon b/explorer/lit_testdata/trace.carbon
deleted file mode 100644
index 529300725e26..000000000000
--- a/explorer/lit_testdata/trace.carbon
+++ /dev/null
@@ -1,66 +0,0 @@
-// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-// Exceptions. See /LICENSE for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-// A lot of output is elided: this is only checking for a few things for simple
-// sanity checking on --parser_debug --trace_file=- along with filter flags output.
-//
-// NOAUTOUPDATE
-// RUN: %{explorer} --parser_debug --trace_file=- -trace_file_context=all | %{FileCheck-allow-unmatched} --check-prefixes=NO-SOURCE,NO-NAMES,NO-PRELUDE,NO-FLOW,NO-TYPE,NO-UNFORMED,NO-DECLS,EXEC,NO-TIMING
-// RUN: %{explorer} --parser_debug --trace_file=- -trace_file_context=all -trace_phase=execution | %{FileCheck-allow-unmatched} --check-prefixes=NO-SOURCE,NO-NAMES,NO-PRELUDE,NO-FLOW,NO-TYPE,NO-UNFORMED,NO-DECLS,EXEC,NO-TIMING
-// RUN: %{explorer} --parser_debug --trace_file=- -trace_file_context=all -trace_phase=source_program | %{FileCheck-allow-unmatched} --check-prefixes=SOURCE,NO-NAMES,NO-PRELUDE,NO-FLOW,NO-TYPE,NO-UNFORMED,NO-DECLS,NO-EXEC,NO-TIMING
-// RUN: %{explorer} --parser_debug --trace_file=- -trace_file_context=all -trace_phase=name_resolution | %{FileCheck-allow-unmatched} --check-prefixes=NO-SOURCE,NAMES,NO-PRELUDE,NO-FLOW,NO-TYPE,NO-UNFORMED,NO-DECLS,NO-EXEC,NO-TIMING
-// RUN: %{explorer} --parser_debug --trace_file=- -trace_file_context=all -trace_phase=control_flow_resolution | %{FileCheck-allow-unmatched} --check-prefixes=NO-SOURCE,NO-NAMES,NO-PRELUDE,FLOW,NO-TYPE,NO-UNFORMED,NO-DECLS,NO-EXEC,NO-TIMING
-// RUN: %{explorer} --parser_debug --trace_file=- -trace_file_context=all -trace_phase=type_checking | %{FileCheck-allow-unmatched} --check-prefixes=NO-SOURCE,NO-NAMES,NO-PRELUDE,NO-FLOW,TYPE,NO-UNFORMED,NO-DECLS,NO-EXEC,NO-TIMING
-// RUN: %{explorer} --parser_debug --trace_file=- -trace_file_context=all -trace_phase=unformed_variables_resolution | %{FileCheck-allow-unmatched} --check-prefixes=NO-SOURCE,NO-NAMES,NO-PRELUDE,NO-FLOW,NO-TYPE,UNFORMED,NO-DECLS,NO-EXEC,NO-TIMING
-// RUN: %{explorer} --parser_debug --trace_file=- -trace_file_context=all -trace_phase=declarations | %{FileCheck-allow-unmatched} --check-prefixes=NO-SOURCE,NO-NAMES,NO-PRELUDE,NO-FLOW,NO-TYPE,NO-UNFORMED,DECLS,NO-EXEC,NO-TIMING
-// RUN: %{explorer} --parser_debug --trace_file=- -trace_file_context=all -trace_phase=timing | %{FileCheck-allow-unmatched} --check-prefixes=NO-SOURCE,NO-NAMES,NO-PRELUDE,NO-FLOW,NO-TYPE,NO-UNFORMED,NO-DECLS,NO-EXEC,TIMING
-// RUN: %{explorer} --parser_debug --trace_file=- -trace_file_context=all -trace_phase=all | %{FileCheck-allow-unmatched} --check-prefixes=SOURCE,NAMES,NO-PRELUDE,FLOW,TYPE,UNFORMED,DECLS,EXEC,TIMING
-// RUN: %{explorer} --parser_debug --trace_file=- -trace_file_context=all -trace_phase=source_program,declarations | %{FileCheck-allow-unmatched} --check-prefixes=SOURCE,NO-NAMES,NO-PRELUDE,NO-FLOW,NO-TYPE,NO-UNFORMED,DECLS,NO-EXEC,NO-TIMING
-// NO-SOURCE-NOT:STDOUT: ********** source program **********
-//        SOURCE:STDOUT: ********** source program **********
-// NO-SOURCE-NOT:STDOUT: interface TestInterface {
-//        SOURCE:STDOUT: interface TestInterface {
-// NO-NAMES-NOT:STDOUT: ********** resolving names **********
-//        NAMES:STDOUT: ********** resolving names **********
-// NO-PRELUDE-NOT:STDOUT: interface ImplicitAs {
-// NO-FLOW-NOT:STDOUT: ********** resolving control flow **********
-//        FLOW:STDOUT: ********** resolving control flow **********
-// NO-TYPE-NOT:STDOUT: ********** type checking **********
-//        TYPE:STDOUT: ********** type checking **********
-// NO-TYPE-NOT:STDOUT: ** declaring interface TestInterface
-//        TYPE:STDOUT: ** declaring interface TestInterface
-// NO-UNFORMED-NOT:STDOUT: ********** resolving unformed variables **********
-//        UNFORMED:STDOUT: ********** resolving unformed variables **********
-// NO-DECLS-NOT:STDOUT: ********** printing declarations **********
-//        DECLS:STDOUT: ********** printing declarations **********
-// NO-DECLS-NOT:STDOUT: interface TestInterface {
-//        DECLS:STDOUT: interface TestInterface {
-// NO-EXEC-NOT:STDOUT: ********** starting execution **********
-//        EXEC:STDOUT: ********** starting execution **********
-// NO-EXEC-NOT:STDOUT: ********** initializing globals **********
-//        EXEC:STDOUT: ********** initializing globals **********
-// NO-EXEC-NOT:STDOUT: ********** calling main function **********
-//        EXEC:STDOUT: ********** calling main function **********
-// NO-EXEC-NOT:STDOUT: --- step exp Main() .0. (<Main()>:0) --->
-//        EXEC:STDOUT: --- step exp Main() .0. (<Main()>:0) --->
-// NO-EXEC-NOT:STDOUT: interpreter result: 0
-//        EXEC:STDOUT: interpreter result: 0
-// NO-TIMING-NOT:STDOUT: ********** printing timing **********
-//        TIMING:STDOUT: ********** printing timing **********
-// NO-TIMING-NOT:STDOUT: Time elapsed in ExecProgram: {{[0-9]+}}ms
-//        TIMING:STDOUT: Time elapsed in ExecProgram: {{[0-9]+}}ms
-// NO-TIMING-NOT:STDOUT: Time elapsed in AnalyzeProgram: {{[0-9]+}}ms
-//        TIMING:STDOUT: Time elapsed in AnalyzeProgram: {{[0-9]+}}ms
-// NO-TIMING-NOT:STDOUT: Time elapsed in AddPrelude: {{[0-9]+}}ms
-//        TIMING:STDOUT: Time elapsed in AddPrelude: {{[0-9]+}}ms
-// NO-TIMING-NOT:STDOUT: Time elapsed in Parse: {{[0-9]+}}ms
-//        TIMING:STDOUT: Time elapsed in Parse: {{[0-9]+}}ms
-
-package ExplorerTest api;
-
-interface TestInterface {}
-
-fn Main() -> i32 {
-  return 0;
-}
diff --git a/explorer/lit_testdata/trace_file_filters.carbon b/explorer/lit_testdata/trace_file_filters.carbon
deleted file mode 100644
index dc75164af8bc..000000000000
--- a/explorer/lit_testdata/trace_file_filters.carbon
+++ /dev/null
@@ -1,31 +0,0 @@
-// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-// Exceptions. See /LICENSE for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-// A lot of output is elided: this is only checking for a few things for simple
-// sanity checking on --parser_debug --trace_file=- along with file context
-// filter flags output.
-//
-// NOAUTOUPDATE
-// RUN: %{explorer} --parser_debug --trace_file=- -trace_phase=all -trace_file_context=main | %{FileCheck-allow-unmatched} --check-prefixes=MAIN,NO-PRELUDE
-// RUN: %{explorer} --parser_debug --trace_file=- -trace_phase=all -trace_file_context=prelude | %{FileCheck-allow-unmatched} --check-prefixes=NO-MAIN,PRELUDE
-// RUN: %{explorer} --parser_debug --trace_file=- -trace_phase=all -trace_file_context=all | %{FileCheck-allow-unmatched} --check-prefixes=MAIN,PRELUDE
-// RUN: %{explorer} --parser_debug --trace_file=- -trace_phase=all -trace_file_context=main,prelude | %{FileCheck-allow-unmatched} --check-prefixes=MAIN,PRELUDE
-// NO-PRELUDE-NOT:STDOUT: ** declaring interface As
-//        PRELUDE:STDOUT: ** declaring interface As
-// NO-MAIN-NOT:STDOUT: ** declaring function Main
-//        MAIN:STDOUT: ** declaring function Main
-// NO-PRELUDE-NOT:STDOUT: interface As {
-//        PRELUDE:STDOUT: interface As {
-// NO-MAIN-NOT:STDOUT: fn Main ()-> i32 {
-//        MAIN:STDOUT: fn Main ()-> i32 {
-// NO-MAIN-NOT:STDOUT: --- step exp Main() .0. (<Main()>:0) --->
-//        MAIN:STDOUT: --- step exp Main() .0. (<Main()>:0) --->
-
-package ExplorerTest api;
-
-interface TestInterface {}
-
-fn Main() -> i32 {
-  return 0;
-}
diff --git a/explorer/testdata/assert/fail_assert.carbon b/explorer/testdata/assert/fail_assert.carbon
index adee17e917ba..577c96767a8d 100644
--- a/explorer/testdata/assert/fail_assert.carbon
+++ b/explorer/testdata/assert/fail_assert.carbon
@@ -3,7 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
-// CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/data/prelude.carbon:{{.*}}: "HALLO WELT"
+// CHECK:STDERR: RUNTIME ERROR: /explorer/data/prelude.carbon:{{\d+}}: "HALLO WELT"
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/assert/fail_convert.carbon b/explorer/testdata/assert/fail_convert.carbon
index 14850834eb79..4b4633105444 100644
--- a/explorer/testdata/assert/fail_convert.carbon
+++ b/explorer/testdata/assert/fail_convert.carbon
@@ -3,7 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
-// CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/data/prelude.carbon:{{.*}}: "Fail"
+// CHECK:STDERR: RUNTIME ERROR: /explorer/data/prelude.carbon:{{\d+}}: "Fail"
 
 package ExplorerTest api;
 
diff --git a/explorer/lit_testdata/zero.carbon b/explorer/testdata/basic_syntax/zero.carbon
similarity index 70%
rename from explorer/lit_testdata/zero.carbon
rename to explorer/testdata/basic_syntax/zero.carbon
index 792edc95a15d..0dd08dd99e6e 100644
--- a/explorer/lit_testdata/zero.carbon
+++ b/explorer/testdata/basic_syntax/zero.carbon
@@ -3,8 +3,6 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
-// RUN: %{explorer} | %{FileCheck-strict}
-// RUN: %{explorer} --parser_debug --trace_file=- | %{FileCheck-allow-unmatched}
 // CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
diff --git a/explorer/testdata/destructor/fail_delete_base_without_virtual_destructor.carbon b/explorer/testdata/destructor/fail_delete_base_without_virtual_destructor.carbon
index b41544187c28..f93b2ffc0a6c 100644
--- a/explorer/testdata/destructor/fail_delete_base_without_virtual_destructor.carbon
+++ b/explorer/testdata/destructor/fail_delete_base_without_virtual_destructor.carbon
@@ -3,7 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
-// CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/data/prelude.carbon:{{.*}}: Deallocating a derived class from base class pointer requires a virtual destructor
+// CHECK:STDERR: RUNTIME ERROR: /explorer/data/prelude.carbon:{{\d+}}: Deallocating a derived class from base class pointer requires a virtual destructor
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/fail_left_shift_large_rhs.carbon b/explorer/testdata/operators/fail_left_shift_large_rhs.carbon
index 86ed85194971..3eb36e6f090c 100644
--- a/explorer/testdata/operators/fail_left_shift_large_rhs.carbon
+++ b/explorer/testdata/operators/fail_left_shift_large_rhs.carbon
@@ -3,7 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
-// CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/data/prelude.carbon:{{.*}}: Integer overflow
+// CHECK:STDERR: RUNTIME ERROR: /explorer/data/prelude.carbon:{{\d+}}: Integer overflow
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/fail_left_shift_negative_rhs.carbon b/explorer/testdata/operators/fail_left_shift_negative_rhs.carbon
index a633e90577ac..9866b0e71bfc 100644
--- a/explorer/testdata/operators/fail_left_shift_negative_rhs.carbon
+++ b/explorer/testdata/operators/fail_left_shift_negative_rhs.carbon
@@ -3,7 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
-// CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/data/prelude.carbon:{{.*}}: Integer overflow
+// CHECK:STDERR: RUNTIME ERROR: /explorer/data/prelude.carbon:{{\d+}}: Integer overflow
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/fail_right_shift_large_rhs.carbon b/explorer/testdata/operators/fail_right_shift_large_rhs.carbon
index 22bb572a3b58..8aa0c384e940 100644
--- a/explorer/testdata/operators/fail_right_shift_large_rhs.carbon
+++ b/explorer/testdata/operators/fail_right_shift_large_rhs.carbon
@@ -3,7 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
-// CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/data/prelude.carbon:{{.*}}: Integer overflow
+// CHECK:STDERR: RUNTIME ERROR: /explorer/data/prelude.carbon:{{\d+}}: Integer overflow
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/operators/fail_right_shift_negative_rhs.carbon b/explorer/testdata/operators/fail_right_shift_negative_rhs.carbon
index c0a99e5d691c..9cc9ab6a15bb 100644
--- a/explorer/testdata/operators/fail_right_shift_negative_rhs.carbon
+++ b/explorer/testdata/operators/fail_right_shift_negative_rhs.carbon
@@ -3,7 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
-// CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/data/prelude.carbon:{{.*}}: Integer overflow
+// CHECK:STDERR: RUNTIME ERROR: /explorer/data/prelude.carbon:{{\d+}}: Integer overflow
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/optional/fail_optional_get_empty.carbon b/explorer/testdata/optional/fail_optional_get_empty.carbon
index f1aab77d6144..c2f0d1560a69 100644
--- a/explorer/testdata/optional/fail_optional_get_empty.carbon
+++ b/explorer/testdata/optional/fail_optional_get_empty.carbon
@@ -3,7 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
-// CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/data/prelude.carbon:{{.*}}: "Attempted to unwrap empty Optional"
+// CHECK:STDERR: RUNTIME ERROR: /explorer/data/prelude.carbon:{{\d+}}: "Attempted to unwrap empty Optional"
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/random/fail_empty_range.carbon b/explorer/testdata/random/fail_empty_range.carbon
index 03b4c42e8317..81b7d839cd57 100644
--- a/explorer/testdata/random/fail_empty_range.carbon
+++ b/explorer/testdata/random/fail_empty_range.carbon
@@ -3,7 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
-// CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/data/prelude.carbon:{{.*}}: Rand inputs must be ordered for a non-empty range: 0 must be less than 0
+// CHECK:STDERR: RUNTIME ERROR: /explorer/data/prelude.carbon:{{\d+}}: Rand inputs must be ordered for a non-empty range: 0 must be less than 0
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/random/fail_reverse_range.carbon b/explorer/testdata/random/fail_reverse_range.carbon
index 23b782ea3b4c..f8f6ece01961 100644
--- a/explorer/testdata/random/fail_reverse_range.carbon
+++ b/explorer/testdata/random/fail_reverse_range.carbon
@@ -3,7 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
-// CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/data/prelude.carbon:{{.*}}: Rand inputs must be ordered for a non-empty range: 1 must be less than -1
+// CHECK:STDERR: RUNTIME ERROR: /explorer/data/prelude.carbon:{{\d+}}: Rand inputs must be ordered for a non-empty range: 1 must be less than -1
 
 package ExplorerTest api;
 
diff --git a/explorer/testdata/trace/context_all.carbon b/explorer/testdata/trace/context_all.carbon
new file mode 100644
index 000000000000..77aa4d1bb895
--- /dev/null
+++ b/explorer/testdata/trace/context_all.carbon
@@ -0,0 +1,22 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+package ExplorerTest api;
+
+interface TestInterface {}
+
+fn Main() -> i32 {
+  return 0;
+}
+
+// Place checks after code so that line numbers are stable, reducing merge
+// conflicts.
+// ARGS: --trace_file=- --trace_phase=all --trace_file_context=all %s
+// NOAUTOUPDATE
+// SET-CHECK-SUBSET
+// CHECK:STDOUT: ** declaring interface As
+// CHECK:STDOUT: ** declaring function Main
+// CHECK:STDOUT: interface As {
+// CHECK:STDOUT: fn Main ()-> i32 {
+// CHECK:STDOUT: --- step exp Main() .0. (<Main()>:0) --->
diff --git a/explorer/testdata/trace/context_main.carbon b/explorer/testdata/trace/context_main.carbon
new file mode 100644
index 000000000000..a74d35275475
--- /dev/null
+++ b/explorer/testdata/trace/context_main.carbon
@@ -0,0 +1,160 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+package ExplorerTest api;
+
+interface TestInterface {}
+
+fn Main() -> i32 {
+  return 0;
+}
+
+// Place checks after code so that line numbers are stable, reducing merge
+// conflicts.
+// ARGS: --trace_file=- --trace_phase=all --trace_file_context=main %s
+// AUTOUPDATE
+// CHECK:STDOUT: ********** source program **********
+// CHECK:STDOUT: interface TestInterface {
+// CHECK:STDOUT: }
+// CHECK:STDOUT: fn Main ()-> i32 {
+// CHECK:STDOUT: {
+// CHECK:STDOUT: return 0;
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: }
+// CHECK:STDOUT: ********** resolving names **********
+// CHECK:STDOUT: --- declared `TestInterface` as `interface TestInterface` in `package` (context_main.carbon:7)
+// CHECK:STDOUT: --- declared `Main` as `fn Main` in `package` (context_main.carbon:11)
+// CHECK:STDOUT: ** resolving decl `interface TestInterface` (context_main.carbon:7)
+// CHECK:STDOUT: --- marked `TestInterface` declared but not usable in `package`
+// CHECK:STDOUT: --- marked `TestInterface` usable in `package`
+// CHECK:STDOUT: --- declared `Self` as `Self` in `interface TestInterface` (context_main.carbon:7)
+// CHECK:STDOUT: ** finished resolving decl `interface TestInterface` (context_main.carbon:7)
+// CHECK:STDOUT: ** resolving decl `fn Main` (context_main.carbon:11)
+// CHECK:STDOUT: --- marked `Main` declared but not usable in `package`
+// CHECK:STDOUT: --- marked `Main` usable in `package`
+// CHECK:STDOUT: ** resolving stmt `{return 0;}` (context_main.carbon:11)
+// CHECK:STDOUT: ** resolving stmt `return 0;` (context_main.carbon:10)
+// CHECK:STDOUT: ** finished resolving stmt `return 0;` (context_main.carbon:10)
+// CHECK:STDOUT: ** finished resolving stmt `{return 0;}` (context_main.carbon:11)
+// CHECK:STDOUT: ** finished resolving decl `fn Main` (context_main.carbon:11)
+// CHECK:STDOUT: --- resolved `Main` as `fn Main` in `package` (<Main()>:0)
+// CHECK:STDOUT: ********** resolving control flow **********
+// CHECK:STDOUT: --- flow-resolved return statement `return 0;` in `fn Main` (context_main.carbon:10)
+// CHECK:STDOUT: ********** type checking **********
+// CHECK:STDOUT: ** declaring interface TestInterface
+// CHECK:STDOUT: ** finished declaring interface TestInterface
+// CHECK:STDOUT: checking InterfaceDeclaration
+// CHECK:STDOUT: ** checking interface TestInterface
+// CHECK:STDOUT: impl declarations:{{ }}
+// CHECK:STDOUT: impl declarations: bool as interface EqWith(U = bool), i32 as interface EqWith(U = i32), String as interface EqWith(U = String), i32 as interface CompareWith(U = i32), String as interface CompareWith(U = String), i32 as interface LessWith(U = i32), String as interface LessWith(U = String), i32 as interface LessEqWith(U = i32), String as interface LessEqWith(U = String), i32 as interface GreaterWith(U = i32), String as interface GreaterWith(U = String), i32 as interface GreaterEqWith(U = i32), String as interface GreaterEqWith(U = String), i32 as interface Negate, i32 as interface AddWith(U = i32), i32 as interface SubWith(U = i32), i32 as interface MulWith(U = i32), i32 as interface DivWith(U = i32), i32 as interface ModWith(U = i32), i32 as interface BitComplement, i32 as interface BitAndWith(U = i32), i32 as interface BitOrWith(U = i32), i32 as interface BitXorWith(U = i32), i32 as interface LeftShiftWith(U = i32), i32 as interface RightShiftWith(U = i32), i32 as interface Inc, i32 as interface Dec, U as interface __EqualConverter [0], (T1,) as interface As(T = (U1,)) [0, 0, 0; 1, 1, 0, 0, 0], (T1, T2) as interface As(T = (U1, U2)) [0, 0, 0; 0, 0, 1; 1, 1, 0, 0, 0; 1, 1, 0, 0, 1], (T1, U1) as interface EqWith(U = (T2, U2)) [0, 0, 0; 0, 0, 1; 1, 1, 0, 0, 0; 1, 1, 0, 0, 1], (T1, T2, T3) as interface As(T = (U1, U2, U3)) [0, 0, 0; 0, 0, 1; 0, 0, 2; 1, 1, 0, 0, 0; 1, 1, 0, 0, 1; 1, 1, 0, 0, 2], T as interface ImplicitAs(T = U) [0; 1, 1, 0], T as interface As(T = U) [0; 1, 1, 0], T as interface ImplicitAs(T = U) [0; 1, 1, 0], T as interface As(T = U) [0; 1, 1, 0], T as interface AssignWith(U = U) [0; 1, 1, 0], T as interface AddAssignWith(U = U) [0; 1, 1, 0], T as interface SubAssignWith(U = U) [0; 1, 1, 0], T as interface MulAssignWith(U = U) [0; 1, 1, 0], T as interface DivAssignWith(U = U) [0; 1, 1, 0], T as interface ModAssignWith(U = U) [0; 1, 1, 0], T as interface BitAndAssignWith(U = U) [0; 1, 1, 0], T as interface BitOrAssignWith(U = U) [0; 1, 1, 0], T as interface BitXorAssignWith(U = U) [0; 1, 1, 0], T as interface LeftShiftAssignWith(U = U) [0; 1, 1, 0], T as interface RightShiftAssignWith(U = U) [0; 1, 1, 0]
+// CHECK:STDOUT: ** finished checking interface TestInterface
+// CHECK:STDOUT: ** declaring function Main
+// CHECK:STDOUT: checking TuplePattern ()
+// CHECK:STDOUT: checking IntTypeLiteral i32
+// CHECK:STDOUT: (+) stack-push: i32 .0.
+// CHECK:STDOUT: (+) stack-push: i32 .0.
+// CHECK:STDOUT: --- step exp i32 .0. (context_main.carbon:9) --->
+// CHECK:STDOUT: (-) stack-pop: i32 .0.
+// CHECK:STDOUT: (-) stack-pop: i32 .1. {{\[\[}}i32]]
+// CHECK:STDOUT: ** finished declaring function Main of type fn () -> i32
+// CHECK:STDOUT: checking FunctionDeclaration
+// CHECK:STDOUT: ** checking function Main
+// CHECK:STDOUT: impl declarations:{{ }}
+// CHECK:STDOUT: impl declarations: bool as interface EqWith(U = bool), i32 as interface EqWith(U = i32), String as interface EqWith(U = String), i32 as interface CompareWith(U = i32), String as interface CompareWith(U = String), i32 as interface LessWith(U = i32), String as interface LessWith(U = String), i32 as interface LessEqWith(U = i32), String as interface LessEqWith(U = String), i32 as interface GreaterWith(U = i32), String as interface GreaterWith(U = String), i32 as interface GreaterEqWith(U = i32), String as interface GreaterEqWith(U = String), i32 as interface Negate, i32 as interface AddWith(U = i32), i32 as interface SubWith(U = i32), i32 as interface MulWith(U = i32), i32 as interface DivWith(U = i32), i32 as interface ModWith(U = i32), i32 as interface BitComplement, i32 as interface BitAndWith(U = i32), i32 as interface BitOrWith(U = i32), i32 as interface BitXorWith(U = i32), i32 as interface LeftShiftWith(U = i32), i32 as interface RightShiftWith(U = i32), i32 as interface Inc, i32 as interface Dec, U as interface __EqualConverter [0], (T1,) as interface As(T = (U1,)) [0, 0, 0; 1, 1, 0, 0, 0], (T1, T2) as interface As(T = (U1, U2)) [0, 0, 0; 0, 0, 1; 1, 1, 0, 0, 0; 1, 1, 0, 0, 1], (T1, U1) as interface EqWith(U = (T2, U2)) [0, 0, 0; 0, 0, 1; 1, 1, 0, 0, 0; 1, 1, 0, 0, 1], (T1, T2, T3) as interface As(T = (U1, U2, U3)) [0, 0, 0; 0, 0, 1; 0, 0, 2; 1, 1, 0, 0, 0; 1, 1, 0, 0, 1; 1, 1, 0, 0, 2], T as interface ImplicitAs(T = U) [0; 1, 1, 0], T as interface As(T = U) [0; 1, 1, 0], T as interface ImplicitAs(T = U) [0; 1, 1, 0], T as interface As(T = U) [0; 1, 1, 0], T as interface AssignWith(U = U) [0; 1, 1, 0], T as interface AddAssignWith(U = U) [0; 1, 1, 0], T as interface SubAssignWith(U = U) [0; 1, 1, 0], T as interface MulAssignWith(U = U) [0; 1, 1, 0], T as interface DivAssignWith(U = U) [0; 1, 1, 0], T as interface ModAssignWith(U = U) [0; 1, 1, 0], T as interface BitAndAssignWith(U = U) [0; 1, 1, 0], T as interface BitOrAssignWith(U = U) [0; 1, 1, 0], T as interface BitXorAssignWith(U = U) [0; 1, 1, 0], T as interface LeftShiftAssignWith(U = U) [0; 1, 1, 0], T as interface RightShiftAssignWith(U = U) [0; 1, 1, 0]
+// CHECK:STDOUT: checking Block {
+// CHECK:STDOUT: return 0;
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: checking ReturnExpression return 0;
+// CHECK:STDOUT: checking IntLiteral 0
+// CHECK:STDOUT: ** finished checking function Main
+// CHECK:STDOUT: checking CallExpression Main()
+// CHECK:STDOUT: checking IdentifierExpression Main
+// CHECK:STDOUT: checking TupleLiteral ()
+// CHECK:STDOUT: checking call to function of type fn () -> i32
+// CHECK:STDOUT: with arguments of type: ()
+// CHECK:STDOUT: performing argument deduction for bindings:{{ }}
+// CHECK:STDOUT: deduction succeeded with results: {}
+// CHECK:STDOUT: ********** resolving unformed variables **********
+// CHECK:STDOUT: *** resolving-unformed in decl `interface TestInterface` (context_main.carbon:7)
+// CHECK:STDOUT: *** resolving-unformed in decl `fn Main` (context_main.carbon:11)
+// CHECK:STDOUT: *** resolving-unformed in stmt `{return 0;}` (context_main.carbon:11)
+// CHECK:STDOUT: *** resolving-unformed in stmt `return 0;` (context_main.carbon:10)
+// CHECK:STDOUT: ********** printing declarations **********
+// CHECK:STDOUT: interface TestInterface {
+// CHECK:STDOUT: }
+// CHECK:STDOUT: fn Main ()-> i32 {
+// CHECK:STDOUT: {
+// CHECK:STDOUT: return 0;
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: }
+// CHECK:STDOUT: ********** starting execution **********
+// CHECK:STDOUT: ********** initializing globals **********
+// CHECK:STDOUT: (+) stack-push: interface TestInterface {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:  .0.
+// CHECK:STDOUT: --- step decl interface TestInterface .0. (context_main.carbon:7) --->
+// CHECK:STDOUT: (-) stack-pop: interface TestInterface {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:  .0.
+// CHECK:STDOUT: (+) stack-push: fn Main ()-> i32 {
+// CHECK:STDOUT: {
+// CHECK:STDOUT: return 0;
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: }
+// CHECK:STDOUT:  .0.
+// CHECK:STDOUT: --- step decl fn Main .0. (context_main.carbon:11) --->
+// CHECK:STDOUT: (-) stack-pop: fn Main ()-> i32 {
+// CHECK:STDOUT: {
+// CHECK:STDOUT: return 0;
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: }
+// CHECK:STDOUT:  .0.
+// CHECK:STDOUT: ********** calling main function **********
+// CHECK:STDOUT: (+) stack-push: Main() .0.
+// CHECK:STDOUT: (+) stack-push: Main() .0.
+// CHECK:STDOUT: --- step exp Main() .0. (<Main()>:0) --->
+// CHECK:STDOUT: (+) stack-push: Main .0.
+// CHECK:STDOUT: (+) stack-push: Main .0.
+// CHECK:STDOUT: --- step exp Main .0. (<Main()>:0) --->
+// CHECK:STDOUT: (-) stack-pop: Main .0.
+// CHECK:STDOUT: (-) stack-pop: Main .1. {{\[\[}}fun<Main>]]
+// CHECK:STDOUT: --- step exp Main() .1. (<Main()>:0) --->
+// CHECK:STDOUT: calling function: fun<Main>
+// CHECK:STDOUT: match pattern ()
+// CHECK:STDOUT: from value expression with value ()
+// CHECK:STDOUT: (+) stack-push: .0. {}
+// CHECK:STDOUT: (+) stack-push: {return 0;} .0.
+// CHECK:STDOUT: --- step stmt {return 0;} .0. (context_main.carbon:11) --->
+// CHECK:STDOUT: (+) stack-push: return 0; .0.
+// CHECK:STDOUT: --- step stmt return 0; .0. (context_main.carbon:10) --->
+// CHECK:STDOUT: (+) stack-push: 0 .0.
+// CHECK:STDOUT: (+) stack-push: 0 .0.
+// CHECK:STDOUT: --- step exp 0 .0. (context_main.carbon:10) --->
+// CHECK:STDOUT: (-) stack-pop: 0 .0.
+// CHECK:STDOUT: (-) stack-pop: 0 .1. {{\[\[}}0]]
+// CHECK:STDOUT: --- step stmt return 0; .1. (context_main.carbon:10) --->
+// CHECK:STDOUT: (-) stack-pop: return 0; .1. {{\[\[}}0]]
+// CHECK:STDOUT: (-) stack-pop: {return 0;} .1. {}
+// CHECK:STDOUT: (-) stack-pop: .0. {}
+// CHECK:STDOUT: (+) stack-push: clean up.0. {}
+// CHECK:STDOUT: (+) stack-push: clean up.0. {}
+// CHECK:STDOUT: (-) stack-pop: clean up.0. {}
+// CHECK:STDOUT: (-) stack-pop: clean up.0. {}
+// CHECK:STDOUT: --- step exp Main() .2. (<Main()>:0) --->
+// CHECK:STDOUT: (-) stack-pop: Main() .2. {{\[\[}}fun<Main>, 0]] {}
+// CHECK:STDOUT: (+) stack-push: clean up.0. {}
+// CHECK:STDOUT: (-) stack-pop: clean up.0. {}
+// CHECK:STDOUT: (-) stack-pop: Main() .1. {{\[\[}}0]]
+// CHECK:STDOUT: interpreter result: 0
+// CHECK:STDOUT: ********** printing timing **********
+// CHECK:STDOUT: Time elapsed in ExecProgram: {{\d+}}ms
+// CHECK:STDOUT: Time elapsed in AnalyzeProgram: {{\d+}}ms
+// CHECK:STDOUT: Time elapsed in AddPrelude: {{\d+}}ms
+// CHECK:STDOUT: Time elapsed in Parse: {{\d+}}ms
+// CHECK:STDOUT: result: 0
diff --git a/explorer/testdata/trace/context_prelude.carbon b/explorer/testdata/trace/context_prelude.carbon
new file mode 100644
index 000000000000..c152b4207511
--- /dev/null
+++ b/explorer/testdata/trace/context_prelude.carbon
@@ -0,0 +1,19 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+package ExplorerTest api;
+
+interface TestInterface {}
+
+fn Main() -> i32 {
+  return 0;
+}
+
+// Place checks after code so that line numbers are stable, reducing merge
+// conflicts.
+// ARGS: --trace_file=- --trace_phase=all --trace_file_context=prelude %s
+// NOAUTOUPDATE
+// SET-CHECK-SUBSET
+// CHECK:STDOUT: ** declaring interface As
+// CHECK:STDOUT: interface As {
diff --git a/explorer/trace_testdata/full_trace.carbon b/explorer/testdata/trace/phase_all.carbon
similarity index 76%
rename from explorer/trace_testdata/full_trace.carbon
rename to explorer/testdata/trace/phase_all.carbon
index 619184430ee1..ea198f8a708c 100644
--- a/explorer/trace_testdata/full_trace.carbon
+++ b/explorer/testdata/trace/phase_all.carbon
@@ -17,7 +17,8 @@ fn Main() -> i32 {
   return x;
 }
 
-// Place checks after code so that line numbers are stable, reducing merge conflicts.
+// Place checks after code so that line numbers are stable, reducing merge
+// conflicts.
 // ARGS: --trace_file=- --trace_phase=all %s
 // AUTOUPDATE
 // CHECK:STDOUT: ********** source program **********
@@ -37,48 +38,48 @@ fn Main() -> i32 {
 // CHECK:STDOUT:
 // CHECK:STDOUT: }
 // CHECK:STDOUT: ********** resolving names **********
-// CHECK:STDOUT: --- declared `TestInterface` as `interface TestInterface` in `package` (full_trace.carbon:7)
-// CHECK:STDOUT: --- declared `N` as `namespace N` in `package` (full_trace.carbon:9)
-// CHECK:STDOUT: --- resolved `N` as `namespace N` in `package` (full_trace.carbon:11)
-// CHECK:STDOUT: --- declared `Foo` as `fn N.Foo` in `namespace N` (full_trace.carbon:13)
-// CHECK:STDOUT: --- declared `Main` as `fn Main` in `package` (full_trace.carbon:18)
-// CHECK:STDOUT: ** resolving decl `interface TestInterface` (full_trace.carbon:7)
+// CHECK:STDOUT: --- declared `TestInterface` as `interface TestInterface` in `package` (phase_all.carbon:7)
+// CHECK:STDOUT: --- declared `N` as `namespace N` in `package` (phase_all.carbon:9)
+// CHECK:STDOUT: --- resolved `N` as `namespace N` in `package` (phase_all.carbon:11)
+// CHECK:STDOUT: --- declared `Foo` as `fn N.Foo` in `namespace N` (phase_all.carbon:13)
+// CHECK:STDOUT: --- declared `Main` as `fn Main` in `package` (phase_all.carbon:18)
+// CHECK:STDOUT: ** resolving decl `interface TestInterface` (phase_all.carbon:7)
 // CHECK:STDOUT: --- marked `TestInterface` declared but not usable in `package`
 // CHECK:STDOUT: --- marked `TestInterface` usable in `package`
-// CHECK:STDOUT: --- declared `Self` as `Self` in `interface TestInterface` (full_trace.carbon:7)
-// CHECK:STDOUT: ** finished resolving decl `interface TestInterface` (full_trace.carbon:7)
-// CHECK:STDOUT: ** resolving decl `namespace N` (full_trace.carbon:9)
+// CHECK:STDOUT: --- declared `Self` as `Self` in `interface TestInterface` (phase_all.carbon:7)
+// CHECK:STDOUT: ** finished resolving decl `interface TestInterface` (phase_all.carbon:7)
+// CHECK:STDOUT: ** resolving decl `namespace N` (phase_all.carbon:9)
 // CHECK:STDOUT: --- marked `N` usable in `package`
-// CHECK:STDOUT: ** finished resolving decl `namespace N` (full_trace.carbon:9)
-// CHECK:STDOUT: ** resolving decl `fn N.Foo` (full_trace.carbon:13)
-// CHECK:STDOUT: --- resolved `N` as `namespace N` in `package` (full_trace.carbon:11)
+// CHECK:STDOUT: ** finished resolving decl `namespace N` (phase_all.carbon:9)
+// CHECK:STDOUT: ** resolving decl `fn N.Foo` (phase_all.carbon:13)
+// CHECK:STDOUT: --- resolved `N` as `namespace N` in `package` (phase_all.carbon:11)
 // CHECK:STDOUT: --- marked `Foo` declared but not usable in `namespace N`
-// CHECK:STDOUT: --- declared `n` as `n` in `fn N.Foo` (full_trace.carbon:11)
+// CHECK:STDOUT: --- declared `n` as `n` in `fn N.Foo` (phase_all.carbon:11)
 // CHECK:STDOUT: --- marked `Foo` usable in `namespace N`
-// CHECK:STDOUT: ** resolving stmt `{return (n + 1);}` (full_trace.carbon:13)
-// CHECK:STDOUT: ** resolving stmt `return (n + 1);` (full_trace.carbon:12)
-// CHECK:STDOUT: --- resolved `n` as `n` in `fn N.Foo` (full_trace.carbon:12)
-// CHECK:STDOUT: ** finished resolving stmt `return (n + 1);` (full_trace.carbon:12)
-// CHECK:STDOUT: ** finished resolving stmt `{return (n + 1);}` (full_trace.carbon:13)
-// CHECK:STDOUT: ** finished resolving decl `fn N.Foo` (full_trace.carbon:13)
-// CHECK:STDOUT: ** resolving decl `fn Main` (full_trace.carbon:18)
+// CHECK:STDOUT: ** resolving stmt `{return (n + 1);}` (phase_all.carbon:13)
+// CHECK:STDOUT: ** resolving stmt `return (n + 1);` (phase_all.carbon:12)
+// CHECK:STDOUT: --- resolved `n` as `n` in `fn N.Foo` (phase_all.carbon:12)
+// CHECK:STDOUT: ** finished resolving stmt `return (n + 1);` (phase_all.carbon:12)
+// CHECK:STDOUT: ** finished resolving stmt `{return (n + 1);}` (phase_all.carbon:13)
+// CHECK:STDOUT: ** finished resolving decl `fn N.Foo` (phase_all.carbon:13)
+// CHECK:STDOUT: ** resolving decl `fn Main` (phase_all.carbon:18)
 // CHECK:STDOUT: --- marked `Main` declared but not usable in `package`
 // CHECK:STDOUT: --- marked `Main` usable in `package`
-// CHECK:STDOUT: ** resolving stmt `{var x: i32 = N.Foo(0);return x;}` (full_trace.carbon:18)
-// CHECK:STDOUT: ** resolving stmt `var x: i32 = N.Foo(0);` (full_trace.carbon:16)
-// CHECK:STDOUT: --- resolved `N` as `namespace N` in `package` (full_trace.carbon:16)
-// CHECK:STDOUT: --- resolved `Foo` as `fn N.Foo` in `namespace N` (full_trace.carbon:16)
-// CHECK:STDOUT: --- declared `x` as `x` in `{var x: i32 = N.Foo(0);return x;}` (full_trace.carbon:16)
-// CHECK:STDOUT: ** finished resolving stmt `var x: i32 = N.Foo(0);` (full_trace.carbon:16)
-// CHECK:STDOUT: ** resolving stmt `return x;` (full_trace.carbon:17)
-// CHECK:STDOUT: --- resolved `x` as `x` in `{var x: i32 = N.Foo(0);return x;}` (full_trace.carbon:17)
-// CHECK:STDOUT: ** finished resolving stmt `return x;` (full_trace.carbon:17)
-// CHECK:STDOUT: ** finished resolving stmt `{var x: i32 = N.Foo(0);return x;}` (full_trace.carbon:18)
-// CHECK:STDOUT: ** finished resolving decl `fn Main` (full_trace.carbon:18)
+// CHECK:STDOUT: ** resolving stmt `{var x: i32 = N.Foo(0);return x;}` (phase_all.carbon:18)
+// CHECK:STDOUT: ** resolving stmt `var x: i32 = N.Foo(0);` (phase_all.carbon:16)
+// CHECK:STDOUT: --- resolved `N` as `namespace N` in `package` (phase_all.carbon:16)
+// CHECK:STDOUT: --- resolved `Foo` as `fn N.Foo` in `namespace N` (phase_all.carbon:16)
+// CHECK:STDOUT: --- declared `x` as `x` in `{var x: i32 = N.Foo(0);return x;}` (phase_all.carbon:16)
+// CHECK:STDOUT: ** finished resolving stmt `var x: i32 = N.Foo(0);` (phase_all.carbon:16)
+// CHECK:STDOUT: ** resolving stmt `return x;` (phase_all.carbon:17)
+// CHECK:STDOUT: --- resolved `x` as `x` in `{var x: i32 = N.Foo(0);return x;}` (phase_all.carbon:17)
+// CHECK:STDOUT: ** finished resolving stmt `return x;` (phase_all.carbon:17)
+// CHECK:STDOUT: ** finished resolving stmt `{var x: i32 = N.Foo(0);return x;}` (phase_all.carbon:18)
+// CHECK:STDOUT: ** finished resolving decl `fn Main` (phase_all.carbon:18)
 // CHECK:STDOUT: --- resolved `Main` as `fn Main` in `package` (<Main()>:0)
 // CHECK:STDOUT: ********** resolving control flow **********
-// CHECK:STDOUT: --- flow-resolved return statement `return (n + 1);` in `fn N.Foo` (full_trace.carbon:12)
-// CHECK:STDOUT: --- flow-resolved return statement `return x;` in `fn Main` (full_trace.carbon:17)
+// CHECK:STDOUT: --- flow-resolved return statement `return (n + 1);` in `fn N.Foo` (phase_all.carbon:12)
+// CHECK:STDOUT: --- flow-resolved return statement `return x;` in `fn Main` (phase_all.carbon:17)
 // CHECK:STDOUT: ********** type checking **********
 // CHECK:STDOUT: ** declaring interface TestInterface
 // CHECK:STDOUT: ** finished declaring interface TestInterface
@@ -95,16 +96,16 @@ fn Main() -> i32 {
 // CHECK:STDOUT: checking IntTypeLiteral i32
 // CHECK:STDOUT: (+) stack-push: i32 .0.
 // CHECK:STDOUT: (+) stack-push: i32 .0.
-// CHECK:STDOUT: --- step exp i32 .0. (full_trace.carbon:11) --->
+// CHECK:STDOUT: --- step exp i32 .0. (phase_all.carbon:11) --->
 // CHECK:STDOUT: (-) stack-pop: i32 .0.
-// CHECK:STDOUT: (-) stack-pop: i32 .1. {{[[][[]}}i32]]
+// CHECK:STDOUT: (-) stack-pop: i32 .1. {{\[\[}}i32]]
 // CHECK:STDOUT: finished checking tuple pattern field n: i32
 // CHECK:STDOUT: checking IntTypeLiteral i32
 // CHECK:STDOUT: (+) stack-push: i32 .0.
 // CHECK:STDOUT: (+) stack-push: i32 .0.
-// CHECK:STDOUT: --- step exp i32 .0. (full_trace.carbon:11) --->
+// CHECK:STDOUT: --- step exp i32 .0. (phase_all.carbon:11) --->
 // CHECK:STDOUT: (-) stack-pop: i32 .0.
-// CHECK:STDOUT: (-) stack-pop: i32 .1. {{[[][[]}}i32]]
+// CHECK:STDOUT: (-) stack-pop: i32 .1. {{\[\[}}i32]]
 // CHECK:STDOUT: ** finished declaring function Foo of type fn (i32,) -> i32
 // CHECK:STDOUT: checking FunctionDeclaration
 // CHECK:STDOUT: ** checking function Foo
@@ -124,9 +125,9 @@ fn Main() -> i32 {
 // CHECK:STDOUT: checking IntTypeLiteral i32
 // CHECK:STDOUT: (+) stack-push: i32 .0.
 // CHECK:STDOUT: (+) stack-push: i32 .0.
-// CHECK:STDOUT: --- step exp i32 .0. (full_trace.carbon:15) --->
+// CHECK:STDOUT: --- step exp i32 .0. (phase_all.carbon:15) --->
 // CHECK:STDOUT: (-) stack-pop: i32 .0.
-// CHECK:STDOUT: (-) stack-pop: i32 .1. {{[[][[]}}i32]]
+// CHECK:STDOUT: (-) stack-pop: i32 .1. {{\[\[}}i32]]
 // CHECK:STDOUT: ** finished declaring function Main of type fn () -> i32
 // CHECK:STDOUT: checking FunctionDeclaration
 // CHECK:STDOUT: ** checking function Main
@@ -153,9 +154,9 @@ fn Main() -> i32 {
 // CHECK:STDOUT: checking IntTypeLiteral i32
 // CHECK:STDOUT: (+) stack-push: i32 .0.
 // CHECK:STDOUT: (+) stack-push: i32 .0.
-// CHECK:STDOUT: --- step exp i32 .0. (full_trace.carbon:16) --->
+// CHECK:STDOUT: --- step exp i32 .0. (phase_all.carbon:16) --->
 // CHECK:STDOUT: (-) stack-pop: i32 .0.
-// CHECK:STDOUT: (-) stack-pop: i32 .1. {{[[][[]}}i32]]
+// CHECK:STDOUT: (-) stack-pop: i32 .1. {{\[\[}}i32]]
 // CHECK:STDOUT: checking ReturnExpression return x;
 // CHECK:STDOUT: checking IdentifierExpression x
 // CHECK:STDOUT: ** finished checking function Main
@@ -167,18 +168,18 @@ fn Main() -> i32 {
 // CHECK:STDOUT: performing argument deduction for bindings:{{ }}
 // CHECK:STDOUT: deduction succeeded with results: {}
 // CHECK:STDOUT: ********** resolving unformed variables **********
-// CHECK:STDOUT: *** resolving-unformed in decl `interface TestInterface` (full_trace.carbon:7)
-// CHECK:STDOUT: *** resolving-unformed in decl `namespace N` (full_trace.carbon:9)
-// CHECK:STDOUT: *** resolving-unformed in decl `fn N.Foo` (full_trace.carbon:13)
-// CHECK:STDOUT: *** resolving-unformed in stmt `{return (n + 1);}` (full_trace.carbon:13)
-// CHECK:STDOUT: *** resolving-unformed in stmt `return (n + 1);` (full_trace.carbon:12)
-// CHECK:STDOUT: --- check `n` (full_trace.carbon:12)
-// CHECK:STDOUT: *** resolving-unformed in decl `fn Main` (full_trace.carbon:18)
-// CHECK:STDOUT: *** resolving-unformed in stmt `{var x: i32 = N.Foo(0);return x;}` (full_trace.carbon:18)
-// CHECK:STDOUT: *** resolving-unformed in stmt `var x: i32 = N.Foo(0);` (full_trace.carbon:16)
-// CHECK:STDOUT: --- add init `x` (full_trace.carbon:16)
-// CHECK:STDOUT: *** resolving-unformed in stmt `return x;` (full_trace.carbon:17)
-// CHECK:STDOUT: --- check `x` (full_trace.carbon:17)
+// CHECK:STDOUT: *** resolving-unformed in decl `interface TestInterface` (phase_all.carbon:7)
+// CHECK:STDOUT: *** resolving-unformed in decl `namespace N` (phase_all.carbon:9)
+// CHECK:STDOUT: *** resolving-unformed in decl `fn N.Foo` (phase_all.carbon:13)
+// CHECK:STDOUT: *** resolving-unformed in stmt `{return (n + 1);}` (phase_all.carbon:13)
+// CHECK:STDOUT: *** resolving-unformed in stmt `return (n + 1);` (phase_all.carbon:12)
+// CHECK:STDOUT: --- check `n` (phase_all.carbon:12)
+// CHECK:STDOUT: *** resolving-unformed in decl `fn Main` (phase_all.carbon:18)
+// CHECK:STDOUT: *** resolving-unformed in stmt `{var x: i32 = N.Foo(0);return x;}` (phase_all.carbon:18)
+// CHECK:STDOUT: *** resolving-unformed in stmt `var x: i32 = N.Foo(0);` (phase_all.carbon:16)
+// CHECK:STDOUT: --- add init `x` (phase_all.carbon:16)
+// CHECK:STDOUT: *** resolving-unformed in stmt `return x;` (phase_all.carbon:17)
+// CHECK:STDOUT: --- check `x` (phase_all.carbon:17)
 // CHECK:STDOUT: ********** printing declarations **********
 // CHECK:STDOUT: interface TestInterface {
 // CHECK:STDOUT: }
@@ -200,12 +201,12 @@ fn Main() -> i32 {
 // CHECK:STDOUT: (+) stack-push: interface TestInterface {
 // CHECK:STDOUT: }
 // CHECK:STDOUT:  .0.
-// CHECK:STDOUT: --- step decl interface TestInterface .0. (full_trace.carbon:7) --->
+// CHECK:STDOUT: --- step decl interface TestInterface .0. (phase_all.carbon:7) --->
 // CHECK:STDOUT: (-) stack-pop: interface TestInterface {
 // CHECK:STDOUT: }
 // CHECK:STDOUT:  .0.
 // CHECK:STDOUT: (+) stack-push: namespace N; .0.
-// CHECK:STDOUT: --- step decl namespace N .0. (full_trace.carbon:9) --->
+// CHECK:STDOUT: --- step decl namespace N .0. (phase_all.carbon:9) --->
 // CHECK:STDOUT: (-) stack-pop: namespace N; .0.
 // CHECK:STDOUT: (+) stack-push: fn Foo (n: i32)-> i32 {
 // CHECK:STDOUT: {
@@ -214,7 +215,7 @@ fn Main() -> i32 {
 // CHECK:STDOUT:
 // CHECK:STDOUT: }
 // CHECK:STDOUT:  .0.
-// CHECK:STDOUT: --- step decl fn N.Foo .0. (full_trace.carbon:13) --->
+// CHECK:STDOUT: --- step decl fn N.Foo .0. (phase_all.carbon:13) --->
 // CHECK:STDOUT: (-) stack-pop: fn Foo (n: i32)-> i32 {
 // CHECK:STDOUT: {
 // CHECK:STDOUT: return (n + 1);
@@ -230,7 +231,7 @@ fn Main() -> i32 {
 // CHECK:STDOUT:
 // CHECK:STDOUT: }
 // CHECK:STDOUT:  .0.
-// CHECK:STDOUT: --- step decl fn Main .0. (full_trace.carbon:18) --->
+// CHECK:STDOUT: --- step decl fn Main .0. (phase_all.carbon:18) --->
 // CHECK:STDOUT: (-) stack-pop: fn Main ()-> i32 {
 // CHECK:STDOUT: {
 // CHECK:STDOUT: var x: i32 = N.Foo(0);
@@ -247,32 +248,32 @@ fn Main() -> i32 {
 // CHECK:STDOUT: (+) stack-push: Main .0.
 // CHECK:STDOUT: --- step exp Main .0. (<Main()>:0) --->
 // CHECK:STDOUT: (-) stack-pop: Main .0.
-// CHECK:STDOUT: (-) stack-pop: Main .1. {{[[][[]}}fun<Main>]]
+// CHECK:STDOUT: (-) stack-pop: Main .1. {{\[\[}}fun<Main>]]
 // CHECK:STDOUT: --- step exp Main() .1. (<Main()>:0) --->
 // CHECK:STDOUT: calling function: fun<Main>
 // CHECK:STDOUT: match pattern ()
 // CHECK:STDOUT: from value expression with value ()
 // CHECK:STDOUT: (+) stack-push: .0. {}
 // CHECK:STDOUT: (+) stack-push: {var x: i32 = N.Foo(0);return x;} .0.
-// CHECK:STDOUT: --- step stmt {var x: i32 = N.Foo(0);return x;} .0. (full_trace.carbon:18) --->
+// CHECK:STDOUT: --- step stmt {var x: i32 = N.Foo(0);return x;} .0. (phase_all.carbon:18) --->
 // CHECK:STDOUT: (+) stack-push: var x: i32 = N.Foo(0); .0.
-// CHECK:STDOUT: --- step stmt var x: i32 = N.Foo(0); .0. (full_trace.carbon:16) --->
+// CHECK:STDOUT: --- step stmt var x: i32 = N.Foo(0); .0. (phase_all.carbon:16) --->
 // CHECK:STDOUT: (+) memory-alloc: #1 `Uninit<i32>` uninitialized
 // CHECK:STDOUT: (+) stack-push: N.Foo(0) .0.
-// CHECK:STDOUT: --- step exp N.Foo(0) .0. (full_trace.carbon:16) --->
+// CHECK:STDOUT: --- step exp N.Foo(0) .0. (phase_all.carbon:16) --->
 // CHECK:STDOUT: (+) stack-push: N.Foo .0.
 // CHECK:STDOUT: (+) stack-push: N.Foo .0.
-// CHECK:STDOUT: --- step exp N.Foo .0. (full_trace.carbon:16) --->
+// CHECK:STDOUT: --- step exp N.Foo .0. (phase_all.carbon:16) --->
 // CHECK:STDOUT: (-) stack-pop: N.Foo .0.
 // CHECK:STDOUT: (+) stack-push: Foo .0.
-// CHECK:STDOUT: --- step exp Foo .0. (full_trace.carbon:16) --->
+// CHECK:STDOUT: --- step exp Foo .0. (phase_all.carbon:16) --->
 // CHECK:STDOUT: (-) stack-pop: Foo .0.
-// CHECK:STDOUT: (-) stack-pop: N.Foo .1. {{[[][[]}}fun<N.Foo>]]
-// CHECK:STDOUT: --- step exp N.Foo(0) .1. (full_trace.carbon:16) --->
+// CHECK:STDOUT: (-) stack-pop: N.Foo .1. {{\[\[}}fun<N.Foo>]]
+// CHECK:STDOUT: --- step exp N.Foo(0) .1. (phase_all.carbon:16) --->
 // CHECK:STDOUT: (+) stack-push: 0 .0.
-// CHECK:STDOUT: --- step exp 0 .0. (full_trace.carbon:16) --->
+// CHECK:STDOUT: --- step exp 0 .0. (phase_all.carbon:16) --->
 // CHECK:STDOUT: (-) stack-pop: 0 .0.
-// CHECK:STDOUT: --- step exp N.Foo(0) .2. (full_trace.carbon:16) --->
+// CHECK:STDOUT: --- step exp N.Foo(0) .2. (phase_all.carbon:16) --->
 // CHECK:STDOUT: calling function: fun<N.Foo>
 // CHECK:STDOUT: match pattern (Placeholder<n>,)
 // CHECK:STDOUT: from value expression with value (0,)
@@ -280,55 +281,55 @@ fn Main() -> i32 {
 // CHECK:STDOUT: from value expression with value 0
 // CHECK:STDOUT: (+) stack-push: .0. {n: i32: 0}
 // CHECK:STDOUT: (+) stack-push: {return (n + 1);} .0.
-// CHECK:STDOUT: --- step stmt {return (n + 1);} .0. (full_trace.carbon:13) --->
+// CHECK:STDOUT: --- step stmt {return (n + 1);} .0. (phase_all.carbon:13) --->
 // CHECK:STDOUT: (+) stack-push: return (n + 1); .0.
-// CHECK:STDOUT: --- step stmt return (n + 1); .0. (full_trace.carbon:12) --->
+// CHECK:STDOUT: --- step stmt return (n + 1); .0. (phase_all.carbon:12) --->
 // CHECK:STDOUT: (+) stack-push: (n + 1) .0.
 // CHECK:STDOUT: (+) stack-push: (n + 1) .0.
-// CHECK:STDOUT: --- step exp (n + 1) .0. (full_trace.carbon:12) --->
+// CHECK:STDOUT: --- step exp (n + 1) .0. (phase_all.carbon:12) --->
 // CHECK:STDOUT: (+) stack-push: n .0.
 // CHECK:STDOUT: (+) stack-push: n .0.
-// CHECK:STDOUT: --- step exp n .0. (full_trace.carbon:12) --->
+// CHECK:STDOUT: --- step exp n .0. (phase_all.carbon:12) --->
 // CHECK:STDOUT: (-) stack-pop: n .0.
-// CHECK:STDOUT: (-) stack-pop: n .1. {{[[][[]}}0]]
-// CHECK:STDOUT: --- step exp (n + 1) .1. (full_trace.carbon:12) --->
+// CHECK:STDOUT: (-) stack-pop: n .1. {{\[\[}}0]]
+// CHECK:STDOUT: --- step exp (n + 1) .1. (phase_all.carbon:12) --->
 // CHECK:STDOUT: (+) stack-push: 1 .0.
 // CHECK:STDOUT: (+) stack-push: 1 .0.
-// CHECK:STDOUT: --- step exp 1 .0. (full_trace.carbon:12) --->
+// CHECK:STDOUT: --- step exp 1 .0. (phase_all.carbon:12) --->
 // CHECK:STDOUT: (-) stack-pop: 1 .0.
-// CHECK:STDOUT: (-) stack-pop: 1 .1. {{[[][[]}}1]]
-// CHECK:STDOUT: --- step exp (n + 1) .2. (full_trace.carbon:12) --->
-// CHECK:STDOUT: (-) stack-pop: (n + 1) .2. {{[[][[]}}0, 1]]
-// CHECK:STDOUT: (-) stack-pop: (n + 1) .1. {{[[][[]}}1]]
-// CHECK:STDOUT: --- step stmt return (n + 1); .1. (full_trace.carbon:12) --->
+// CHECK:STDOUT: (-) stack-pop: 1 .1. {{\[\[}}1]]
+// CHECK:STDOUT: --- step exp (n + 1) .2. (phase_all.carbon:12) --->
+// CHECK:STDOUT: (-) stack-pop: (n + 1) .2. {{\[\[}}0, 1]]
+// CHECK:STDOUT: (-) stack-pop: (n + 1) .1. {{\[\[}}1]]
+// CHECK:STDOUT: --- step stmt return (n + 1); .1. (phase_all.carbon:12) --->
 // CHECK:STDOUT: +++ memory-write: #1 `1`
-// CHECK:STDOUT: (-) stack-pop: return (n + 1); .1. {{[[][[]}}1]]
+// CHECK:STDOUT: (-) stack-pop: return (n + 1); .1. {{\[\[}}1]]
 // CHECK:STDOUT: (-) stack-pop: {return (n + 1);} .1. {}
 // CHECK:STDOUT: (-) stack-pop: .0. {n: i32: 0}
 // CHECK:STDOUT: (+) stack-push: clean up.0. {n: i32: 0}
 // CHECK:STDOUT: (+) stack-push: clean up.0. {}
 // CHECK:STDOUT: (-) stack-pop: clean up.0. {}
 // CHECK:STDOUT: (-) stack-pop: clean up.0. {n: i32: 0}
-// CHECK:STDOUT: --- step exp N.Foo(0) .3. (full_trace.carbon:16) --->
-// CHECK:STDOUT: (-) stack-pop: N.Foo(0) .3. {{[[][[]}}fun<N.Foo>, 0, 1]] {}
+// CHECK:STDOUT: --- step exp N.Foo(0) .3. (phase_all.carbon:16) --->
+// CHECK:STDOUT: (-) stack-pop: N.Foo(0) .3. {{\[\[}}fun<N.Foo>, 0, 1]] {}
 // CHECK:STDOUT: (+) stack-push: clean up.0. {}
 // CHECK:STDOUT: (-) stack-pop: clean up.0. {}
-// CHECK:STDOUT: --- step stmt var x: i32 = N.Foo(0); .1. (full_trace.carbon:16) --->
+// CHECK:STDOUT: --- step stmt var x: i32 = N.Foo(0); .1. (phase_all.carbon:16) --->
 // CHECK:STDOUT: +++ memory-read: #1 `1`
 // CHECK:STDOUT: match pattern Placeholder<x>
 // CHECK:STDOUT: from initializing expression with value 1
-// CHECK:STDOUT: (-) stack-pop: var x: i32 = N.Foo(0); .1. {{[[][[]}}1]]
-// CHECK:STDOUT: --- step stmt {var x: i32 = N.Foo(0);return x;} .1. (full_trace.carbon:18) --->
+// CHECK:STDOUT: (-) stack-pop: var x: i32 = N.Foo(0); .1. {{\[\[}}1]]
+// CHECK:STDOUT: --- step stmt {var x: i32 = N.Foo(0);return x;} .1. (phase_all.carbon:18) --->
 // CHECK:STDOUT: (+) stack-push: return x; .0.
-// CHECK:STDOUT: --- step stmt return x; .0. (full_trace.carbon:17) --->
+// CHECK:STDOUT: --- step stmt return x; .0. (phase_all.carbon:17) --->
 // CHECK:STDOUT: (+) stack-push: x .0.
 // CHECK:STDOUT: (+) stack-push: x .0.
-// CHECK:STDOUT: --- step exp x .0. (full_trace.carbon:17) --->
+// CHECK:STDOUT: --- step exp x .0. (phase_all.carbon:17) --->
 // CHECK:STDOUT: +++ memory-read: #1 `1`
 // CHECK:STDOUT: (-) stack-pop: x .0.
-// CHECK:STDOUT: (-) stack-pop: x .1. {{[[][[]}}ref_expr<Allocation(1)>]]
-// CHECK:STDOUT: --- step stmt return x; .1. (full_trace.carbon:17) --->
-// CHECK:STDOUT: (-) stack-pop: return x; .1. {{[[][[]}}1]]
+// CHECK:STDOUT: (-) stack-pop: x .1. {{\[\[}}ref_expr<Allocation(1)>]]
+// CHECK:STDOUT: --- step stmt return x; .1. (phase_all.carbon:17) --->
+// CHECK:STDOUT: (-) stack-pop: return x; .1. {{\[\[}}1]]
 // CHECK:STDOUT: (-) stack-pop: {var x: i32 = N.Foo(0);return x;} .2. {x: i32: lval<Allocation(1)>}
 // CHECK:STDOUT: (-) stack-pop: .0. {}
 // CHECK:STDOUT: (+) stack-push: clean up.0. {}
@@ -340,14 +341,14 @@ fn Main() -> i32 {
 // CHECK:STDOUT: (-) stack-pop: clean up.2. {x: i32: lval<Allocation(1)>}
 // CHECK:STDOUT: (-) stack-pop: clean up.0. {}
 // CHECK:STDOUT: --- step exp Main() .2. (<Main()>:0) --->
-// CHECK:STDOUT: (-) stack-pop: Main() .2. {{[[][[]}}fun<Main>, 1]] {}
+// CHECK:STDOUT: (-) stack-pop: Main() .2. {{\[\[}}fun<Main>, 1]] {}
 // CHECK:STDOUT: (+) stack-push: clean up.0. {}
 // CHECK:STDOUT: (-) stack-pop: clean up.0. {}
-// CHECK:STDOUT: (-) stack-pop: Main() .1. {{[[][[]}}1]]
+// CHECK:STDOUT: (-) stack-pop: Main() .1. {{\[\[}}1]]
 // CHECK:STDOUT: interpreter result: 1
 // CHECK:STDOUT: ********** printing timing **********
-// CHECK:STDOUT: Time elapsed in ExecProgram: {{[0-9]+}}ms
-// CHECK:STDOUT: Time elapsed in AnalyzeProgram: {{[0-9]+}}ms
-// CHECK:STDOUT: Time elapsed in AddPrelude: {{[0-9]+}}ms
-// CHECK:STDOUT: Time elapsed in Parse: {{[0-9]+}}ms
+// CHECK:STDOUT: Time elapsed in ExecProgram: {{\d+}}ms
+// CHECK:STDOUT: Time elapsed in AnalyzeProgram: {{\d+}}ms
+// CHECK:STDOUT: Time elapsed in AddPrelude: {{\d+}}ms
+// CHECK:STDOUT: Time elapsed in Parse: {{\d+}}ms
 // CHECK:STDOUT: result: 1
diff --git a/explorer/testdata/trace/phase_control_flow_resolution.carbon b/explorer/testdata/trace/phase_control_flow_resolution.carbon
new file mode 100644
index 000000000000..e954c5e99d60
--- /dev/null
+++ b/explorer/testdata/trace/phase_control_flow_resolution.carbon
@@ -0,0 +1,19 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+package ExplorerTest api;
+
+interface TestInterface {}
+
+fn Main() -> i32 {
+  return 0;
+}
+
+// Place checks after code so that line numbers are stable, reducing merge
+// conflicts.
+// ARGS: --trace_file=- --trace_phase=control_flow_resolution %s
+// AUTOUPDATE
+// CHECK:STDOUT: ********** resolving control flow **********
+// CHECK:STDOUT: --- flow-resolved return statement `return 0;` in `fn Main` (phase_control_flow_resolution.carbon:10)
+// CHECK:STDOUT: result: 0
diff --git a/explorer/testdata/trace/phase_declarations.carbon b/explorer/testdata/trace/phase_declarations.carbon
new file mode 100644
index 000000000000..4bde91f5ae57
--- /dev/null
+++ b/explorer/testdata/trace/phase_declarations.carbon
@@ -0,0 +1,26 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+package ExplorerTest api;
+
+interface TestInterface {}
+
+fn Main() -> i32 {
+  return 0;
+}
+
+// Place checks after code so that line numbers are stable, reducing merge
+// conflicts.
+// ARGS: --trace_file=- --trace_phase=declarations %s
+// AUTOUPDATE
+// CHECK:STDOUT: ********** printing declarations **********
+// CHECK:STDOUT: interface TestInterface {
+// CHECK:STDOUT: }
+// CHECK:STDOUT: fn Main ()-> i32 {
+// CHECK:STDOUT: {
+// CHECK:STDOUT: return 0;
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: }
+// CHECK:STDOUT: result: 0
diff --git a/explorer/testdata/trace/phase_execution.carbon b/explorer/testdata/trace/phase_execution.carbon
new file mode 100644
index 000000000000..d941627f5ea9
--- /dev/null
+++ b/explorer/testdata/trace/phase_execution.carbon
@@ -0,0 +1,78 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+package ExplorerTest api;
+
+interface TestInterface {}
+
+fn Main() -> i32 {
+  return 0;
+}
+
+// Place checks after code so that line numbers are stable, reducing merge
+// conflicts.
+// ARGS: --trace_file=- --trace_phase=execution %s
+// AUTOUPDATE
+// CHECK:STDOUT: ********** starting execution **********
+// CHECK:STDOUT: ********** initializing globals **********
+// CHECK:STDOUT: (+) stack-push: interface TestInterface {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:  .0.
+// CHECK:STDOUT: --- step decl interface TestInterface .0. (phase_execution.carbon:7) --->
+// CHECK:STDOUT: (-) stack-pop: interface TestInterface {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:  .0.
+// CHECK:STDOUT: (+) stack-push: fn Main ()-> i32 {
+// CHECK:STDOUT: {
+// CHECK:STDOUT: return 0;
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: }
+// CHECK:STDOUT:  .0.
+// CHECK:STDOUT: --- step decl fn Main .0. (phase_execution.carbon:11) --->
+// CHECK:STDOUT: (-) stack-pop: fn Main ()-> i32 {
+// CHECK:STDOUT: {
+// CHECK:STDOUT: return 0;
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: }
+// CHECK:STDOUT:  .0.
+// CHECK:STDOUT: ********** calling main function **********
+// CHECK:STDOUT: (+) stack-push: Main() .0.
+// CHECK:STDOUT: (+) stack-push: Main() .0.
+// CHECK:STDOUT: --- step exp Main() .0. (<Main()>:0) --->
+// CHECK:STDOUT: (+) stack-push: Main .0.
+// CHECK:STDOUT: (+) stack-push: Main .0.
+// CHECK:STDOUT: --- step exp Main .0. (<Main()>:0) --->
+// CHECK:STDOUT: (-) stack-pop: Main .0.
+// CHECK:STDOUT: (-) stack-pop: Main .1. {{\[\[}}fun<Main>]]
+// CHECK:STDOUT: --- step exp Main() .1. (<Main()>:0) --->
+// CHECK:STDOUT: calling function: fun<Main>
+// CHECK:STDOUT: match pattern ()
+// CHECK:STDOUT: from value expression with value ()
+// CHECK:STDOUT: (+) stack-push: .0. {}
+// CHECK:STDOUT: (+) stack-push: {return 0;} .0.
+// CHECK:STDOUT: --- step stmt {return 0;} .0. (phase_execution.carbon:11) --->
+// CHECK:STDOUT: (+) stack-push: return 0; .0.
+// CHECK:STDOUT: --- step stmt return 0; .0. (phase_execution.carbon:10) --->
+// CHECK:STDOUT: (+) stack-push: 0 .0.
+// CHECK:STDOUT: (+) stack-push: 0 .0.
+// CHECK:STDOUT: --- step exp 0 .0. (phase_execution.carbon:10) --->
+// CHECK:STDOUT: (-) stack-pop: 0 .0.
+// CHECK:STDOUT: (-) stack-pop: 0 .1. {{\[\[}}0]]
+// CHECK:STDOUT: --- step stmt return 0; .1. (phase_execution.carbon:10) --->
+// CHECK:STDOUT: (-) stack-pop: return 0; .1. {{\[\[}}0]]
+// CHECK:STDOUT: (-) stack-pop: {return 0;} .1. {}
+// CHECK:STDOUT: (-) stack-pop: .0. {}
+// CHECK:STDOUT: (+) stack-push: clean up.0. {}
+// CHECK:STDOUT: (+) stack-push: clean up.0. {}
+// CHECK:STDOUT: (-) stack-pop: clean up.0. {}
+// CHECK:STDOUT: (-) stack-pop: clean up.0. {}
+// CHECK:STDOUT: --- step exp Main() .2. (<Main()>:0) --->
+// CHECK:STDOUT: (-) stack-pop: Main() .2. {{\[\[}}fun<Main>, 0]] {}
+// CHECK:STDOUT: (+) stack-push: clean up.0. {}
+// CHECK:STDOUT: (-) stack-pop: clean up.0. {}
+// CHECK:STDOUT: (-) stack-pop: Main() .1. {{\[\[}}0]]
+// CHECK:STDOUT: interpreter result: 0
+// CHECK:STDOUT: result: 0
diff --git a/explorer/testdata/trace/phase_name_resolution.carbon b/explorer/testdata/trace/phase_name_resolution.carbon
new file mode 100644
index 000000000000..237d2fcce1ff
--- /dev/null
+++ b/explorer/testdata/trace/phase_name_resolution.carbon
@@ -0,0 +1,34 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+package ExplorerTest api;
+
+interface TestInterface {}
+
+fn Main() -> i32 {
+  return 0;
+}
+
+// Place checks after code so that line numbers are stable, reducing merge
+// conflicts.
+// ARGS: --trace_file=- --trace_phase=name_resolution %s
+// AUTOUPDATE
+// CHECK:STDOUT: ********** resolving names **********
+// CHECK:STDOUT: --- declared `TestInterface` as `interface TestInterface` in `package` (phase_name_resolution.carbon:7)
+// CHECK:STDOUT: --- declared `Main` as `fn Main` in `package` (phase_name_resolution.carbon:11)
+// CHECK:STDOUT: ** resolving decl `interface TestInterface` (phase_name_resolution.carbon:7)
+// CHECK:STDOUT: --- marked `TestInterface` declared but not usable in `package`
+// CHECK:STDOUT: --- marked `TestInterface` usable in `package`
+// CHECK:STDOUT: --- declared `Self` as `Self` in `interface TestInterface` (phase_name_resolution.carbon:7)
+// CHECK:STDOUT: ** finished resolving decl `interface TestInterface` (phase_name_resolution.carbon:7)
+// CHECK:STDOUT: ** resolving decl `fn Main` (phase_name_resolution.carbon:11)
+// CHECK:STDOUT: --- marked `Main` declared but not usable in `package`
+// CHECK:STDOUT: --- marked `Main` usable in `package`
+// CHECK:STDOUT: ** resolving stmt `{return 0;}` (phase_name_resolution.carbon:11)
+// CHECK:STDOUT: ** resolving stmt `return 0;` (phase_name_resolution.carbon:10)
+// CHECK:STDOUT: ** finished resolving stmt `return 0;` (phase_name_resolution.carbon:10)
+// CHECK:STDOUT: ** finished resolving stmt `{return 0;}` (phase_name_resolution.carbon:11)
+// CHECK:STDOUT: ** finished resolving decl `fn Main` (phase_name_resolution.carbon:11)
+// CHECK:STDOUT: --- resolved `Main` as `fn Main` in `package` (<Main()>:0)
+// CHECK:STDOUT: result: 0
diff --git a/explorer/testdata/trace/phase_source_program.carbon b/explorer/testdata/trace/phase_source_program.carbon
new file mode 100644
index 000000000000..d56b30b0e0ac
--- /dev/null
+++ b/explorer/testdata/trace/phase_source_program.carbon
@@ -0,0 +1,26 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+package ExplorerTest api;
+
+interface TestInterface {}
+
+fn Main() -> i32 {
+  return 0;
+}
+
+// Place checks after code so that line numbers are stable, reducing merge
+// conflicts.
+// ARGS: --trace_file=- --trace_phase=source_program %s
+// AUTOUPDATE
+// CHECK:STDOUT: ********** source program **********
+// CHECK:STDOUT: interface TestInterface {
+// CHECK:STDOUT: }
+// CHECK:STDOUT: fn Main ()-> i32 {
+// CHECK:STDOUT: {
+// CHECK:STDOUT: return 0;
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: }
+// CHECK:STDOUT: result: 0
diff --git a/explorer/testdata/trace/phase_timing.carbon b/explorer/testdata/trace/phase_timing.carbon
new file mode 100644
index 000000000000..6d8a9f61c3de
--- /dev/null
+++ b/explorer/testdata/trace/phase_timing.carbon
@@ -0,0 +1,22 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+package ExplorerTest api;
+
+interface TestInterface {}
+
+fn Main() -> i32 {
+  return 0;
+}
+
+// Place checks after code so that line numbers are stable, reducing merge
+// conflicts.
+// ARGS: --trace_file=- --trace_phase=timing %s
+// AUTOUPDATE
+// CHECK:STDOUT: ********** printing timing **********
+// CHECK:STDOUT: Time elapsed in ExecProgram: {{\d+}}ms
+// CHECK:STDOUT: Time elapsed in AnalyzeProgram: {{\d+}}ms
+// CHECK:STDOUT: Time elapsed in AddPrelude: {{\d+}}ms
+// CHECK:STDOUT: Time elapsed in Parse: {{\d+}}ms
+// CHECK:STDOUT: result: 0
diff --git a/explorer/testdata/trace/phase_type_checking.carbon b/explorer/testdata/trace/phase_type_checking.carbon
new file mode 100644
index 000000000000..7c535ae0a9f0
--- /dev/null
+++ b/explorer/testdata/trace/phase_type_checking.carbon
@@ -0,0 +1,52 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+package ExplorerTest api;
+
+interface TestInterface {}
+
+fn Main() -> i32 {
+  return 0;
+}
+
+// Place checks after code so that line numbers are stable, reducing merge
+// conflicts.
+// ARGS: --trace_file=- --trace_phase=type_checking %s
+// AUTOUPDATE
+// CHECK:STDOUT: ********** type checking **********
+// CHECK:STDOUT: ** declaring interface TestInterface
+// CHECK:STDOUT: ** finished declaring interface TestInterface
+// CHECK:STDOUT: checking InterfaceDeclaration
+// CHECK:STDOUT: ** checking interface TestInterface
+// CHECK:STDOUT: impl declarations:{{ }}
+// CHECK:STDOUT: impl declarations: bool as interface EqWith(U = bool), i32 as interface EqWith(U = i32), String as interface EqWith(U = String), i32 as interface CompareWith(U = i32), String as interface CompareWith(U = String), i32 as interface LessWith(U = i32), String as interface LessWith(U = String), i32 as interface LessEqWith(U = i32), String as interface LessEqWith(U = String), i32 as interface GreaterWith(U = i32), String as interface GreaterWith(U = String), i32 as interface GreaterEqWith(U = i32), String as interface GreaterEqWith(U = String), i32 as interface Negate, i32 as interface AddWith(U = i32), i32 as interface SubWith(U = i32), i32 as interface MulWith(U = i32), i32 as interface DivWith(U = i32), i32 as interface ModWith(U = i32), i32 as interface BitComplement, i32 as interface BitAndWith(U = i32), i32 as interface BitOrWith(U = i32), i32 as interface BitXorWith(U = i32), i32 as interface LeftShiftWith(U = i32), i32 as interface RightShiftWith(U = i32), i32 as interface Inc, i32 as interface Dec, U as interface __EqualConverter [0], (T1,) as interface As(T = (U1,)) [0, 0, 0; 1, 1, 0, 0, 0], (T1, T2) as interface As(T = (U1, U2)) [0, 0, 0; 0, 0, 1; 1, 1, 0, 0, 0; 1, 1, 0, 0, 1], (T1, U1) as interface EqWith(U = (T2, U2)) [0, 0, 0; 0, 0, 1; 1, 1, 0, 0, 0; 1, 1, 0, 0, 1], (T1, T2, T3) as interface As(T = (U1, U2, U3)) [0, 0, 0; 0, 0, 1; 0, 0, 2; 1, 1, 0, 0, 0; 1, 1, 0, 0, 1; 1, 1, 0, 0, 2], T as interface ImplicitAs(T = U) [0; 1, 1, 0], T as interface As(T = U) [0; 1, 1, 0], T as interface ImplicitAs(T = U) [0; 1, 1, 0], T as interface As(T = U) [0; 1, 1, 0], T as interface AssignWith(U = U) [0; 1, 1, 0], T as interface AddAssignWith(U = U) [0; 1, 1, 0], T as interface SubAssignWith(U = U) [0; 1, 1, 0], T as interface MulAssignWith(U = U) [0; 1, 1, 0], T as interface DivAssignWith(U = U) [0; 1, 1, 0], T as interface ModAssignWith(U = U) [0; 1, 1, 0], T as interface BitAndAssignWith(U = U) [0; 1, 1, 0], T as interface BitOrAssignWith(U = U) [0; 1, 1, 0], T as interface BitXorAssignWith(U = U) [0; 1, 1, 0], T as interface LeftShiftAssignWith(U = U) [0; 1, 1, 0], T as interface RightShiftAssignWith(U = U) [0; 1, 1, 0]
+// CHECK:STDOUT: ** finished checking interface TestInterface
+// CHECK:STDOUT: ** declaring function Main
+// CHECK:STDOUT: checking TuplePattern ()
+// CHECK:STDOUT: checking IntTypeLiteral i32
+// CHECK:STDOUT: (+) stack-push: i32 .0.
+// CHECK:STDOUT: (+) stack-push: i32 .0.
+// CHECK:STDOUT: --- step exp i32 .0. (phase_type_checking.carbon:9) --->
+// CHECK:STDOUT: (-) stack-pop: i32 .0.
+// CHECK:STDOUT: (-) stack-pop: i32 .1. {{\[\[}}i32]]
+// CHECK:STDOUT: ** finished declaring function Main of type fn () -> i32
+// CHECK:STDOUT: checking FunctionDeclaration
+// CHECK:STDOUT: ** checking function Main
+// CHECK:STDOUT: impl declarations:{{ }}
+// CHECK:STDOUT: impl declarations: bool as interface EqWith(U = bool), i32 as interface EqWith(U = i32), String as interface EqWith(U = String), i32 as interface CompareWith(U = i32), String as interface CompareWith(U = String), i32 as interface LessWith(U = i32), String as interface LessWith(U = String), i32 as interface LessEqWith(U = i32), String as interface LessEqWith(U = String), i32 as interface GreaterWith(U = i32), String as interface GreaterWith(U = String), i32 as interface GreaterEqWith(U = i32), String as interface GreaterEqWith(U = String), i32 as interface Negate, i32 as interface AddWith(U = i32), i32 as interface SubWith(U = i32), i32 as interface MulWith(U = i32), i32 as interface DivWith(U = i32), i32 as interface ModWith(U = i32), i32 as interface BitComplement, i32 as interface BitAndWith(U = i32), i32 as interface BitOrWith(U = i32), i32 as interface BitXorWith(U = i32), i32 as interface LeftShiftWith(U = i32), i32 as interface RightShiftWith(U = i32), i32 as interface Inc, i32 as interface Dec, U as interface __EqualConverter [0], (T1,) as interface As(T = (U1,)) [0, 0, 0; 1, 1, 0, 0, 0], (T1, T2) as interface As(T = (U1, U2)) [0, 0, 0; 0, 0, 1; 1, 1, 0, 0, 0; 1, 1, 0, 0, 1], (T1, U1) as interface EqWith(U = (T2, U2)) [0, 0, 0; 0, 0, 1; 1, 1, 0, 0, 0; 1, 1, 0, 0, 1], (T1, T2, T3) as interface As(T = (U1, U2, U3)) [0, 0, 0; 0, 0, 1; 0, 0, 2; 1, 1, 0, 0, 0; 1, 1, 0, 0, 1; 1, 1, 0, 0, 2], T as interface ImplicitAs(T = U) [0; 1, 1, 0], T as interface As(T = U) [0; 1, 1, 0], T as interface ImplicitAs(T = U) [0; 1, 1, 0], T as interface As(T = U) [0; 1, 1, 0], T as interface AssignWith(U = U) [0; 1, 1, 0], T as interface AddAssignWith(U = U) [0; 1, 1, 0], T as interface SubAssignWith(U = U) [0; 1, 1, 0], T as interface MulAssignWith(U = U) [0; 1, 1, 0], T as interface DivAssignWith(U = U) [0; 1, 1, 0], T as interface ModAssignWith(U = U) [0; 1, 1, 0], T as interface BitAndAssignWith(U = U) [0; 1, 1, 0], T as interface BitOrAssignWith(U = U) [0; 1, 1, 0], T as interface BitXorAssignWith(U = U) [0; 1, 1, 0], T as interface LeftShiftAssignWith(U = U) [0; 1, 1, 0], T as interface RightShiftAssignWith(U = U) [0; 1, 1, 0]
+// CHECK:STDOUT: checking Block {
+// CHECK:STDOUT: return 0;
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: checking ReturnExpression return 0;
+// CHECK:STDOUT: checking IntLiteral 0
+// CHECK:STDOUT: ** finished checking function Main
+// CHECK:STDOUT: checking CallExpression Main()
+// CHECK:STDOUT: checking IdentifierExpression Main
+// CHECK:STDOUT: checking TupleLiteral ()
+// CHECK:STDOUT: checking call to function of type fn () -> i32
+// CHECK:STDOUT: with arguments of type: ()
+// CHECK:STDOUT: performing argument deduction for bindings:{{ }}
+// CHECK:STDOUT: deduction succeeded with results: {}
+// CHECK:STDOUT: result: 0
diff --git a/explorer/testdata/trace/phase_unformed_variables_resolution.carbon b/explorer/testdata/trace/phase_unformed_variables_resolution.carbon
new file mode 100644
index 000000000000..c3bda2856da5
--- /dev/null
+++ b/explorer/testdata/trace/phase_unformed_variables_resolution.carbon
@@ -0,0 +1,22 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+package ExplorerTest api;
+
+interface TestInterface {}
+
+fn Main() -> i32 {
+  return 0;
+}
+
+// Place checks after code so that line numbers are stable, reducing merge
+// conflicts.
+// ARGS: --trace_file=- --trace_phase=unformed_variables_resolution %s
+// AUTOUPDATE
+// CHECK:STDOUT: ********** resolving unformed variables **********
+// CHECK:STDOUT: *** resolving-unformed in decl `interface TestInterface` (phase_unformed_variables_resolution.carbon:7)
+// CHECK:STDOUT: *** resolving-unformed in decl `fn Main` (phase_unformed_variables_resolution.carbon:11)
+// CHECK:STDOUT: *** resolving-unformed in stmt `{return 0;}` (phase_unformed_variables_resolution.carbon:11)
+// CHECK:STDOUT: *** resolving-unformed in stmt `return 0;` (phase_unformed_variables_resolution.carbon:10)
+// CHECK:STDOUT: result: 0
diff --git a/testing/file_test/autoupdate.cpp b/testing/file_test/autoupdate.cpp
index 6122c27ad4e3..f2c305f68f47 100644
--- a/testing/file_test/autoupdate.cpp
+++ b/testing/file_test/autoupdate.cpp
@@ -19,12 +19,23 @@ namespace Carbon::Testing {
 // Put helper classes in an anonymous namespace.
 namespace {
 
+// Converts a matched line number to an int, trimming whitespace.
+static auto ParseLineNumber(absl::string_view matched_line_number) -> int {
+  llvm::StringRef trimmed = matched_line_number;
+  trimmed = trimmed.trim();
+  // NOLINTNEXTLINE(google-runtime-int): API requirement.
+  long long val;
+  CARBON_CHECK(!llvm::getAsSignedInteger(trimmed, 10, val));
+  return val;
+}
+
 class CheckLine : public FileTestLineBase {
  public:
   // RE2 is passed by a pointer because it doesn't support std::optional.
-  explicit CheckLine(int line_number, const RE2* line_number_re,
-                     std::string line)
+  explicit CheckLine(int line_number, bool line_number_re_has_file,
+                     const RE2* line_number_re, std::string line)
       : FileTestLineBase(line_number),
+        line_number_re_has_file_(line_number_re_has_file),
         line_number_re_(line_number_re),
         line_(std::move(line)) {}
 
@@ -41,20 +52,50 @@ class CheckLine : public FileTestLineBase {
 
   // When the location of all lines in a file are known, we can set the line
   // offset based on the target line.
-  auto SetRemappedLine(const std::string& sub_for_formatv,
-                       int target_line_number) -> void {
-    // Should only be called when we have a regex.
-    CARBON_CHECK(line_number_re_);
+  auto RemapLineNumbers(const llvm::DenseMap<int, int>& output_line_remap,
+                        const std::string& line_formatv) -> void {
+    // Only need to do remappings when there's a regex.
+    if (!line_number_re_) {
+      return;
+    }
 
-    int offset = target_line_number - output_line_number_;
-    const char* offset_prefix = offset < 0 ? "" : "+";
-    std::string replacement =
-        llvm::formatv(sub_for_formatv.data(),
-                      llvm::formatv("[[@LINE{0}{1}]]", offset_prefix, offset));
-    RE2::Replace(&line_, *line_number_re_, replacement);
+    bool found_one = false;
+    while (true) {
+      // Look for a line number to replace. There may be multiple, so we
+      // repeatedly check.
+      absl::string_view matched_line_number;
+      if (line_number_re_has_file_) {
+        RE2::PartialMatch(line_, *line_number_re_, nullptr,
+                          &matched_line_number);
+      } else {
+        RE2::PartialMatch(line_, *line_number_re_, &matched_line_number);
+      }
+      if (matched_line_number.empty()) {
+        CARBON_CHECK(found_one) << line_;
+        return;
+      }
+      found_one = true;
+
+      // Calculate the offset from the CHECK line to the new line number
+      // (possibly with new CHECK lines added, or some removed).
+      // NOLINTNEXTLINE(google-runtime-int): API requirement.
+      auto remapped =
+          output_line_remap.find(ParseLineNumber(matched_line_number));
+      CARBON_CHECK(remapped != output_line_remap.end());
+      int offset = remapped->second - output_line_number_;
+
+      // Update the line offset in the CHECK line.
+      const char* offset_prefix = offset < 0 ? "" : "+";
+      std::string replacement = llvm::formatv(
+          line_formatv.c_str(),
+          llvm::formatv("[[@LINE{0}{1}]]", offset_prefix, offset));
+      line_.replace(matched_line_number.data() - line_.data(),
+                    matched_line_number.size(), replacement);
+    }
   }
 
  private:
+  bool line_number_re_has_file_;
   const RE2* line_number_re_;
   std::string line_;
   llvm::StringRef indent_;
@@ -90,10 +131,19 @@ static auto AddCheckLines(
     lines.pop_back();
   }
 
+  // `{{` and `[[` are escaped as a regex matcher.
+  RE2 double_brace_re(R"(\{\{)");
+  RE2 double_square_bracket_re(R"(\[\[)");
+  // End-of-line whitespace is replaced with a regex matcher to make it visible.
+  RE2 end_of_line_whitespace_re(R"((\s+)$)");
+
   int append_to = 0;
   for (const auto& line : lines) {
     std::string check_line = llvm::formatv("// CHECK:{0}:{1}{2}", label,
                                            line.empty() ? "" : " ", line);
+    RE2::Replace(&check_line, double_brace_re, R"({{\\{\\{}})");
+    RE2::Replace(&check_line, double_square_bracket_re, R"({{\\[\\[}})");
+    RE2::Replace(&check_line, end_of_line_whitespace_re, R"({{\1}})");
 
     // Ignore TEST_TMPDIR in output.
     if (auto pos = check_line.find(tmpdir); pos != std::string::npos) {
@@ -102,10 +152,10 @@ static auto AddCheckLines(
 
     do_extra_check_replacements(check_line);
 
-    // Look for line information in the output. line_number is only set if the
-    // match is correct.
-    int line_number = -1;
-    int match_line_number;
+    // Look for line information in the output. use_line_number is only set if
+    // the match is correct.
+    std::optional<llvm::StringRef> use_line_number;
+    absl::string_view match_line_number;
     if (line_number_re_has_file) {
       absl::string_view match_filename;
       if (RE2::PartialMatch(check_line, line_number_re, &match_filename,
@@ -119,22 +169,25 @@ static auto AddCheckLines(
           if (pos != filenames.end()) {
             remaining_filenames = pos + 1;
             append_to = pos - filenames.begin();
-            line_number = match_line_number;
+            use_line_number = match_line_number;
           }
         } else {
           // The line applies to the current file.
-          line_number = match_line_number;
+          use_line_number = match_line_number;
         }
       }
     } else {
       // There's no file association, so we only look at the line.
       if (RE2::PartialMatch(check_line, line_number_re, &match_line_number)) {
-        line_number = match_line_number;
+        use_line_number = match_line_number;
       }
     }
+    // NOLINTNEXTLINE(google-runtime-int): API requirement.
+    long long line_number =
+        use_line_number ? ParseLineNumber(*use_line_number) : -1;
     check_lines[append_to].push_back(
-        CheckLine(line_number, line_number == -1 ? nullptr : &line_number_re,
-                  check_line));
+        CheckLine(line_number, line_number_re_has_file,
+                  use_line_number ? &line_number_re : nullptr, check_line));
   }
 }
 
@@ -212,12 +265,8 @@ auto AutoupdateFileTest(
 
     // Update all remapped lines in CHECK output.
     for (auto& offset_check_line : check_file) {
-      if (offset_check_line.line_number() >= 1) {
-        auto new_line = output_line_remap.find(offset_check_line.line_number());
-        CARBON_CHECK(new_line != output_line_remap.end());
-        offset_check_line.SetRemappedLine(
-            line_number_replacement.sub_for_formatv, new_line->second);
-      }
+      offset_check_line.RemapLineNumbers(output_line_remap,
+                                         line_number_replacement.line_formatv);
     }
   }
 
diff --git a/testing/file_test/autoupdate.h b/testing/file_test/autoupdate.h
index 0683610f2272..c0c81165795a 100644
--- a/testing/file_test/autoupdate.h
+++ b/testing/file_test/autoupdate.h
@@ -20,8 +20,8 @@ struct FileTestLineNumberReplacement {
   // should have a file and line group; otherwise, only a line group.
   std::string pattern;
 
-  // sub_for_formatv should provide {0} to substitute with [[@LINE...]] deltas.
-  std::string sub_for_formatv;
+  // line_formatv should provide {0} to substitute with [[@LINE...]] deltas.
+  std::string line_formatv;
 };
 
 // Automatically updates CHECKs in the provided file. Returns true if updated.
diff --git a/testing/file_test/file_test_base.cpp b/testing/file_test/file_test_base.cpp
index 7dd6becb674f..1a55f3453bb6 100644
--- a/testing/file_test/file_test_base.cpp
+++ b/testing/file_test/file_test_base.cpp
@@ -52,15 +52,11 @@ static auto SplitOutput(llvm::StringRef output)
 // Runs a test and compares output. This keeps output split by line so that
 // issues are a little easier to identify by the different line.
 auto FileTestBase::TestBody() -> void {
-  const char* src_dir = getenv("TEST_SRCDIR");
-  CARBON_CHECK(src_dir);
-  std::string test_file = path().lexically_relative(
-      std::filesystem::path(src_dir).append("carbon"));
   const char* target = getenv("TEST_TARGET");
   CARBON_CHECK(target);
   // This advice overrides the --file_tests flag provided by the file_test rule.
   llvm::errs() << "\nTo test this file alone, run:\n  bazel test " << target
-               << " --test_arg=--file_tests=" << test_file << "\n\n";
+               << " --test_arg=--file_tests=" << GetTestFilename() << "\n\n";
 
   TestContext context;
   auto run_result = ProcessTestFileAndRun(context);
@@ -86,10 +82,13 @@ auto FileTestBase::TestBody() -> void {
   }
 }
 
-auto FileTestBase::Autoupdate() -> bool {
+auto FileTestBase::Autoupdate() -> ErrorOr<bool> {
   TestContext context;
   auto run_result = ProcessTestFileAndRun(context);
-  CARBON_CHECK(run_result.ok()) << run_result.error();
+  if (!run_result.ok()) {
+    return ErrorBuilder() << "Error updating " << GetTestFilename() << ": "
+                          << run_result.error();
+  }
   if (!context.autoupdate_line_number) {
     return false;
   }
@@ -120,8 +119,8 @@ auto FileTestBase::GetLineNumberReplacement(
     llvm::ArrayRef<llvm::StringRef> filenames) -> LineNumberReplacement {
   return {
       .has_file = true,
-      .pattern = llvm::formatv(R"(({0}):(\d+):)", llvm::join(filenames, "|")),
-      .sub_for_formatv = R"(\1:{0}:)"};
+      .pattern = llvm::formatv(R"(({0}):(\d+))", llvm::join(filenames, "|")),
+      .line_formatv = R"({0})"};
 }
 
 auto FileTestBase::ProcessTestFileAndRun(TestContext& context)
@@ -258,15 +257,21 @@ auto FileTestBase::ProcessTestFile(TestContext& context) -> ErrorOr<Success> {
 
     // Process expectations when found.
     if (line_trimmed.consume_front("// CHECK")) {
-      llvm::SmallVector<Matcher<std::string>>* expected = nullptr;
-      if (line_trimmed.consume_front(":STDOUT:")) {
-        expected = &context.expected_stdout;
-      } else if (line_trimmed.consume_front(":STDERR:")) {
-        expected = &context.expected_stderr;
-      } else {
-        return ErrorBuilder() << "Unexpected CHECK in input: " << line.str();
+      // Don't build expectations when doing an autoupdate. We don't want to
+      // break the autoupdate on an invalid CHECK line.
+      if (!absl::GetFlag(FLAGS_autoupdate)) {
+        llvm::SmallVector<Matcher<std::string>>* expected = nullptr;
+        if (line_trimmed.consume_front(":STDOUT:")) {
+          expected = &context.expected_stdout;
+        } else if (line_trimmed.consume_front(":STDERR:")) {
+          expected = &context.expected_stderr;
+        } else {
+          return ErrorBuilder() << "Unexpected CHECK in input: " << line.str();
+        }
+        CARBON_ASSIGN_OR_RETURN(Matcher<std::string> check_matcher,
+                                TransformExpectation(line_index, line_trimmed));
+        expected->push_back(check_matcher);
       }
-      expected->push_back(TransformExpectation(line_index, line_trimmed));
     } else {
       context.non_check_lines.back().push_back(FileTestLine(line_index, line));
       if (line_trimmed.consume_front("// ARGS: ")) {
@@ -330,11 +335,13 @@ auto FileTestBase::ProcessTestFile(TestContext& context) -> ErrorOr<Success> {
 }
 
 auto FileTestBase::TransformExpectation(int line_index, llvm::StringRef in)
-    -> Matcher<std::string> {
+    -> ErrorOr<Matcher<std::string>> {
   if (in.empty()) {
-    return StrEq("");
+    return Matcher<std::string>{StrEq("")};
+  }
+  if (in[0] != ' ') {
+    return ErrorBuilder() << "Malformated CHECK line: " << in;
   }
-  CARBON_CHECK(in[0] == ' ') << "Malformated input: " << in;
   std::string str = in.substr(1).str();
   for (int pos = 0; pos < static_cast<int>(str.size());) {
     switch (str[pos]) {
@@ -364,18 +371,20 @@ auto FileTestBase::TransformExpectation(int line_index, llvm::StringRef in)
             line_keyword_cursor.consume_front("+");
             int offset;
             // consumeInteger returns true for errors, not false.
-            CARBON_CHECK(!line_keyword_cursor.consumeInteger(10, offset) &&
-                         line_keyword_cursor.consume_front("]]"))
-                << "Unexpected @LINE offset at `"
-                << line_keyword_cursor.substr(0, 5) << "` in: " << in;
+            if (line_keyword_cursor.consumeInteger(10, offset) ||
+                !line_keyword_cursor.consume_front("]]")) {
+              return ErrorBuilder()
+                     << "Unexpected @LINE offset at `"
+                     << line_keyword_cursor.substr(0, 5) << "` in: " << in;
+            }
             std::string int_str = llvm::Twine(line_index + offset).str();
             int remove_len = (line_keyword_cursor.data() - str.data()) - pos;
             str.replace(pos, remove_len, int_str);
             pos += int_str.size();
           } else {
-            CARBON_FATAL() << "Unexpected [[, should be {{\\[\\[}} at `"
-                           << line_keyword_cursor.substr(0, 5)
-                           << "` in: " << in;
+            return ErrorBuilder()
+                   << "Unexpected [[, should be {{\\[\\[}} at `"
+                   << line_keyword_cursor.substr(0, 5) << "` in: " << in;
           }
         } else {
           // Escape the `[`.
@@ -408,7 +417,14 @@ auto FileTestBase::TransformExpectation(int line_index, llvm::StringRef in)
     }
   }
 
-  return MatchesRegex(str);
+  return Matcher<std::string>{MatchesRegex(str)};
+}
+
+auto FileTestBase::GetTestFilename() -> std::string {
+  const char* src_dir = getenv("TEST_SRCDIR");
+  CARBON_CHECK(src_dir);
+  return path().lexically_relative(
+      std::filesystem::path(src_dir).append("carbon"));
 }
 
 }  // namespace Carbon::Testing
@@ -429,7 +445,7 @@ auto main(int argc, char** argv) -> int {
 
   // Configure the base directory for test names.
   const char* target = getenv("TEST_TARGET");
-  CARBON_CHECK(target != nullptr);
+  CARBON_CHECK(target);
   llvm::StringRef target_dir = target;
   std::error_code ec;
   std::filesystem::path working_dir = std::filesystem::current_path(ec);
@@ -450,7 +466,9 @@ auto main(int argc, char** argv) -> int {
     if (absl::GetFlag(FLAGS_autoupdate)) {
       std::unique_ptr<Carbon::Testing::FileTestBase> test(
           test_factory.factory_fn(path));
-      llvm::errs() << (test->Autoupdate() ? "!" : ".");
+      auto result = test->Autoupdate();
+      llvm::errs() << (result.ok() ? (*result ? "!" : ".")
+                                   : result.error().message());
     } else {
       std::string test_name = path.string().substr(base_dir.size());
       testing::RegisterTest(test_factory.name, test_name.c_str(), nullptr,
diff --git a/testing/file_test/file_test_base.h b/testing/file_test/file_test_base.h
index 93667e8cc017..e5f8865e39ca 100644
--- a/testing/file_test/file_test_base.h
+++ b/testing/file_test/file_test_base.h
@@ -79,7 +79,7 @@ class FileTestBase : public testing::Test {
   auto TestBody() -> void final;
 
   // Runs the test and autoupdates checks. Returns true if updated.
-  auto Autoupdate() -> bool;
+  auto Autoupdate() -> ErrorOr<bool>;
 
   // Returns the full path of the file being tested.
   auto path() -> const std::filesystem::path& { return path_; };
@@ -134,10 +134,13 @@ class FileTestBase : public testing::Test {
   // Processes the test input, producing test files and expected output.
   auto ProcessTestFile(TestContext& context) -> ErrorOr<Success>;
 
+  // Gets the test filename, relative to the target directory.
+  auto GetTestFilename() -> std::string;
+
   // Transforms an expectation on a given line from `FileCheck` syntax into a
   // standard regex matcher.
   static auto TransformExpectation(int line_index, llvm::StringRef in)
-      -> testing::Matcher<std::string>;
+      -> ErrorOr<testing::Matcher<std::string>>;
 
   const std::filesystem::path path_;
 };
diff --git a/testing/lit_test/__init__.py b/testing/lit_test/__init__.py
deleted file mode 100644
index e69de29bb2d1..000000000000
diff --git a/testing/lit_test/lit.cfg.py b/testing/lit_test/lit.cfg.py
deleted file mode 100644
index 8f5483251e65..000000000000
--- a/testing/lit_test/lit.cfg.py
+++ /dev/null
@@ -1,58 +0,0 @@
-__copyright__ = """
-Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-Exceptions. See /LICENSE for license information.
-SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-"""
-
-import lit.formats
-import os
-from pathlib import Path
-
-
-# This is a provided variable, ignore the undefined name warning.
-config = config  # noqa: F821
-
-
-def fullpath(relative_path):
-    return Path(os.environ["TEST_SRCDIR"]).joinpath(relative_path)
-
-
-def add_substitution(before, after):
-    """Adds a substitution to the config. Wraps before as `%{before}`."""
-    config.substitutions.append((f"%{{{before}}}", after))
-
-
-def add_substitutions():
-    """Adds required substitutions to the config."""
-    tools = {
-        "carbon": fullpath("carbon/toolchain/driver/carbon"),
-        "explorer": fullpath("carbon/explorer/explorer"),
-        "explorer_prelude": fullpath("carbon/explorer/data/prelude.carbon"),
-        "filecheck": fullpath("llvm-project/llvm/FileCheck"),
-        "not": fullpath("llvm-project/llvm/not"),
-        "merge_output": fullpath("carbon/testing/lit_test/merge_output"),
-    }
-
-    run_carbon = f"{tools['merge_output']} {tools['carbon']}"
-    run_explorer = (
-        f"{tools['merge_output']} {tools['explorer']} %s "
-        f"--prelude={tools['explorer_prelude']}"
-    )
-    filecheck_allow_unmatched = (
-        f"{tools['filecheck']} %s --match-full-lines --strict-whitespace"
-    )
-    filecheck_strict = (
-        f"{filecheck_allow_unmatched} --implicit-check-not={{{{.}}}}"
-    )
-
-    add_substitution("carbon", f"{run_carbon}")
-    add_substitution("explorer", f"{run_explorer}")
-    add_substitution("FileCheck-allow-unmatched", filecheck_allow_unmatched)
-    add_substitution("FileCheck-strict", filecheck_strict)
-    add_substitution("not", tools["not"])
-
-
-config.name = "lit"
-config.suffixes = [".carbon"]
-config.test_format = lit.formats.ShTest()
-add_substitutions()
diff --git a/testing/lit_test/lit_test.py b/testing/lit_test/lit_test.py
deleted file mode 100644
index 7d7313de7233..000000000000
--- a/testing/lit_test/lit_test.py
+++ /dev/null
@@ -1,57 +0,0 @@
-"""Runs `lit` for testing."""
-
-__copyright__ = """
-Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-Exceptions. See /LICENSE for license information.
-SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-"""
-
-import argparse
-import os
-from pathlib import Path
-import shutil
-import subprocess
-
-
-def _parse_args():
-    """Parses command line arguments, returning the result."""
-    arg_parser = argparse.ArgumentParser(description=__doc__)
-    arg_parser.add_argument(
-        "--package_name", help="The directory containing tests to run."
-    )
-    arg_parser.add_argument(
-        "lit_args", nargs="*", help="Arguments to pass through to lit."
-    )
-    return arg_parser.parse_args()
-
-
-def main():
-    parsed_args = _parse_args()
-
-    args = [
-        str(Path(os.environ["TEST_SRCDIR"]).joinpath("llvm-project/llvm/lit")),
-        str(Path.cwd().joinpath(parsed_args.package_name)),
-        "-v",
-    ]
-
-    # Force tests to be explicit about command paths. Some tools are required by
-    # bazel's py_binary output in order to find python3, so we add these. The
-    # list of tools may prove to be fragile: if so, we may need to change or
-    # remove the PATH hiding.
-    system_tools = Path(os.environ["TEST_TMPDIR"]).joinpath("system_tools")
-    system_tools.mkdir()
-    for tool in ("env", "grep", "python3", "sh", "which"):
-        system_tools.joinpath(tool).symlink_to(shutil.which(tool))
-    env = os.environ.copy()
-    env["PATH"] = str(system_tools)
-
-    # Run lit.
-    try:
-        subprocess.check_call(args=args + parsed_args.lit_args, env=env)
-    except subprocess.CalledProcessError as e:
-        # Print without the stack trace.
-        exit(e)
-
-
-if __name__ == "__main__":
-    exit(main())
diff --git a/testing/lit_test/merge_output.py b/testing/lit_test/merge_output.py
deleted file mode 100644
index f57aaf91cd12..000000000000
--- a/testing/lit_test/merge_output.py
+++ /dev/null
@@ -1,38 +0,0 @@
-"""Merges stdout and stderr into a single stream with labels."""
-
-__copyright__ = """
-Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-Exceptions. See /LICENSE for license information.
-SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-"""
-
-import subprocess
-import sys
-
-
-def label_output(label: str, output: str) -> None:
-    """Prints output with labels.
-
-    This mirrors label_output in scripts/autoupdate_testdata_base.py and should
-    be kept in sync. They're separate in order to avoid a subprocess or import
-    complexity.
-    """
-    if output:
-        for line in output.splitlines():
-            print(" ".join(filter(None, (label, line))))
-
-
-def main() -> None:
-    p = subprocess.run(
-        sys.argv[1:],
-        stdout=subprocess.PIPE,
-        stderr=subprocess.PIPE,
-        encoding="utf-8",
-    )
-    label_output("STDOUT:", p.stdout)
-    label_output("STDERR:", p.stderr)
-    exit(p.returncode)
-
-
-if __name__ == "__main__":
-    main()
diff --git a/testing/lit_test/rules.bzl b/testing/lit_test/rules.bzl
deleted file mode 100644
index c0ac72d18895..000000000000
--- a/testing/lit_test/rules.bzl
+++ /dev/null
@@ -1,66 +0,0 @@
-# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-# Exceptions. See /LICENSE for license information.
-# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-
-"""Rule for a lit test."""
-
-def glob_lit_tests(name, driver, data, test_file_exts, exclude = None, **kwargs):
-    """Runs `lit` on test_dir.
-
-    `lit` reference:
-      https://llvm.org/docs/CommandGuide/lit.html
-
-    This will generate three types of rules:
-    1. For each test file, a rule with ".test" at the end. For example,
-       "foo/bar/baz.carbon" will generate ":foo/bar/baz.carbon.test"
-    2. If any test exists in subdirectory, a test_suite is generated for each
-       directory. For example, ":foo" will run all tests under directory "foo",
-       and ":foo/bar" will run all tests under "foo/bar".
-    3. A test_suite containing all tests. The name of this will be the "name"
-       arg passed to the glob_lit_tests rule.
-
-    Args:
-      name: The name of the test_suite rule to generate for running all tests.
-      driver: The path to the lit config.
-      data: A list of tools to provide to the tests. These will be aliased for
-        execution.
-      test_file_exts: A list of extensions to use for tests.
-      exclude: A list of paths to exclude from the glob.
-      **kwargs: Any additional parameters for the generated py_test.
-    """
-    if not exclude:
-        exclude = []
-    test_files = native.glob(
-        ["**"],
-        exclude = exclude,
-        exclude_directories = 1,
-    )
-    data.append("@llvm-project//llvm:lit")
-    suites = dict()
-    all_tests = list()
-    for f in test_files:
-        if f.split(".")[-1] not in test_file_exts:
-            continue
-        test = "%s.test" % f
-        all_tests.append(test)
-        native.py_test(
-            name = test,
-            srcs = ["//testing/lit_test:lit_test.py"],
-            main = "//testing/lit_test:lit_test.py",
-            data = data + [driver, f],
-            args = ["--package_name=%s" % native.package_name(), "--"],
-            size = "small",
-            **kwargs
-        )
-
-        # Cluster tests by directory in order to produce suites. For example,
-        # foo/bar/baz.carbon.test is added to suites :foo and :foo/bar.
-        dirs = f.split("/")[:-1]
-        for num_parts in range(1, 1 + len(dirs)):
-            dir = "/".join(dirs[:num_parts])
-            if dir not in suites:
-                suites[dir] = []
-            suites[dir].append(test)
-    native.test_suite(name = name, tests = all_tests)
-    for suite, tests in suites.items():
-        native.test_suite(name = suite, tests = tests)
diff --git a/testing/scripts/__init__.py b/testing/scripts/__init__.py
deleted file mode 100644
index e69de29bb2d1..000000000000
diff --git a/testing/scripts/autoupdate_testdata_base.py b/testing/scripts/autoupdate_testdata_base.py
deleted file mode 100755
index 7de5d66a5de9..000000000000
--- a/testing/scripts/autoupdate_testdata_base.py
+++ /dev/null
@@ -1,542 +0,0 @@
-#!/usr/bin/env python3
-
-"""Updates the CHECK: lines in tests with an AUTOUPDATE line."""
-
-__copyright__ = """
-Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-Exceptions. See /LICENSE for license information.
-SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-"""
-
-from abc import ABC, abstractmethod
-import argparse
-from concurrent import futures
-import os
-from pathlib import Path
-import re
-import subprocess
-from typing import (
-    Any,
-    Dict,
-    List,
-    Match,
-    NamedTuple,
-    Optional,
-    Pattern,
-    Set,
-    Tuple,
-)
-
-# A prefix followed by a command to run for autoupdating checked output.
-AUTOUPDATE_MARKER = "// AUTOUPDATE"
-
-# Indicates no autoupdate is requested.
-NOAUTOUPDATE_MARKER = "// NOAUTOUPDATE"
-
-# Supported tools.
-TOOLS = {
-    "carbon": "//toolchain/driver:carbon",
-    "explorer": "//explorer:explorer",
-}
-
-
-class ParsedArgs(NamedTuple):
-    autoupdate_args: List[str]
-    build_mode: str
-    extra_check_replacements: List[Tuple[Pattern, Pattern, str]]
-    line_number_delta_prefix: str
-    line_number_pattern: Pattern
-    lit_run: List[str]
-    testdata: str
-    tests: List[Path]
-    tool: str
-
-
-def parse_args() -> ParsedArgs:
-    """Parses command-line arguments and flags."""
-    parser = argparse.ArgumentParser(description=__doc__)
-    parser.add_argument("tests", nargs="*")
-    parser.add_argument(
-        "--autoupdate_arg",
-        metavar="COMMAND",
-        default=[],
-        action="append",
-        help="Optional arguments to pass to the autoupdate command.",
-    )
-    parser.add_argument(
-        "--build_mode",
-        metavar="MODE",
-        default="opt",
-        help="The build mode to use. Defaults to opt for faster execution.",
-    )
-    parser.add_argument(
-        "--extra_check_replacement",
-        nargs=3,
-        metavar=("MATCHING", "BEFORE", "AFTER"),
-        default=[],
-        action="append",
-        help="On a CHECK line with MATCHING, does a regex replacement of "
-        "BEFORE with AFTER.",
-    )
-    parser.add_argument(
-        "--line_number_delta_prefix",
-        metavar="PREFIX",
-        default="",
-        help="An optional prefix to add before the [[@LINE+delta]] marker.",
-    )
-    parser.add_argument(
-        "--line_number_pattern",
-        metavar="PATTERN",
-        default=r"(?P<prefix>(?P<filename>\w+\.carbon):)"
-        r"(?P<line>\d+)(?P<suffix>(?:\D|$))",
-        help="A regular expression which matches line numbers to update as its "
-        "only group. Capture groups 'prefix', 'line', and 'suffix' are "
-        "required for structure. The 'filename' capture group is optional and "
-        "should be provided when lines may belong to different files.",
-    )
-    parser.add_argument(
-        "--lit_run",
-        metavar="COMMAND",
-        default=[],
-        required=False,
-        action="append",
-        help="RUN lines to set.",
-    )
-    parser.add_argument(
-        "--testdata",
-        metavar="PATH",
-        required=True,
-        help="The path to the testdata to update, relative to the workspace "
-        "root.",
-    )
-    parser.add_argument(
-        "--tool",
-        metavar="TOOL",
-        required=True,
-        choices=TOOLS.keys(),
-        help="The tool being tested.",
-    )
-    parsed_args = parser.parse_args()
-    extra_check_replacements = [
-        (re.compile(line_matcher), re.compile(before), after)
-        for line_matcher, before, after in parsed_args.extra_check_replacement
-    ]
-    return ParsedArgs(
-        autoupdate_args=parsed_args.autoupdate_arg,
-        build_mode=parsed_args.build_mode,
-        extra_check_replacements=extra_check_replacements,
-        line_number_delta_prefix=parsed_args.line_number_delta_prefix,
-        line_number_pattern=re.compile(parsed_args.line_number_pattern),
-        lit_run=parsed_args.lit_run,
-        testdata=parsed_args.testdata,
-        tests=[Path(test).resolve() for test in parsed_args.tests],
-        tool=parsed_args.tool,
-    )
-
-
-def get_tests(testdata: str) -> Set[Path]:
-    """Get the list of tests from the filesystem."""
-    tests = set()
-    for root, _, files in os.walk(testdata):
-        for f in files:
-            if f in {"lit.cfg.py", "BUILD", "README.md"}:
-                # Ignore the lit config.
-                continue
-            if os.path.splitext(f)[1] == ".carbon":
-                tests.add(Path(root).joinpath(f))
-            else:
-                exit(f"Unrecognized file type in testdata: {f}")
-    return tests
-
-
-class Line(ABC):
-    """A line that may appear in the resulting test file."""
-
-    @abstractmethod
-    def format(
-        self, *, output_line_number: int, line_number_remap: Dict[int, int]
-    ) -> str:
-        raise NotImplementedError
-
-
-class OriginalLine(Line):
-    """A line that was copied from the original test file."""
-
-    def __init__(self, line_number: int, text: str) -> None:
-        self.line_number = line_number
-        self.text = text
-
-    def format(self, **kwargs: Any) -> str:
-        return self.text
-
-
-class RunLine(Line):
-    """A RUN line."""
-
-    def __init__(self, text: str) -> None:
-        self.text = text
-
-    def format(self, **kwargs: Any) -> str:
-        return self.text
-
-
-class CheckLine(Line):
-    """A `// CHECK:` line generated from the test output.
-
-    If there's a line number, it'll be fixed up after we've figured out which
-    lines to include in the resulting test file and in what order, because
-    their contents depend on where an original input line appears in the output.
-    """
-
-    def __init__(
-        self,
-        test: str,
-        out_line: str,
-        line_number_delta_prefix: str,
-        line_number_pattern: Pattern,
-    ) -> None:
-        super().__init__()
-        self.filename = Path(test).name
-        self.indent = ""
-        self.out_line = out_line
-        self.line_number_delta_prefix = line_number_delta_prefix
-        self.line_number_pattern = line_number_pattern
-        self.trailing_whitespace_pattern = re.compile(r"(\s+$)")
-
-        # If any match is specific to this file, use the first matched line for
-        # the location of the CHECK comment.
-        self.line_in_file = None
-        for match in line_number_pattern.finditer(self.out_line):
-            if self._matches_filename(match):
-                self.line_in_file = int(match.group("line")) - 1
-                break
-
-    def format(
-        self, *, output_line_number: int, line_number_remap: Dict[int, int]
-    ) -> str:
-        assert self.out_line
-        result = self.out_line
-        while True:
-            line_match = self.line_number_pattern.search(result)
-            trailing_match = self.trailing_whitespace_pattern.search(result)
-            if line_match:
-                if self._matches_filename(line_match):
-                    line_number = int(line_match.group("line")) - 1
-                    delta = line_number_remap[line_number] - output_line_number
-                    # We use `:+d` here to produce `LINE-n` or `LINE+n` as
-                    # appropriate.
-                    result = self.line_number_pattern.sub(
-                        rf"\g<prefix>{self.line_number_delta_prefix}"
-                        rf"[[@LINE{delta:+d}]]\g<suffix>",
-                        result,
-                        count=1,
-                    )
-                else:
-                    result = self.line_number_pattern.sub(
-                        r"\g<prefix>{{.*}}\g<suffix>",
-                        result,
-                        count=1,
-                    )
-            elif trailing_match:
-                result = self.trailing_whitespace_pattern.sub(r"{{\1}}", result)
-            else:
-                break
-
-        return f"{self.indent}// CHECK:{result}\n"
-
-    def _matches_filename(self, match: Match) -> bool:
-        return (
-            "filename" not in match.groupdict()
-            or match.group("filename") == self.filename
-        )
-
-
-def find_autoupdate(test: str, orig_lines: List[str]) -> Optional[int]:
-    """Figures out whether autoupdate should occur.
-
-    For AUTOUPDATE, returns the line. For NOAUTOUPDATE, returns None.
-    """
-    found = 0
-    result = None
-    for line_number, line in enumerate(orig_lines):
-        if line.startswith(AUTOUPDATE_MARKER):
-            found += 1
-            result = line_number
-        elif line.startswith(NOAUTOUPDATE_MARKER):
-            found += 1
-    if found == 0:
-        raise ValueError(
-            f"{test} must have either '{AUTOUPDATE_MARKER}' or "
-            f"'{NOAUTOUPDATE_MARKER}'"
-        )
-    elif found > 1:
-        raise ValueError(
-            f"{test} must have only one of '{AUTOUPDATE_MARKER}' or "
-            f"'{NOAUTOUPDATE_MARKER}'"
-        )
-    return result
-
-
-def replace_all(s: str, replacements: List[Tuple[str, str]]) -> str:
-    """Runs multiple replacements on a string."""
-    for before, after in replacements:
-        s = s.replace(before, after)
-    return s
-
-
-def label_output(label: str, output: str) -> List[str]:
-    """Merges output with labels.
-
-    This mirrors label_output in lit_test/merge_output.py and should
-    be kept in sync. They're separate in order to avoid a subprocess or import
-    complexity.
-    """
-    result = []
-    if output:
-        for line in output.splitlines():
-            result.append(" ".join(filter(None, (label, line))))
-    return result
-
-
-def get_matchable_test_output(
-    autoupdate_args: List[str],
-    extra_check_replacements: List[Tuple[Pattern, Pattern, str]],
-    tool: str,
-    bazel_runfiles: Pattern,
-    llvm_symbolizer: str,
-    test: str,
-) -> List[str]:
-    """Runs the autoupdate command and returns the output lines."""
-    # Run the autoupdate command to generate output.
-    # (`bazel run` would serialize)
-    autoupdate_cmd = Path.cwd().joinpath(
-        TOOLS[tool].replace("//", "./bazel-bin/").replace(":", "/")
-    )
-    p = subprocess.run(
-        [str(autoupdate_cmd)] + autoupdate_args + [Path(test).name],
-        env={"LLVM_SYMBOLIZER_PATH": llvm_symbolizer},
-        stdout=subprocess.PIPE,
-        stderr=subprocess.PIPE,
-        encoding="utf-8",
-        cwd=str(Path(test).parent),
-    )
-
-    out_lines = label_output("STDOUT:", p.stdout)
-    out_lines.extend(label_output("STDERR:", p.stderr))
-
-    for i, line in enumerate(out_lines):
-        # Escape things that mirror FileCheck special characters.
-        line = line.replace("{{", "{{[{][{]}}")
-        line = line.replace("[[", "{{[[][[]}}")
-        line = bazel_runfiles.sub("{{.*}}/", line)
-
-        for line_matcher, before, after in extra_check_replacements:
-            if line_matcher.match(line):
-                line = before.sub(after, line)
-
-        out_lines[i] = line
-
-    return out_lines
-
-
-def is_replaced(line: str) -> bool:
-    """Returns true if autoupdate should replace the line."""
-    line = line.lstrip()
-    return line.startswith("// CHECK") or line.startswith("// RUN:")
-
-
-def merge_lines(
-    line_number_delta_prefix: str,
-    line_number_pattern: Pattern,
-    lit_run: List[str],
-    test: str,
-    autoupdate_line_number: int,
-    raw_orig_lines: List[str],
-    out_lines: List[str],
-) -> List[Line]:
-    """Merges the original output and new lines."""
-    orig_lines = [
-        OriginalLine(i, line)
-        for i, line in enumerate(raw_orig_lines)
-        if not is_replaced(line)
-    ]
-    check_lines = [
-        CheckLine(test, out_line, line_number_delta_prefix, line_number_pattern)
-        for out_line in out_lines
-    ]
-
-    result_lines: List[Line] = []
-    # CHECK lines must go after AUTOUPDATE.
-    while orig_lines and orig_lines[0].line_number <= autoupdate_line_number:
-        result_lines.append(orig_lines.pop(0))
-    for line in lit_run:
-        run_not = ""
-        if Path(test).name.startswith("fail_"):
-            run_not = "%{not} "
-        result_lines.append(RunLine(f"// RUN: {run_not}{line}\n"))
-    # Interleave the original lines and the CHECK: lines.
-    while orig_lines and check_lines:
-        # Original lines go first when the CHECK line is known and later.
-        if (
-            check_lines[0].line_in_file is not None
-            and check_lines[0].line_in_file > orig_lines[0].line_number
-        ):
-            result_lines.append(orig_lines.pop(0))
-        else:
-            check_line = check_lines.pop(0)
-            # Indent to match the next original line.
-            check_line.indent = re.findall("^ *", orig_lines[0].text)[0]
-            result_lines.append(check_line)
-    # One list is non-empty; append remaining lines from both to catch it.
-    result_lines.extend(orig_lines)
-    result_lines.extend(check_lines)
-
-    return result_lines
-
-
-def update_check(
-    parsed_args: ParsedArgs,
-    bazel_runfiles: Pattern,
-    llvm_symbolizer: str,
-    test: Path,
-) -> bool:
-    """Updates the CHECK: lines for `test` by running the tool.
-
-    Returns true if a change was made.
-    """
-    with test.open() as f:
-        orig_lines = f.readlines()
-
-    # Make sure we're supposed to autoupdate.
-    autoupdate_line = find_autoupdate(str(test), orig_lines)
-    if autoupdate_line is None:
-        return False
-
-    # Determine the merged output lines.
-    out_lines = get_matchable_test_output(
-        parsed_args.autoupdate_args,
-        parsed_args.extra_check_replacements,
-        parsed_args.tool,
-        bazel_runfiles,
-        llvm_symbolizer,
-        str(test),
-    )
-    result_lines = merge_lines(
-        parsed_args.line_number_delta_prefix,
-        parsed_args.line_number_pattern,
-        parsed_args.lit_run,
-        str(test),
-        autoupdate_line,
-        orig_lines,
-        out_lines,
-    )
-
-    # Calculate the remap for original lines.
-    line_number_remap = dict(
-        [
-            (line.line_number, i)
-            for i, line in enumerate(result_lines)
-            if isinstance(line, OriginalLine)
-        ]
-    )
-    # If the last line of the original output was a CHECK, replace it with an
-    # empty line.
-    if orig_lines[-1].lstrip().startswith("// CHECK"):
-        line_number_remap[len(orig_lines) - 1] = len(result_lines) - 1
-
-    # Generate contents for any lines that depend on line numbers.
-    formatted_result_lines = [
-        line.format(output_line_number=i, line_number_remap=line_number_remap)
-        for i, line in enumerate(result_lines)
-    ]
-
-    # If nothing's changed, we're done.
-    if formatted_result_lines == orig_lines:
-        return False
-
-    # Interleave the new CHECK: lines with the tested content.
-    with test.open("w") as f:
-        f.writelines(formatted_result_lines)
-        return True
-
-
-def update_checks(
-    parsed_args: ParsedArgs,
-    bazel_runfiles: Pattern,
-    llvm_symbolizer: str,
-    tests: Set[Path],
-) -> None:
-    """Updates CHECK: lines in lit tests."""
-
-    def map_helper(test: Path) -> bool:
-        try:
-            updated = update_check(
-                parsed_args, bazel_runfiles, llvm_symbolizer, test
-            )
-        except Exception as e:
-            raise ValueError(f"Failed to update {test}") from e
-        print(".", end="", flush=True)
-        return updated
-
-    print(f"Updating {len(tests)} lit test(s)...")
-    with futures.ThreadPoolExecutor() as exec:
-        # list() iterates in order to immediately propagate exceptions.
-        results = list(exec.map(map_helper, tests))
-
-    # Each update call indicates progress with a dot without a newline, so put a
-    # newline to wrap.
-    print(f"\nUpdated {results.count(True)} lit test(s).")
-
-
-def main() -> None:
-    # Parse arguments relative to the working directory.
-    parsed_args = parse_args()
-
-    # Remaining script logic should be relative to the repository root.
-    root = Path(__file__).parent.parent.parent
-    os.chdir(root)
-
-    if parsed_args.tests:
-        tests = {test.relative_to(root) for test in parsed_args.tests}
-    else:
-        print(
-            "HINT: run `autoupdate_testdata.py f1 f2 ...` "
-            "to update specific tests"
-        )
-        tests = get_tests(parsed_args.testdata)
-
-    # Build inputs.
-    print(f"Building {parsed_args.tool}...")
-    subprocess.check_call(
-        [
-            "bazel",
-            "build",
-            "-c",
-            parsed_args.build_mode,
-            TOOLS[parsed_args.tool],
-        ]
-    )
-    bazel_bin_dir = subprocess.check_output(
-        ["bazel", "info", "-c", parsed_args.build_mode, "bazel-bin"],
-        encoding="utf-8",
-    ).strip()
-    bazel_runfiles = re.compile(
-        r"{0}/.*\.runfiles/carbon/".format(re.escape(bazel_bin_dir))
-    )
-
-    # Grab the symbolizer.
-    clang_var_content = Path(
-        "bazel-execroot/external/bazel_cc_toolchain/"
-        "clang_detected_variables.bzl"
-    ).read_text()
-    llvm_symbolizer = re.search(
-        '(?m)^llvm_symbolizer = "(.*)"$', clang_var_content
-    )
-    assert llvm_symbolizer is not None
-
-    # Run updates.
-    update_checks(parsed_args, bazel_runfiles, llvm_symbolizer[1], tests)
-
-
-if __name__ == "__main__":
-    main()
diff --git a/toolchain/lexer/lexer_file_test.cpp b/toolchain/lexer/lexer_file_test.cpp
index 0ef0720c4671..54196743357b 100644
--- a/toolchain/lexer/lexer_file_test.cpp
+++ b/toolchain/lexer/lexer_file_test.cpp
@@ -24,9 +24,9 @@ class LexerFileTest : public DriverFileTestBase {
   auto GetLineNumberReplacement(llvm::ArrayRef<llvm::StringRef> /*filenames*/)
       -> LineNumberReplacement override {
     return {.has_file = false,
-            .pattern = R"(line: +(\d+))",
+            .pattern = R"(line: (\s*\d+))",
             // The `{{{{` becomes `{{`.
-            .sub_for_formatv = "line: {{{{ *}}{0}"};
+            .line_formatv = "{{{{ *}}{0}"};
   }
 
   auto DoExtraCheckReplacements(std::string& check_line) -> void override {