From e85f45de10a6e0053d8827848c151f276c2b4d6a Mon Sep 17 00:00:00 2001
From: Richard Smith <richard@metafoo.co.uk>
Date: Tue, 3 May 2022 15:48:56 -0700
Subject: [PATCH] Move CHECK lines in tests next to the line that caused the
 output. (#1224)

Use FileCheck's `[[@LINE+n]]` mechanism to refer to the next line.

This is made awkward by a couple of things:

* We want to keep the `CHECK` lines in the original order.
* Errors are sometimes more than one line long.

The approach we use is to interleave the original lines and the check lines, putting each check line as early as possible subject to two rules:

1) Check lines never precede the 'AUTOUPDATE' line
2) Except when required by rule (1), a check line that refers to a source line by line number is never placed earlier than a source line that precedes that source line.

The actual `[[@LINE+n]]` annotations are created in a second pass after we've interleaved the lines so that we can work out the correct offsets.

Co-authored-by: Jon Meow <jperkins@google.com>
---
 .../auto_return/fail_no_return.carbon         |  19 ++
 .../function/fail_non_exhaustive_match.carbon |  18 ++
 .../testdata/import/fail_order.carbon         |  18 ++
 .../name_lookup/fail_block_duplicate.carbon   |  18 ++
 .../name_lookup/fail_class_duplicate.carbon   |  21 ++
 .../name_lookup/fail_global_duplicate.carbon  |  19 ++
 .../name_lookup/fail_match_duplicate.carbon   |  20 ++
 .../fail_explicit_with_no_return.carbon       |  20 ++
 .../fail_implicit_with_explicit_return.carbon |  21 ++
 .../testdata/array/fail_negative_size.carbon  |   2 +-
 .../testdata/array/fail_size_mismatch.carbon  |   2 +-
 .../fail_assign_to_function.carbon            |   2 +-
 .../basic_syntax/fail_assign_to_rval.carbon   |   2 +-
 .../basic_syntax/fail_invalid_char.carbon     |   2 +-
 .../basic_syntax/fail_invalid_integer.carbon  |   2 +-
 .../fail_invalid_var_expression.carbon        |   2 +-
 .../basic_syntax/fail_missing_var.carbon      |   2 +-
 .../basic_syntax/fail_nested_binding.carbon   |   2 +-
 .../fail_unimplemented_example.carbon         |   2 +-
 .../basic_syntax/fail_var_type.carbon         |   2 +-
 .../not_compare_precedence.carbon             |   2 +-
 .../class/fail_field_access_mismatch.carbon   |   2 +-
 .../testdata/class/fail_field_mismatch.carbon |   2 +-
 .../testdata/class/fail_field_missing.carbon  |   2 +-
 .../class/fail_method_from_class.carbon       |   2 +-
 .../fail_continuation_syntax.carbon           |   2 +-
 .../auto_return/fail_direct_recurse.carbon    |   2 +-
 .../auto_return/fail_multiple_returns.carbon  |   2 +-
 .../auto_return/fail_no_return.carbon         |   2 +-
 .../auto_return/fail_separate_decl.carbon     |   2 +-
 .../function/fail_call_with_tuple.carbon      |   2 +-
 .../function/fail_match_no_return.carbon      |   2 +-
 .../function/fail_match_partial_return.carbon |   2 +-
 .../function/fail_non_exhaustive_match.carbon |   2 +-
 .../function/fail_parameter_type.carbon       |   2 +-
 .../generic_class/fail_args_mismatch.carbon   |   2 +-
 .../fail_argument_deduction.carbon            |   6 +-
 .../fail_bad_parameter_type.carbon            |   2 +-
 .../fail_field_access_on_generic.carbon       |   2 +-
 .../fail_generic_class_arg.carbon             |   2 +-
 .../fail_generic_in_pattern.carbon            |   2 +-
 .../fail_instantiate_non_generic.carbon       |   2 +-
 .../generic_class/fail_point_equal.carbon     |   2 +-
 .../fail_return_type_is_type.carbon           |   2 +-
 .../generic_class/fail_two_arg_lists.carbon   |   2 +-
 .../generic_function/fail_not_addable.carbon  |   6 +-
 .../fail_type_deduction_mismatch.carbon       |   6 +-
 .../fail_type_deduction_unused.carbon         |   4 +-
 .../fail_init_type_mismatch.carbon            |   2 +-
 .../testdata/impl/fail_ambiguous_impl.carbon  |   2 +-
 .../impl/fail_impl_as_not_constraint.carbon   |   2 +-
 .../impl/fail_impl_as_parameterized.carbon    |   2 +-
 explorer/testdata/impl/impl_as.carbon         |   2 +-
 explorer/testdata/import/fail_order.carbon    |   2 +-
 .../interface/fail_impl_bad_member.carbon     |   2 +-
 .../interface/fail_impl_missing_member.carbon |   2 +-
 .../fail_interface_missing_member.carbon      |   2 +-
 .../testdata/interface/fail_no_impl.carbon    |   2 +-
 .../testdata/let/fail_function_args.carbon    |   2 +-
 .../testdata/let/fail_global_assign.carbon    |   2 +-
 .../testdata/let/fail_local_assign.carbon     |   2 +-
 .../testdata/let/fail_match_choice.carbon     |   2 +-
 explorer/testdata/let/fail_method_args.carbon |   2 +-
 .../let/fail_tuple_pattern_let_context.carbon |   2 +-
 ...il_tuple_pattern_let_context_nested.carbon |   2 +-
 .../let/fail_tuple_pattern_let_in_var.carbon  |   2 +-
 .../name_lookup/fail_block_duplicate.carbon   |   2 +-
 .../name_lookup/fail_choice_duplicate.carbon  |   2 +-
 .../name_lookup/fail_class_duplicate.carbon   |   2 +-
 .../name_lookup/fail_global_duplicate.carbon  |   2 +-
 .../name_lookup/fail_match_duplicate.carbon   |   2 +-
 .../fail_use_before_declare.carbon            |   2 +-
 explorer/testdata/package/fail_missing.carbon |   2 +-
 .../pointer/fail_rvalue_addressof.carbon      |   2 +-
 .../fail_explicit_with_no_return.carbon       |   2 +-
 .../fail_explicit_with_plain_return.carbon    |   2 +-
 .../fail_implicit_with_explicit_return.carbon |   2 +-
 .../fail_block_quotes_not_on_own_line.carbon  |   2 +-
 .../testdata/string/fail_hex_lower.carbon     |   2 +-
 .../testdata/string/fail_hex_truncated.carbon |   2 +-
 .../string/fail_invalid_escape.carbon         |   2 +-
 explorer/testdata/string/fail_newline.carbon  |   2 +-
 explorer/testdata/string/fail_octal.carbon    |   2 +-
 explorer/testdata/string/fail_tab.carbon      |   2 +-
 .../testdata/struct/fail_equality_type.carbon |   6 +-
 .../struct/fail_field_access_mismatch.carbon  |   2 +-
 .../testdata/tuple/fail_equality_type.carbon  |   6 +-
 explorer/testdata/tuple/fail_index.carbon     |   2 +-
 explorer/update_checks.py                     | 192 +++++++++++++++---
 89 files changed, 433 insertions(+), 113 deletions(-)
 create mode 100644 executable_semantics/testdata/function/auto_return/fail_no_return.carbon
 create mode 100644 executable_semantics/testdata/function/fail_non_exhaustive_match.carbon
 create mode 100644 executable_semantics/testdata/import/fail_order.carbon
 create mode 100644 executable_semantics/testdata/name_lookup/fail_block_duplicate.carbon
 create mode 100644 executable_semantics/testdata/name_lookup/fail_class_duplicate.carbon
 create mode 100644 executable_semantics/testdata/name_lookup/fail_global_duplicate.carbon
 create mode 100644 executable_semantics/testdata/name_lookup/fail_match_duplicate.carbon
 create mode 100644 executable_semantics/testdata/return/fail_explicit_with_no_return.carbon
 create mode 100644 executable_semantics/testdata/return/fail_implicit_with_explicit_return.carbon

diff --git a/executable_semantics/testdata/function/auto_return/fail_no_return.carbon b/executable_semantics/testdata/function/auto_return/fail_no_return.carbon
new file mode 100644
index 000000000000..6842b3c26c5d
--- /dev/null
+++ b/executable_semantics/testdata/function/auto_return/fail_no_return.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
+//
+// RUN: %{not} %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{executable_semantics} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+
+package ExecutableSemanticsTest api;
+
+fn NoReturn() -> auto {
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/function/auto_return/fail_no_return.carbon:[[@LINE+1]]: control-flow reaches end of function that provides a `->` return type without reaching a return statement
+}
+
+fn Main() {
+  NoReturn();
+}
diff --git a/executable_semantics/testdata/function/fail_non_exhaustive_match.carbon b/executable_semantics/testdata/function/fail_non_exhaustive_match.carbon
new file mode 100644
index 000000000000..8f5df732f63d
--- /dev/null
+++ b/executable_semantics/testdata/function/fail_non_exhaustive_match.carbon
@@ -0,0 +1,18 @@
+// 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
+//
+// RUN: %{not} %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{executable_semantics} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+
+package ExecutableSemanticsTest api;
+
+fn Main() -> i32 {
+  match (0) {
+    case 1 => return 0;
+  }
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/function/fail_non_exhaustive_match.carbon:[[@LINE+1]]: non-exhaustive match may allow control-flow to reach the end of a function that provides a `->` return type
+}
diff --git a/executable_semantics/testdata/import/fail_order.carbon b/executable_semantics/testdata/import/fail_order.carbon
new file mode 100644
index 000000000000..5bbb13c143b0
--- /dev/null
+++ b/executable_semantics/testdata/import/fail_order.carbon
@@ -0,0 +1,18 @@
+// 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
+//
+// RUN: %{not} %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{executable_semantics} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+
+package ExecutableSemanticsTest api;
+
+fn Main() -> i32 {
+  return 0;
+}
+
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/import/fail_order.carbon:[[@LINE+1]]: syntax error, unexpected IMPORT, expecting END_OF_FILE
+import ExecutableSemanticsTest library "Nonexistent";
diff --git a/executable_semantics/testdata/name_lookup/fail_block_duplicate.carbon b/executable_semantics/testdata/name_lookup/fail_block_duplicate.carbon
new file mode 100644
index 000000000000..01fbaf6e801c
--- /dev/null
+++ b/executable_semantics/testdata/name_lookup/fail_block_duplicate.carbon
@@ -0,0 +1,18 @@
+// 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
+//
+// RUN: %{not} %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{executable_semantics} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+
+package ExecutableSemanticsTest api;
+
+fn Main() -> i32 {
+  var x: i32 = 0;
+  // CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/name_lookup/fail_block_duplicate.carbon:[[@LINE+1]]: Duplicate name `x` also found at {{.*}}/executable_semantics/testdata/name_lookup/fail_block_duplicate.carbon:14
+  var x: i32 = 0;
+  return 0;
+}
diff --git a/executable_semantics/testdata/name_lookup/fail_class_duplicate.carbon b/executable_semantics/testdata/name_lookup/fail_class_duplicate.carbon
new file mode 100644
index 000000000000..97907e8fb6c7
--- /dev/null
+++ b/executable_semantics/testdata/name_lookup/fail_class_duplicate.carbon
@@ -0,0 +1,21 @@
+// 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
+//
+// RUN: %{not} %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{executable_semantics} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+
+package ExecutableSemanticsTest api;
+
+class Foo {
+  var x: i32;
+  // CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/name_lookup/fail_class_duplicate.carbon:[[@LINE+1]]: Duplicate name `x` also found at {{.*}}/executable_semantics/testdata/name_lookup/fail_class_duplicate.carbon:14
+  var x: i32;
+}
+
+fn Main() -> i32 {
+  return 0;
+}
diff --git a/executable_semantics/testdata/name_lookup/fail_global_duplicate.carbon b/executable_semantics/testdata/name_lookup/fail_global_duplicate.carbon
new file mode 100644
index 000000000000..d54cb1bb39c0
--- /dev/null
+++ b/executable_semantics/testdata/name_lookup/fail_global_duplicate.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
+//
+// RUN: %{not} %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{executable_semantics} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+
+package ExecutableSemanticsTest api;
+
+var x: i32 = 0;
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/name_lookup/fail_global_duplicate.carbon:[[@LINE+1]]: Duplicate name `x` also found at {{.*}}/executable_semantics/testdata/name_lookup/fail_global_duplicate.carbon:13
+var x: i32 = 0;
+
+fn Main() -> i32 {
+  return 0;
+}
diff --git a/executable_semantics/testdata/name_lookup/fail_match_duplicate.carbon b/executable_semantics/testdata/name_lookup/fail_match_duplicate.carbon
new file mode 100644
index 000000000000..d6be8b18bcd2
--- /dev/null
+++ b/executable_semantics/testdata/name_lookup/fail_match_duplicate.carbon
@@ -0,0 +1,20 @@
+// 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
+//
+// RUN: %{not} %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{executable_semantics} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+
+package ExecutableSemanticsTest api;
+
+fn Main() -> i32 {
+  match (0) {
+    case x: i32 =>
+      // CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/name_lookup/fail_match_duplicate.carbon:[[@LINE+1]]: Duplicate name `x` also found at {{.*}}/executable_semantics/testdata/name_lookup/fail_match_duplicate.carbon:15
+      var x: i32 = 0;
+  }
+  return 0;
+}
diff --git a/executable_semantics/testdata/return/fail_explicit_with_no_return.carbon b/executable_semantics/testdata/return/fail_explicit_with_no_return.carbon
new file mode 100644
index 000000000000..3e0347e9095a
--- /dev/null
+++ b/executable_semantics/testdata/return/fail_explicit_with_no_return.carbon
@@ -0,0 +1,20 @@
+// 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
+//
+// RUN: %{not} %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{executable_semantics} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+
+package ExecutableSemanticsTest api;
+
+fn F() -> () {
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/return/fail_explicit_with_no_return.carbon:[[@LINE+1]]: control-flow reaches end of function that provides a `->` return type without reaching a return statement
+}
+
+fn Main() -> i32 {
+  F();
+  return 0;
+}
diff --git a/executable_semantics/testdata/return/fail_implicit_with_explicit_return.carbon b/executable_semantics/testdata/return/fail_implicit_with_explicit_return.carbon
new file mode 100644
index 000000000000..a0ad7356d887
--- /dev/null
+++ b/executable_semantics/testdata/return/fail_implicit_with_explicit_return.carbon
@@ -0,0 +1,21 @@
+// 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
+//
+// RUN: %{not} %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{executable_semantics} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+
+package ExecutableSemanticsTest api;
+
+fn F() {
+  // CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/return/fail_implicit_with_explicit_return.carbon:[[@LINE+1]]: return (); should not provide a return value, to match the function's signature.
+  return ();
+}
+
+fn Main() -> i32 {
+  F();
+  return 0;
+}
diff --git a/explorer/testdata/array/fail_negative_size.carbon b/explorer/testdata/array/fail_negative_size.carbon
index 373dff5a7714..707e5902ba6b 100644
--- a/explorer/testdata/array/fail_negative_size.carbon
+++ b/explorer/testdata/array/fail_negative_size.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/array/fail_negative_size.carbon:15: Array size cannot be negative
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/array/fail_negative_size.carbon:[[@LINE+1]]: Array size cannot be negative
   var x: [i32; -1] = ();
   return x[0];
 }
diff --git a/explorer/testdata/array/fail_size_mismatch.carbon b/explorer/testdata/array/fail_size_mismatch.carbon
index 960bc17cb7b0..45693b4ae09a 100644
--- a/explorer/testdata/array/fail_size_mismatch.carbon
+++ b/explorer/testdata/array/fail_size_mismatch.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/array/fail_size_mismatch.carbon:15: type error in name binding: '(i32, i32, i32)' is not implicitly convertible to '[i32; 2]'
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/array/fail_size_mismatch.carbon:[[@LINE+1]]: type error in name binding: '(i32, i32, i32)' is not implicitly convertible to '[i32; 2]'
   var x: [i32; 2] = (0, 1, 2);
   return x[0];
 }
diff --git a/explorer/testdata/basic_syntax/fail_assign_to_function.carbon b/explorer/testdata/basic_syntax/fail_assign_to_function.carbon
index 19d4a2ce84ba..3c8cbf50e189 100644
--- a/explorer/testdata/basic_syntax/fail_assign_to_function.carbon
+++ b/explorer/testdata/basic_syntax/fail_assign_to_function.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_assign_to_function.carbon:18: Cannot assign to rvalue 'F'
 
 package ExplorerTest api;
 
@@ -15,6 +14,7 @@ fn F() {}
 fn G() {}
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_assign_to_function.carbon:[[@LINE+1]]: Cannot assign to rvalue 'F'
   F = G;
   return 0;
 }
diff --git a/explorer/testdata/basic_syntax/fail_assign_to_rval.carbon b/explorer/testdata/basic_syntax/fail_assign_to_rval.carbon
index 945e59c15bb0..3c3e33f261b3 100644
--- a/explorer/testdata/basic_syntax/fail_assign_to_rval.carbon
+++ b/explorer/testdata/basic_syntax/fail_assign_to_rval.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_assign_to_rval.carbon:15: Cannot assign to rvalue '1'
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_assign_to_rval.carbon:[[@LINE+1]]: Cannot assign to rvalue '1'
   1 = 0;
   return 0;
 }
diff --git a/explorer/testdata/basic_syntax/fail_invalid_char.carbon b/explorer/testdata/basic_syntax/fail_invalid_char.carbon
index fcea303d007d..7731c3366a5c 100644
--- a/explorer/testdata/basic_syntax/fail_invalid_char.carbon
+++ b/explorer/testdata/basic_syntax/fail_invalid_char.carbon
@@ -7,6 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_invalid_char.carbon:12: invalid character '\xEF' in source file.
 
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_invalid_char.carbon:[[@LINE+1]]: invalid character '\xEF' in source file.
 �
diff --git a/explorer/testdata/basic_syntax/fail_invalid_integer.carbon b/explorer/testdata/basic_syntax/fail_invalid_integer.carbon
index fecffea8e513..afc76a2c26b9 100644
--- a/explorer/testdata/basic_syntax/fail_invalid_integer.carbon
+++ b/explorer/testdata/basic_syntax/fail_invalid_integer.carbon
@@ -7,10 +7,10 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_invalid_integer.carbon:15: Invalid integer literal: 11111111111111111111111111
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_invalid_integer.carbon:[[@LINE+1]]: Invalid integer literal: 11111111111111111111111111
   return 11111111111111111111111111;
 }
diff --git a/explorer/testdata/basic_syntax/fail_invalid_var_expression.carbon b/explorer/testdata/basic_syntax/fail_invalid_var_expression.carbon
index ffcf80a0c5ad..5480e1d1f477 100644
--- a/explorer/testdata/basic_syntax/fail_invalid_var_expression.carbon
+++ b/explorer/testdata/basic_syntax/fail_invalid_var_expression.carbon
@@ -7,10 +7,10 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_invalid_var_expression.carbon:14: Expected expression for variable type
 
 package ExplorerTest api;
 
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_invalid_var_expression.carbon:[[@LINE+1]]: Expected expression for variable type
 var a: auto;
 
 fn Main() -> i32 { return 0; }
diff --git a/explorer/testdata/basic_syntax/fail_missing_var.carbon b/explorer/testdata/basic_syntax/fail_missing_var.carbon
index 6e15e3e85606..7b209be68ba7 100644
--- a/explorer/testdata/basic_syntax/fail_missing_var.carbon
+++ b/explorer/testdata/basic_syntax/fail_missing_var.carbon
@@ -7,12 +7,12 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_missing_var.carbon:16: syntax error, unexpected COLON, expecting EQUAL or SEMICOLON
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   // error
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_missing_var.carbon:[[@LINE+1]]: syntax error, unexpected COLON, expecting EQUAL or SEMICOLON
   x : i32;
   return 1;
 }
diff --git a/explorer/testdata/basic_syntax/fail_nested_binding.carbon b/explorer/testdata/basic_syntax/fail_nested_binding.carbon
index ce558f1b3c96..6b305c7d5878 100644
--- a/explorer/testdata/basic_syntax/fail_nested_binding.carbon
+++ b/explorer/testdata/basic_syntax/fail_nested_binding.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_nested_binding.carbon:15: The type of a binding pattern cannot contain bindings.
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_nested_binding.carbon:[[@LINE+1]]: The type of a binding pattern cannot contain bindings.
   var x: (T: Type) = 1;
   return 1;
 }
diff --git a/explorer/testdata/basic_syntax/fail_unimplemented_example.carbon b/explorer/testdata/basic_syntax/fail_unimplemented_example.carbon
index 05cbdc149732..6cf2b05b0cb2 100644
--- a/explorer/testdata/basic_syntax/fail_unimplemented_example.carbon
+++ b/explorer/testdata/basic_syntax/fail_unimplemented_example.carbon
@@ -7,10 +7,10 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_unimplemented_example.carbon:15: Unimplemented
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_unimplemented_example.carbon:[[@LINE+1]]: Unimplemented
   return 1 __unimplemented_example_infix 2;
 }
diff --git a/explorer/testdata/basic_syntax/fail_var_type.carbon b/explorer/testdata/basic_syntax/fail_var_type.carbon
index 24820752d0c1..b0b21eed5d68 100644
--- a/explorer/testdata/basic_syntax/fail_var_type.carbon
+++ b/explorer/testdata/basic_syntax/fail_var_type.carbon
@@ -7,13 +7,13 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_var_type.carbon:17: Expected a type, but got 42
 
 package ExplorerTest api;
 
 fn Main () -> i32
 {
   // 42 cannot be used as the type of a variable.
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_var_type.carbon:[[@LINE+1]]: Expected a type, but got 42
   var x: 42 = 0;
   return x;
 }
diff --git a/explorer/testdata/basic_syntax/not_compare_precedence.carbon b/explorer/testdata/basic_syntax/not_compare_precedence.carbon
index f67d3cd97a92..3639bb5be7c9 100644
--- a/explorer/testdata/basic_syntax/not_compare_precedence.carbon
+++ b/explorer/testdata/basic_syntax/not_compare_precedence.carbon
@@ -7,10 +7,10 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/not_compare_precedence.carbon:15: syntax error, unexpected EQUAL_EQUAL, expecting SEMICOLON
 
 package ExplorerTest api;
 
 fn CompareBools(a: Bool, b: Bool) -> Bool {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/not_compare_precedence.carbon:[[@LINE+1]]: syntax error, unexpected EQUAL_EQUAL, expecting SEMICOLON
   return not a == b;
 }
diff --git a/explorer/testdata/class/fail_field_access_mismatch.carbon b/explorer/testdata/class/fail_field_access_mismatch.carbon
index 6fa9df6509d1..eb2db096a42f 100644
--- a/explorer/testdata/class/fail_field_access_mismatch.carbon
+++ b/explorer/testdata/class/fail_field_access_mismatch.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_field_access_mismatch.carbon:21: class Point does not have a field named z
 
 package ExplorerTest api;
 
@@ -18,5 +17,6 @@ class Point {
 
 fn Main() -> i32 {
   var p: Point = {.x = 1, .y = 2};
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_field_access_mismatch.carbon:[[@LINE+1]]: class Point does not have a field named z
   return p.z - 1;
 }
diff --git a/explorer/testdata/class/fail_field_mismatch.carbon b/explorer/testdata/class/fail_field_mismatch.carbon
index 32d41ed471da..d1791463a20b 100644
--- a/explorer/testdata/class/fail_field_mismatch.carbon
+++ b/explorer/testdata/class/fail_field_mismatch.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_field_mismatch.carbon:20: type error in name binding: '{.x: i32, .z: i32}' is not implicitly convertible to 'class Point'
 
 package ExplorerTest api;
 
@@ -17,6 +16,7 @@ class Point {
 }
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_field_mismatch.carbon:[[@LINE+1]]: type error in name binding: '{.x: i32, .z: i32}' is not implicitly convertible to 'class Point'
   var p: Point = {.x = 1, .z = 2};
   return p.x - 1;
 }
diff --git a/explorer/testdata/class/fail_field_missing.carbon b/explorer/testdata/class/fail_field_missing.carbon
index c5c4a8b01ada..37d3795386d0 100644
--- a/explorer/testdata/class/fail_field_missing.carbon
+++ b/explorer/testdata/class/fail_field_missing.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_field_missing.carbon:20: type error in name binding: '{.x: i32}' is not implicitly convertible to 'class Point'
 
 package ExplorerTest api;
 
@@ -17,6 +16,7 @@ class Point {
 }
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_field_missing.carbon:[[@LINE+1]]: type error in name binding: '{.x: i32}' is not implicitly convertible to 'class Point'
   var p: Point = {.x = 1};
   return p.x - 1;
 }
diff --git a/explorer/testdata/class/fail_method_from_class.carbon b/explorer/testdata/class/fail_method_from_class.carbon
index 916a435a70fe..027e0b61cabd 100644
--- a/explorer/testdata/class/fail_method_from_class.carbon
+++ b/explorer/testdata/class/fail_method_from_class.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_method_from_class.carbon:28: GetX is not a class function
 
 package ExplorerTest api;
 
@@ -25,5 +24,6 @@ class Point {
 }
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_method_from_class.carbon:[[@LINE+1]]: GetX is not a class function
   return Point.GetX();
 }
diff --git a/explorer/testdata/experimental_continuation/fail_continuation_syntax.carbon b/explorer/testdata/experimental_continuation/fail_continuation_syntax.carbon
index cb2deb0934b4..626dddb6d255 100644
--- a/explorer/testdata/experimental_continuation/fail_continuation_syntax.carbon
+++ b/explorer/testdata/experimental_continuation/fail_continuation_syntax.carbon
@@ -7,12 +7,12 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/experimental_continuation/fail_continuation_syntax.carbon:16: syntax error, unexpected identifier, expecting LEFT_CURLY_BRACE
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var x: i32 = 0;
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/experimental_continuation/fail_continuation_syntax.carbon:[[@LINE+1]]: syntax error, unexpected identifier, expecting LEFT_CURLY_BRACE
   __continuation k x = 3;
   __run k;
   return x;
diff --git a/explorer/testdata/function/auto_return/fail_direct_recurse.carbon b/explorer/testdata/function/auto_return/fail_direct_recurse.carbon
index d84e64034ea4..92fefab0dcc5 100644
--- a/explorer/testdata/function/auto_return/fail_direct_recurse.carbon
+++ b/explorer/testdata/function/auto_return/fail_direct_recurse.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/function/auto_return/fail_direct_recurse.carbon:18: Function calls itself, but has a deduced return type
 
 package ExplorerTest api;
 
@@ -15,6 +14,7 @@ package ExplorerTest api;
 // used.
 fn Recurse(x: i32, do_recurse: Bool) -> auto {
   if (do_recurse) {
+    // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/function/auto_return/fail_direct_recurse.carbon:[[@LINE+1]]: Function calls itself, but has a deduced return type
     Recurse(x, false);
   }
   return x;
diff --git a/explorer/testdata/function/auto_return/fail_multiple_returns.carbon b/explorer/testdata/function/auto_return/fail_multiple_returns.carbon
index dad80f052f5e..e83d30c50c17 100644
--- a/explorer/testdata/function/auto_return/fail_multiple_returns.carbon
+++ b/explorer/testdata/function/auto_return/fail_multiple_returns.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/function/auto_return/fail_multiple_returns.carbon:18: Only one return is allowed in a function with an `auto` return type.
 
 package ExplorerTest api;
 
@@ -15,6 +14,7 @@ fn Add(x: i32, y: i32) -> auto {
   if (x == 0) {
     return x;
   } else if (y == 0) {
+    // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/function/auto_return/fail_multiple_returns.carbon:[[@LINE+1]]: Only one return is allowed in a function with an `auto` return type.
     return y;
   } else {
     return x + y;
diff --git a/explorer/testdata/function/auto_return/fail_no_return.carbon b/explorer/testdata/function/auto_return/fail_no_return.carbon
index 0940525b457e..9fb3a6721833 100644
--- a/explorer/testdata/function/auto_return/fail_no_return.carbon
+++ b/explorer/testdata/function/auto_return/fail_no_return.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/function/auto_return/fail_no_return.carbon:15: control-flow reaches end of function that provides a `->` return type without reaching a return statement
 
 package ExplorerTest api;
 
 fn NoReturn() -> auto {
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/function/auto_return/fail_no_return.carbon:[[@LINE+1]]: control-flow reaches end of function that provides a `->` return type without reaching a return statement
 }
 
 fn Main() {
diff --git a/explorer/testdata/function/auto_return/fail_separate_decl.carbon b/explorer/testdata/function/auto_return/fail_separate_decl.carbon
index f5947ae4d26e..e23d42a98083 100644
--- a/explorer/testdata/function/auto_return/fail_separate_decl.carbon
+++ b/explorer/testdata/function/auto_return/fail_separate_decl.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/function/auto_return/fail_separate_decl.carbon:15: Function declaration has deduced return type but no body
 
 package ExplorerTest api;
 
 // This declaration is not allowed.
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/function/auto_return/fail_separate_decl.carbon:[[@LINE+1]]: Function declaration has deduced return type but no body
 fn Add(x: i32, y: i32) -> auto;
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/function/fail_call_with_tuple.carbon b/explorer/testdata/function/fail_call_with_tuple.carbon
index 6bb8b732214e..5a31faa3d416 100644
--- a/explorer/testdata/function/fail_call_with_tuple.carbon
+++ b/explorer/testdata/function/fail_call_with_tuple.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_call_with_tuple.carbon:19: type error in call: '((i32, i32))' is not implicitly convertible to '(i32, i32)'
 
 package ExplorerTest api;
 
@@ -16,5 +15,6 @@ fn f(x: i32, y: i32) -> i32 { return x + y; }
 fn Main() -> i32 {
   var xy: (i32, i32) = (1, 2);
   // should fail to type-check
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_call_with_tuple.carbon:[[@LINE+1]]: type error in call: '((i32, i32))' is not implicitly convertible to '(i32, i32)'
   return f(xy);
 }
diff --git a/explorer/testdata/function/fail_match_no_return.carbon b/explorer/testdata/function/fail_match_no_return.carbon
index 41016bdf5648..b16e52a6e82c 100644
--- a/explorer/testdata/function/fail_match_no_return.carbon
+++ b/explorer/testdata/function/fail_match_no_return.carbon
@@ -7,13 +7,13 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_match_no_return.carbon:17: control-flow reaches end of function that provides a `->` return type without reaching a return statement
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var x: i32 = 0;
   match(0) {
+    // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_match_no_return.carbon:[[@LINE+1]]: control-flow reaches end of function that provides a `->` return type without reaching a return statement
     case 1 => { x = 1; }
     case _: auto => { x = 2; }
   }
diff --git a/explorer/testdata/function/fail_match_partial_return.carbon b/explorer/testdata/function/fail_match_partial_return.carbon
index cf1c67874f94..87aeacf7ce66 100644
--- a/explorer/testdata/function/fail_match_partial_return.carbon
+++ b/explorer/testdata/function/fail_match_partial_return.carbon
@@ -7,13 +7,13 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_match_partial_return.carbon:17: control-flow reaches end of function that provides a `->` return type without reaching a return statement
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var x: i32 = 0;
   match (0) {
+    // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_match_partial_return.carbon:[[@LINE+1]]: control-flow reaches end of function that provides a `->` return type without reaching a return statement
     case 1 => { x = 1; }
     case _: auto => { return 0; }
   }
diff --git a/explorer/testdata/function/fail_non_exhaustive_match.carbon b/explorer/testdata/function/fail_non_exhaustive_match.carbon
index f1ef9cc63107..688b3b295d1f 100644
--- a/explorer/testdata/function/fail_non_exhaustive_match.carbon
+++ b/explorer/testdata/function/fail_non_exhaustive_match.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_non_exhaustive_match.carbon:18: non-exhaustive match may allow control-flow to reach the end of a function that provides a `->` return type
 
 package ExplorerTest api;
 
@@ -15,4 +14,5 @@ fn Main() -> i32 {
   match (0) {
     case 1 => return 0;
   }
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_non_exhaustive_match.carbon:[[@LINE+1]]: non-exhaustive match may allow control-flow to reach the end of a function that provides a `->` return type
 }
diff --git a/explorer/testdata/function/fail_parameter_type.carbon b/explorer/testdata/function/fail_parameter_type.carbon
index 08a1f5cbcbb7..0d5f48c6260b 100644
--- a/explorer/testdata/function/fail_parameter_type.carbon
+++ b/explorer/testdata/function/fail_parameter_type.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_parameter_type.carbon:15: Expected a type, but got 42
 
 package ExplorerTest api;
 
 // 42 cannot be used as the type of a parameter.
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_parameter_type.carbon:[[@LINE+1]]: Expected a type, but got 42
 fn f(x: 42) -> i32 {
   return x - 1;
 }
diff --git a/explorer/testdata/generic_class/fail_args_mismatch.carbon b/explorer/testdata/generic_class/fail_args_mismatch.carbon
index 5aa4743552fe..325b85d9b704 100644
--- a/explorer/testdata/generic_class/fail_args_mismatch.carbon
+++ b/explorer/testdata/generic_class/fail_args_mismatch.carbon
@@ -5,12 +5,12 @@
 // RUN: %{not} %{explorer} %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false --dump-input=always -v %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_args_mismatch.carbon:14: type error in call: '(Type, Type)' is not implicitly convertible to '(Type)'
 
 package ExplorerTest api;
 
 class Point(T:! Type) {
   // Point(T, T) does not match class declaration
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_args_mismatch.carbon:[[@LINE+1]]: type error in call: '(Type, Type)' is not implicitly convertible to '(Type)'
   fn Origin(zero: T) -> Point(T, T) {
     return {.x = zero, .y = zero};
   }
diff --git a/explorer/testdata/generic_class/fail_argument_deduction.carbon b/explorer/testdata/generic_class/fail_argument_deduction.carbon
index ee4e57a1574d..63c7b7aa3573 100644
--- a/explorer/testdata/generic_class/fail_argument_deduction.carbon
+++ b/explorer/testdata/generic_class/fail_argument_deduction.carbon
@@ -7,9 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_argument_deduction.carbon:28: type error in argument deduction
-// CHECK: expected: i32
-// CHECK: actual: Bool
 
 package ExplorerTest api;
 
@@ -25,5 +22,8 @@ fn FirstOfTwoPoints[T:! Type](a: Point(T), b: Point(T)) -> Point(T) {
 fn Main() -> i32 {
   var p: Point(i32) = {.x = 0, .y = 1};
   var q: Point(Bool) = {.x = true, .y = false};
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_argument_deduction.carbon:[[@LINE+3]]: type error in argument deduction
+  // CHECK: expected: i32
+  // CHECK: actual: Bool
   return FirstOfTwoPoints(p, q).x;
 }
diff --git a/explorer/testdata/generic_class/fail_bad_parameter_type.carbon b/explorer/testdata/generic_class/fail_bad_parameter_type.carbon
index 0a731d29494f..430bfee3a005 100644
--- a/explorer/testdata/generic_class/fail_bad_parameter_type.carbon
+++ b/explorer/testdata/generic_class/fail_bad_parameter_type.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_bad_parameter_type.carbon:28: type error in call: '(Type)' is not implicitly convertible to '(i32)'
 
 package ExplorerTest api;
 
@@ -25,6 +24,7 @@ class Point(T:! i32) {
 }
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_bad_parameter_type.carbon:[[@LINE+1]]: type error in call: '(Type)' is not implicitly convertible to '(i32)'
   var p: Point(i32) = Point(i32).Origin(0);
   return p.GetX();
 }
diff --git a/explorer/testdata/generic_class/fail_field_access_on_generic.carbon b/explorer/testdata/generic_class/fail_field_access_on_generic.carbon
index 1b2570db99c3..0263266e95ff 100644
--- a/explorer/testdata/generic_class/fail_field_access_on_generic.carbon
+++ b/explorer/testdata/generic_class/fail_field_access_on_generic.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_field_access_on_generic.carbon:15: field access, unexpected T:! Type of non-interface type Type in a.x
 
 package ExplorerTest api;
 
 fn BadFieldAccess[T:! Type](a: T) -> T {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_field_access_on_generic.carbon:[[@LINE+1]]: field access, unexpected T:! Type of non-interface type Type in a.x
   return a.x;
 }
 
diff --git a/explorer/testdata/generic_class/fail_generic_class_arg.carbon b/explorer/testdata/generic_class/fail_generic_class_arg.carbon
index 48d9fdd03344..bcfd48d80dc9 100644
--- a/explorer/testdata/generic_class/fail_generic_class_arg.carbon
+++ b/explorer/testdata/generic_class/fail_generic_class_arg.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_generic_class_arg.carbon:24: type error in call: '(Type, Type)' is not implicitly convertible to '(Type)'
 
 package ExplorerTest api;
 
@@ -21,6 +20,7 @@ class Point(T:! Type) {
 }
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_generic_class_arg.carbon:[[@LINE+1]]: type error in call: '(Type, Type)' is not implicitly convertible to '(Type)'
   var p: Point(i32) = Point(i32, i32).Create(0, 1);
   return p.x;
 }
diff --git a/explorer/testdata/generic_class/fail_generic_in_pattern.carbon b/explorer/testdata/generic_class/fail_generic_in_pattern.carbon
index 445c2553c017..879575162c0d 100644
--- a/explorer/testdata/generic_class/fail_generic_in_pattern.carbon
+++ b/explorer/testdata/generic_class/fail_generic_in_pattern.carbon
@@ -7,13 +7,13 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_generic_in_pattern.carbon:17: Generic binding may not occur in pattern with expected type: T:! i32
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var t: auto = 5;
   match (t) {
+    // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_generic_in_pattern.carbon:[[@LINE+1]]: Generic binding may not occur in pattern with expected type: T:! i32
     case T:! i32 =>
       return 0;
     default =>
diff --git a/explorer/testdata/generic_class/fail_instantiate_non_generic.carbon b/explorer/testdata/generic_class/fail_instantiate_non_generic.carbon
index b2981966cfa9..ae914c3224da 100644
--- a/explorer/testdata/generic_class/fail_instantiate_non_generic.carbon
+++ b/explorer/testdata/generic_class/fail_instantiate_non_generic.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_instantiate_non_generic.carbon:23: attempt to instantiate a non-generic class: Point(i32)
 
 package ExplorerTest api;
 
@@ -20,6 +19,7 @@ class Point {
 }
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_instantiate_non_generic.carbon:[[@LINE+1]]: attempt to instantiate a non-generic class: Point(i32)
   var p: Point(i32) = Point.Origin();
   return 0;
 }
diff --git a/explorer/testdata/generic_class/fail_point_equal.carbon b/explorer/testdata/generic_class/fail_point_equal.carbon
index 0276d1a81442..c165f62ab96d 100644
--- a/explorer/testdata/generic_class/fail_point_equal.carbon
+++ b/explorer/testdata/generic_class/fail_point_equal.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_point_equal.carbon:21: type error in name binding: 'class Point(T = i32)' is not implicitly convertible to 'class Point(T = Bool)'
 
 package ExplorerTest api;
 
@@ -18,6 +17,7 @@ class Point(T:! Type) {
 
 fn Main() -> i32 {
   var p: Point(i32) = {.x = 0, .y = 0};
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_point_equal.carbon:[[@LINE+1]]: type error in name binding: 'class Point(T = i32)' is not implicitly convertible to 'class Point(T = Bool)'
   var q: Point(Bool) = p;
   return 0;
 }
diff --git a/explorer/testdata/generic_class/fail_return_type_is_type.carbon b/explorer/testdata/generic_class/fail_return_type_is_type.carbon
index ca3e37f15cb8..5fb74c8b1c8d 100644
--- a/explorer/testdata/generic_class/fail_return_type_is_type.carbon
+++ b/explorer/testdata/generic_class/fail_return_type_is_type.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_return_type_is_type.carbon:18: Expected a type, but got class Point
 
 package ExplorerTest api;
 
@@ -15,6 +14,7 @@ class Point(T:! Type) {
   // The return type should be Point(T). Point by itself is not a type.
   fn Create(x: T, y: T) -> Point {
     return {.x = x, .y = y};
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_return_type_is_type.carbon:[[@LINE+1]]: Expected a type, but got class Point
   }
 
   var x: T;
diff --git a/explorer/testdata/generic_class/fail_two_arg_lists.carbon b/explorer/testdata/generic_class/fail_two_arg_lists.carbon
index 342aecb872ca..440f026fb4fa 100644
--- a/explorer/testdata/generic_class/fail_two_arg_lists.carbon
+++ b/explorer/testdata/generic_class/fail_two_arg_lists.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_two_arg_lists.carbon:15: attempt to instantiate an already instantiated generic class: Point(T)(T)
 
 package ExplorerTest api;
 
 class Point(T:! Type) {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_class/fail_two_arg_lists.carbon:[[@LINE+1]]: attempt to instantiate an already instantiated generic class: Point(T)(T)
   fn Origin(zero: T) -> Point(T)(T) {
     return {.x = zero, .y = zero};
   }
diff --git a/explorer/testdata/generic_function/fail_not_addable.carbon b/explorer/testdata/generic_function/fail_not_addable.carbon
index 45cea88691e0..48f3d54a6bbc 100644
--- a/explorer/testdata/generic_function/fail_not_addable.carbon
+++ b/explorer/testdata/generic_function/fail_not_addable.carbon
@@ -7,13 +7,13 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_not_addable.carbon:17: type error in addition(1)
-// CHECK: expected: i32
-// CHECK: actual: T:! Type
 
 package ExplorerTest api;
 
 fn id[T:! Type](x: T) -> T {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_not_addable.carbon:[[@LINE+3]]: type error in addition(1)
+  // CHECK: expected: i32
+  // CHECK: actual: T:! Type
   return x + 0;
 }
 
diff --git a/explorer/testdata/generic_function/fail_type_deduction_mismatch.carbon b/explorer/testdata/generic_function/fail_type_deduction_mismatch.carbon
index 04772a56b79e..8639f6ea982a 100644
--- a/explorer/testdata/generic_function/fail_type_deduction_mismatch.carbon
+++ b/explorer/testdata/generic_function/fail_type_deduction_mismatch.carbon
@@ -7,9 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_type_deduction_mismatch.carbon:21: type error in argument deduction
-// CHECK: expected: i32
-// CHECK: actual: Bool
 
 package ExplorerTest api;
 
@@ -18,5 +15,8 @@ fn fst[T:! Type](x: T, y: T) -> T {
 }
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_type_deduction_mismatch.carbon:[[@LINE+3]]: type error in argument deduction
+  // CHECK: expected: i32
+  // CHECK: actual: Bool
   return fst(0, true);
 }
diff --git a/explorer/testdata/generic_function/fail_type_deduction_unused.carbon b/explorer/testdata/generic_function/fail_type_deduction_unused.carbon
index f8ce4ddbdd4c..8de1b49bc8ce 100644
--- a/explorer/testdata/generic_function/fail_type_deduction_unused.carbon
+++ b/explorer/testdata/generic_function/fail_type_deduction_unused.carbon
@@ -7,8 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_type_deduction_unused.carbon:20: could not deduce type argument for type parameter T
-// CHECK: in id(0)
 
 package ExplorerTest api;
 
@@ -17,5 +15,7 @@ fn id[T:! Type](x: i32) -> i32 {
 }
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/generic_function/fail_type_deduction_unused.carbon:[[@LINE+2]]: could not deduce type argument for type parameter T
+  // CHECK: in id(0)
   return id(0);
 }
diff --git a/explorer/testdata/global_variable/fail_init_type_mismatch.carbon b/explorer/testdata/global_variable/fail_init_type_mismatch.carbon
index 1dffc8edd1af..559c51983b6c 100644
--- a/explorer/testdata/global_variable/fail_init_type_mismatch.carbon
+++ b/explorer/testdata/global_variable/fail_init_type_mismatch.carbon
@@ -7,12 +7,12 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/global_variable/fail_init_type_mismatch.carbon:16: type error in initializer of variable: 'Bool' is not implicitly convertible to 'i32'
 
 package ExplorerTest api;
 
 // Test type checking of global variable. Error expected.
 
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/global_variable/fail_init_type_mismatch.carbon:[[@LINE+1]]: type error in initializer of variable: 'Bool' is not implicitly convertible to 'i32'
 var flag: i32 = true;
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/impl/fail_ambiguous_impl.carbon b/explorer/testdata/impl/fail_ambiguous_impl.carbon
index ab873d0af064..7655f6044fa8 100644
--- a/explorer/testdata/impl/fail_ambiguous_impl.carbon
+++ b/explorer/testdata/impl/fail_ambiguous_impl.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_ambiguous_impl.carbon:50: ambiguous implementations of interface Vector for class Point
 
 
 package ExplorerTest api;
@@ -47,6 +46,7 @@ fn AddAndScaleGeneric[T:! Vector](a: T, b: T, s: i32) -> T {
 fn Main() -> i32 {
   var a: Point = {.x = 1, .y = 1};
   var b: Point = {.x = 2, .y = 3};
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_ambiguous_impl.carbon:[[@LINE+1]]: ambiguous implementations of interface Vector for class Point
   var p: Point = AddAndScaleGeneric(a, b, 5);
   return p.x - 15;
 }
diff --git a/explorer/testdata/impl/fail_impl_as_not_constraint.carbon b/explorer/testdata/impl/fail_impl_as_not_constraint.carbon
index 1f4b1a90bf68..682ea6dafd5f 100644
--- a/explorer/testdata/impl/fail_impl_as_not_constraint.carbon
+++ b/explorer/testdata/impl/fail_impl_as_not_constraint.carbon
@@ -7,10 +7,10 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_impl_as_not_constraint.carbon:14: expected constraint after `as`, found value of type i32
 
 package ExplorerTest api;
 
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_impl_as_not_constraint.carbon:[[@LINE+1]]: expected constraint after `as`, found value of type i32
 external impl i32 as i32 {}
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/impl/fail_impl_as_parameterized.carbon b/explorer/testdata/impl/fail_impl_as_parameterized.carbon
index 99e2a60f0cfa..bf5991277e09 100644
--- a/explorer/testdata/impl/fail_impl_as_parameterized.carbon
+++ b/explorer/testdata/impl/fail_impl_as_parameterized.carbon
@@ -7,13 +7,13 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_impl_as_parameterized.carbon:17: missing arguments for parameterized interface
 
 package ExplorerTest api;
 
 interface Vector(Scalar:! Type) {
 }
 
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/impl/fail_impl_as_parameterized.carbon:[[@LINE+1]]: missing arguments for parameterized interface
 external impl i32 as Vector {}
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/impl/impl_as.carbon b/explorer/testdata/impl/impl_as.carbon
index 53d8652a6d8c..c80bfa9c3d2c 100644
--- a/explorer/testdata/impl/impl_as.carbon
+++ b/explorer/testdata/impl/impl_as.carbon
@@ -7,10 +7,10 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/impl/impl_as.carbon:14: expected constraint after `as`, found value of type i32
 
 package ExplorerTest api;
 
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/impl/impl_as.carbon:[[@LINE+1]]: expected constraint after `as`, found value of type i32
 external impl i32 as i32 {}
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/import/fail_order.carbon b/explorer/testdata/import/fail_order.carbon
index 36b80e7f8ed3..073437acf1de 100644
--- a/explorer/testdata/import/fail_order.carbon
+++ b/explorer/testdata/import/fail_order.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/import/fail_order.carbon:18: syntax error, unexpected IMPORT, expecting END_OF_FILE
 
 package ExplorerTest api;
 
@@ -15,4 +14,5 @@ fn Main() -> i32 {
   return 0;
 }
 
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/import/fail_order.carbon:[[@LINE+1]]: syntax error, unexpected IMPORT, expecting END_OF_FILE
 import ExplorerTest library "Nonexistent";
diff --git a/explorer/testdata/interface/fail_impl_bad_member.carbon b/explorer/testdata/interface/fail_impl_bad_member.carbon
index 4e5cee916cb8..25386c4d3bff 100644
--- a/explorer/testdata/interface/fail_impl_bad_member.carbon
+++ b/explorer/testdata/interface/fail_impl_bad_member.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_impl_bad_member.carbon:28: type error in member of implementation: 'fn (i32) -> i32' is not implicitly convertible to 'fn (i32) -> class Point'
 
 package ExplorerTest api;
 
@@ -25,6 +24,7 @@ class Point {
     }
     fn Scale[me: Point](v: i32) -> i32 {
       return 0;
+    // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_impl_bad_member.carbon:[[@LINE+1]]: type error in member of implementation: 'fn (i32) -> i32' is not implicitly convertible to 'fn (i32) -> class Point'
     }
   }
 }
diff --git a/explorer/testdata/interface/fail_impl_missing_member.carbon b/explorer/testdata/interface/fail_impl_missing_member.carbon
index 0e5c74e7141d..4d7b419e828b 100644
--- a/explorer/testdata/interface/fail_impl_missing_member.carbon
+++ b/explorer/testdata/interface/fail_impl_missing_member.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_impl_missing_member.carbon:26: implementation missing Scale
 
 package ExplorerTest api;
 
@@ -23,6 +22,7 @@ class Point {
     fn Add[me: Point](b: Point) -> Point {
       return {.x = me.x + b.x, .y = me.y + b.y};
     }
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_impl_missing_member.carbon:[[@LINE+1]]: implementation missing Scale
   }
 }
 
diff --git a/explorer/testdata/interface/fail_interface_missing_member.carbon b/explorer/testdata/interface/fail_interface_missing_member.carbon
index f51b7908ddc5..2920b5ad9143 100644
--- a/explorer/testdata/interface/fail_interface_missing_member.carbon
+++ b/explorer/testdata/interface/fail_interface_missing_member.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_interface_missing_member.carbon:19: field access, Scale not in Vector
 
 package ExplorerTest api;
 
@@ -16,6 +15,7 @@ interface Vector {
 }
 
 fn ScaleGeneric[T:! Vector](a: T, s: i32) -> T {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_interface_missing_member.carbon:[[@LINE+1]]: field access, Scale not in Vector
   return a.Scale(s);
 }
 
diff --git a/explorer/testdata/interface/fail_no_impl.carbon b/explorer/testdata/interface/fail_no_impl.carbon
index 0ef0ac73025c..db752f278856 100644
--- a/explorer/testdata/interface/fail_no_impl.carbon
+++ b/explorer/testdata/interface/fail_no_impl.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_no_impl.carbon:31: could not find implementation of interface Vector for class Point
 
 package ExplorerTest api;
 
@@ -28,6 +27,7 @@ fn AddAndScaleGeneric[T:! Vector](a: T, b: T, s: i32) -> T {
 fn Main() -> i32 {
   var a: Point = {.x = 0, .y = 0};
   var b: Point = {.x = 2, .y = 3};
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/interface/fail_no_impl.carbon:[[@LINE+1]]: could not find implementation of interface Vector for class Point
   var p: Point = AddAndScaleGeneric(a, b, 3);
   return p.x - 6;
 }
diff --git a/explorer/testdata/let/fail_function_args.carbon b/explorer/testdata/let/fail_function_args.carbon
index 18e37da0b087..1212434d9fe2 100644
--- a/explorer/testdata/let/fail_function_args.carbon
+++ b/explorer/testdata/let/fail_function_args.carbon
@@ -7,12 +7,12 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_function_args.carbon:16: Cannot assign to rvalue 'y'
 
 package ExplorerTest api;
 
 fn f((var x: i32, y: i32)) -> i32 {
   x = 0;
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_function_args.carbon:[[@LINE+1]]: Cannot assign to rvalue 'y'
   y = 0;
   return x - 1;
 }
diff --git a/explorer/testdata/let/fail_global_assign.carbon b/explorer/testdata/let/fail_global_assign.carbon
index 7560f82da10a..b8299ba57df2 100644
--- a/explorer/testdata/let/fail_global_assign.carbon
+++ b/explorer/testdata/let/fail_global_assign.carbon
@@ -7,13 +7,13 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_global_assign.carbon:17: Cannot assign to rvalue 'x'
 
 package ExplorerTest api;
 
 let x: i32 = 10;
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_global_assign.carbon:[[@LINE+1]]: Cannot assign to rvalue 'x'
   x = 0;
   return 0;
 }
diff --git a/explorer/testdata/let/fail_local_assign.carbon b/explorer/testdata/let/fail_local_assign.carbon
index 1297fff45c88..d7c7ee1b86db 100644
--- a/explorer/testdata/let/fail_local_assign.carbon
+++ b/explorer/testdata/let/fail_local_assign.carbon
@@ -7,12 +7,12 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_local_assign.carbon:16: Cannot assign to rvalue 'x'
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   let x: auto = 10;
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_local_assign.carbon:[[@LINE+1]]: Cannot assign to rvalue 'x'
   x = 0;
   return 0;
 }
diff --git a/explorer/testdata/let/fail_match_choice.carbon b/explorer/testdata/let/fail_match_choice.carbon
index 150c27042aec..4895d57ee20d 100644
--- a/explorer/testdata/let/fail_match_choice.carbon
+++ b/explorer/testdata/let/fail_match_choice.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_match_choice.carbon:29: Cannot assign to rvalue 'x'
 
 package ExplorerTest api;
 
@@ -26,6 +25,7 @@ fn Main() -> i32 {
     case Ints.None() =>
       n = n - 1;
     case Ints.Two(x: auto, y: auto) => {
+      // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_match_choice.carbon:[[@LINE+1]]: Cannot assign to rvalue 'x'
       x = 0;
     }
   }
diff --git a/explorer/testdata/let/fail_method_args.carbon b/explorer/testdata/let/fail_method_args.carbon
index d19a83e5d500..f0c36a058a57 100644
--- a/explorer/testdata/let/fail_method_args.carbon
+++ b/explorer/testdata/let/fail_method_args.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_method_args.carbon:20: Cannot assign to rvalue 'x'
 
 package ExplorerTest api;
 
@@ -17,6 +16,7 @@ class Point {
   }
 
   fn SetX[me: Point](x: i32) {
+    // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_method_args.carbon:[[@LINE+1]]: Cannot assign to rvalue 'x'
     x = 10;
   }
 
diff --git a/explorer/testdata/let/fail_tuple_pattern_let_context.carbon b/explorer/testdata/let/fail_tuple_pattern_let_context.carbon
index 5b0f3abff2cb..1914ea97cac5 100644
--- a/explorer/testdata/let/fail_tuple_pattern_let_context.carbon
+++ b/explorer/testdata/let/fail_tuple_pattern_let_context.carbon
@@ -7,7 +7,6 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_tuple_pattern_let_context.carbon:18: Cannot assign to rvalue 'c'
 
 package ExplorerTest api;
 
@@ -15,6 +14,7 @@ fn Main() -> i32 {
   let (var a: auto, b: auto, c: auto, d: auto) = (1, 2, 3, 4);
   a = 0;
   // should fail
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_tuple_pattern_let_context.carbon:[[@LINE+1]]: Cannot assign to rvalue 'c'
   c = 0;
   return 0;
 }
diff --git a/explorer/testdata/let/fail_tuple_pattern_let_context_nested.carbon b/explorer/testdata/let/fail_tuple_pattern_let_context_nested.carbon
index bcae1367785f..b3097b4d2951 100644
--- a/explorer/testdata/let/fail_tuple_pattern_let_context_nested.carbon
+++ b/explorer/testdata/let/fail_tuple_pattern_let_context_nested.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_tuple_pattern_let_context_nested.carbon:15: syntax error, unexpected LET
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_tuple_pattern_let_context_nested.carbon:[[@LINE+1]]: syntax error, unexpected LET
   let (var a: auto, b: auto, c: auto, var (d: auto, let e: auto)) =
       (1, 2, 3, (4, 5));
   return 0;
diff --git a/explorer/testdata/let/fail_tuple_pattern_let_in_var.carbon b/explorer/testdata/let/fail_tuple_pattern_let_in_var.carbon
index 86925b76dda0..7a847f6bd613 100644
--- a/explorer/testdata/let/fail_tuple_pattern_let_in_var.carbon
+++ b/explorer/testdata/let/fail_tuple_pattern_let_in_var.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_tuple_pattern_let_in_var.carbon:15: syntax error, unexpected LET
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/let/fail_tuple_pattern_let_in_var.carbon:[[@LINE+1]]: syntax error, unexpected LET
   var (var a: auto, b: auto, let c: auto, d: auto) = (1, 2, 3, 4);
   return 0;
 }
diff --git a/explorer/testdata/name_lookup/fail_block_duplicate.carbon b/explorer/testdata/name_lookup/fail_block_duplicate.carbon
index 05fea016141c..cb394809df37 100644
--- a/explorer/testdata/name_lookup/fail_block_duplicate.carbon
+++ b/explorer/testdata/name_lookup/fail_block_duplicate.carbon
@@ -7,12 +7,12 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_block_duplicate.carbon:16: Duplicate name `x` also found at {{.*}}/explorer/testdata/name_lookup/fail_block_duplicate.carbon:15
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var x: i32 = 0;
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_block_duplicate.carbon:[[@LINE+1]]: Duplicate name `x` also found at {{.*}}/explorer/testdata/name_lookup/fail_block_duplicate.carbon:14
   var x: i32 = 0;
   return 0;
 }
diff --git a/explorer/testdata/name_lookup/fail_choice_duplicate.carbon b/explorer/testdata/name_lookup/fail_choice_duplicate.carbon
index 3d53b8f59c2c..0de0d5ee4df2 100644
--- a/explorer/testdata/name_lookup/fail_choice_duplicate.carbon
+++ b/explorer/testdata/name_lookup/fail_choice_duplicate.carbon
@@ -7,12 +7,12 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_choice_duplicate.carbon:16: Duplicate name `None` in choice type
 
 package ExplorerTest api;
 
 choice Ints {
   None,
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_choice_duplicate.carbon:[[@LINE+1]]: Duplicate name `None` in choice type
   None,
 }
 
diff --git a/explorer/testdata/name_lookup/fail_class_duplicate.carbon b/explorer/testdata/name_lookup/fail_class_duplicate.carbon
index 7869a55038ba..3178ac1de59c 100644
--- a/explorer/testdata/name_lookup/fail_class_duplicate.carbon
+++ b/explorer/testdata/name_lookup/fail_class_duplicate.carbon
@@ -7,12 +7,12 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_class_duplicate.carbon:16: Duplicate name `x` also found at {{.*}}/explorer/testdata/name_lookup/fail_class_duplicate.carbon:15
 
 package ExplorerTest api;
 
 class Foo {
   var x: i32;
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_class_duplicate.carbon:[[@LINE+1]]: Duplicate name `x` also found at {{.*}}/explorer/testdata/name_lookup/fail_class_duplicate.carbon:14
   var x: i32;
 }
 
diff --git a/explorer/testdata/name_lookup/fail_global_duplicate.carbon b/explorer/testdata/name_lookup/fail_global_duplicate.carbon
index ffdc84e31208..319a13416134 100644
--- a/explorer/testdata/name_lookup/fail_global_duplicate.carbon
+++ b/explorer/testdata/name_lookup/fail_global_duplicate.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_global_duplicate.carbon:15: Duplicate name `x` also found at {{.*}}/explorer/testdata/name_lookup/fail_global_duplicate.carbon:14
 
 package ExplorerTest api;
 
 var x: i32 = 0;
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_global_duplicate.carbon:[[@LINE+1]]: Duplicate name `x` also found at {{.*}}/explorer/testdata/name_lookup/fail_global_duplicate.carbon:13
 var x: i32 = 0;
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/name_lookup/fail_match_duplicate.carbon b/explorer/testdata/name_lookup/fail_match_duplicate.carbon
index 694fa3ad8d88..e1aa641a9d78 100644
--- a/explorer/testdata/name_lookup/fail_match_duplicate.carbon
+++ b/explorer/testdata/name_lookup/fail_match_duplicate.carbon
@@ -7,13 +7,13 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_match_duplicate.carbon:17: Duplicate name `x` also found at {{.*}}/explorer/testdata/name_lookup/fail_match_duplicate.carbon:16
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   match (0) {
     case x: i32 =>
+      // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_match_duplicate.carbon:[[@LINE+1]]: Duplicate name `x` also found at {{.*}}/explorer/testdata/name_lookup/fail_match_duplicate.carbon:15
       var x: i32 = 0;
   }
   return 0;
diff --git a/explorer/testdata/name_lookup/fail_use_before_declare.carbon b/explorer/testdata/name_lookup/fail_use_before_declare.carbon
index 309f11914567..e693f772db5f 100644
--- a/explorer/testdata/name_lookup/fail_use_before_declare.carbon
+++ b/explorer/testdata/name_lookup/fail_use_before_declare.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_use_before_declare.carbon:15: could not resolve 'x'
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/name_lookup/fail_use_before_declare.carbon:[[@LINE+1]]: could not resolve 'x'
   x = 0;
   var x: i32 = 0;
   return 0;
diff --git a/explorer/testdata/package/fail_missing.carbon b/explorer/testdata/package/fail_missing.carbon
index fa4cd4f2785c..ef3260ee95cd 100644
--- a/explorer/testdata/package/fail_missing.carbon
+++ b/explorer/testdata/package/fail_missing.carbon
@@ -7,8 +7,8 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/package/fail_missing.carbon:12: syntax error, unexpected FN, expecting PACKAGE
 
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/package/fail_missing.carbon:[[@LINE+1]]: syntax error, unexpected FN, expecting PACKAGE
 fn Main() -> i32 {
   return 0;
 }
diff --git a/explorer/testdata/pointer/fail_rvalue_addressof.carbon b/explorer/testdata/pointer/fail_rvalue_addressof.carbon
index 700788f7fa61..f7ad7fff062c 100644
--- a/explorer/testdata/pointer/fail_rvalue_addressof.carbon
+++ b/explorer/testdata/pointer/fail_rvalue_addressof.carbon
@@ -7,12 +7,12 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/pointer/fail_rvalue_addressof.carbon:16: Argument to & should be an lvalue.
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var x: i32 = 5;
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/pointer/fail_rvalue_addressof.carbon:[[@LINE+1]]: Argument to & should be an lvalue.
   var p: i32* = &5;
   return 0;
 }
diff --git a/explorer/testdata/return/fail_explicit_with_no_return.carbon b/explorer/testdata/return/fail_explicit_with_no_return.carbon
index deba307b960f..4ab9407e50c9 100644
--- a/explorer/testdata/return/fail_explicit_with_no_return.carbon
+++ b/explorer/testdata/return/fail_explicit_with_no_return.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/return/fail_explicit_with_no_return.carbon:15: control-flow reaches end of function that provides a `->` return type without reaching a return statement
 
 package ExplorerTest api;
 
 fn F() -> () {
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/return/fail_explicit_with_no_return.carbon:[[@LINE+1]]: control-flow reaches end of function that provides a `->` return type without reaching a return statement
 }
 
 fn Main() -> i32 {
diff --git a/explorer/testdata/return/fail_explicit_with_plain_return.carbon b/explorer/testdata/return/fail_explicit_with_plain_return.carbon
index 3f70dd54b9b7..d6beb5242d58 100644
--- a/explorer/testdata/return/fail_explicit_with_plain_return.carbon
+++ b/explorer/testdata/return/fail_explicit_with_plain_return.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/return/fail_explicit_with_plain_return.carbon:15: return; should provide a return value, to match the function's signature.
 
 package ExplorerTest api;
 
 fn F() -> () {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/return/fail_explicit_with_plain_return.carbon:[[@LINE+1]]: return; should provide a return value, to match the function's signature.
   return;
 }
 
diff --git a/explorer/testdata/return/fail_implicit_with_explicit_return.carbon b/explorer/testdata/return/fail_implicit_with_explicit_return.carbon
index cb5be1a55887..ef4fc3b3c35e 100644
--- a/explorer/testdata/return/fail_implicit_with_explicit_return.carbon
+++ b/explorer/testdata/return/fail_implicit_with_explicit_return.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/return/fail_implicit_with_explicit_return.carbon:15: return (); should not provide a return value, to match the function's signature.
 
 package ExplorerTest api;
 
 fn F() {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/return/fail_implicit_with_explicit_return.carbon:[[@LINE+1]]: return (); should not provide a return value, to match the function's signature.
   return ();
 }
 
diff --git a/explorer/testdata/string/fail_block_quotes_not_on_own_line.carbon b/explorer/testdata/string/fail_block_quotes_not_on_own_line.carbon
index 57d0fa581273..98411802bc18 100644
--- a/explorer/testdata/string/fail_block_quotes_not_on_own_line.carbon
+++ b/explorer/testdata/string/fail_block_quotes_not_on_own_line.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/string/fail_block_quotes_not_on_own_line.carbon:15: Invalid block string: Should end with triple quotes: error: closing """
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/string/fail_block_quotes_not_on_own_line.carbon:[[@LINE+1]]: Invalid block string: Should end with triple quotes: error: closing """
   var s: String = """
     error: closing """ is not on its own line.
   """;
diff --git a/explorer/testdata/string/fail_hex_lower.carbon b/explorer/testdata/string/fail_hex_lower.carbon
index 009723f401e6..1e5bf629c5f3 100644
--- a/explorer/testdata/string/fail_hex_lower.carbon
+++ b/explorer/testdata/string/fail_hex_lower.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/string/fail_hex_lower.carbon:15: Invalid escaping in string: "str\xaa"
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/string/fail_hex_lower.carbon:[[@LINE+1]]: Invalid escaping in string: "str\xaa"
   Print("str\xaa");
   return 0;
 }
diff --git a/explorer/testdata/string/fail_hex_truncated.carbon b/explorer/testdata/string/fail_hex_truncated.carbon
index 9769c975393d..d480dfde3f14 100644
--- a/explorer/testdata/string/fail_hex_truncated.carbon
+++ b/explorer/testdata/string/fail_hex_truncated.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/string/fail_hex_truncated.carbon:15: Invalid escaping in string: "str\x"
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/string/fail_hex_truncated.carbon:[[@LINE+1]]: Invalid escaping in string: "str\x"
   Print("str\x");
   return 0;
 }
diff --git a/explorer/testdata/string/fail_invalid_escape.carbon b/explorer/testdata/string/fail_invalid_escape.carbon
index 7863df635d45..6c143987a089 100644
--- a/explorer/testdata/string/fail_invalid_escape.carbon
+++ b/explorer/testdata/string/fail_invalid_escape.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/string/fail_invalid_escape.carbon:15: Invalid escaping in string: "str\e"
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/string/fail_invalid_escape.carbon:[[@LINE+1]]: Invalid escaping in string: "str\e"
   Print("str\e");
   return 0;
 }
diff --git a/explorer/testdata/string/fail_newline.carbon b/explorer/testdata/string/fail_newline.carbon
index a249f883499e..5e3ea57ddad8 100644
--- a/explorer/testdata/string/fail_newline.carbon
+++ b/explorer/testdata/string/fail_newline.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/string/fail_newline.carbon:15: invalid character '\x22' in source file.
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/string/fail_newline.carbon:[[@LINE+1]]: invalid character '\x22' in source file.
   Print("new
 line");
   return 0;
diff --git a/explorer/testdata/string/fail_octal.carbon b/explorer/testdata/string/fail_octal.carbon
index b949f8850182..efdb5a5f815f 100644
--- a/explorer/testdata/string/fail_octal.carbon
+++ b/explorer/testdata/string/fail_octal.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/string/fail_octal.carbon:15: Invalid escaping in string: "str\01"
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/string/fail_octal.carbon:[[@LINE+1]]: Invalid escaping in string: "str\01"
   Print("str\01");
   return 0;
 }
diff --git a/explorer/testdata/string/fail_tab.carbon b/explorer/testdata/string/fail_tab.carbon
index c7e94bbbe1f6..5a031c5fbe91 100644
--- a/explorer/testdata/string/fail_tab.carbon
+++ b/explorer/testdata/string/fail_tab.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/string/fail_tab.carbon:15: Invalid escaping in string: "new	line"
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/string/fail_tab.carbon:[[@LINE+1]]: Invalid escaping in string: "new	line"
   Print("new	line");
   return 0;
 }
diff --git a/explorer/testdata/struct/fail_equality_type.carbon b/explorer/testdata/struct/fail_equality_type.carbon
index 1451111b2989..2637ecfe8e4d 100644
--- a/explorer/testdata/struct/fail_equality_type.carbon
+++ b/explorer/testdata/struct/fail_equality_type.carbon
@@ -4,15 +4,15 @@
 //
 // RUN: %{not} %{explorer} %s 2>&1 2>&1 | %{FileCheck} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/struct/fail_equality_type.carbon:16: type error in ==
-// CHECK: expected: {.x: i32, .y: i32}
-// CHECK: actual: {.x: i32}
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var t1: {.x: i32, .y: i32} = {.x = 5, .y = 2};
   var t2: {.x: i32,} = {.x = 5,};
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/struct/fail_equality_type.carbon:[[@LINE+3]]: type error in ==
+  // CHECK: expected: {.x: i32, .y: i32}
+  // CHECK: actual: {.x: i32}
   if (t1 == t2) {
     return 1;
   } else {
diff --git a/explorer/testdata/struct/fail_field_access_mismatch.carbon b/explorer/testdata/struct/fail_field_access_mismatch.carbon
index 449448fb183f..67eba881dfad 100644
--- a/explorer/testdata/struct/fail_field_access_mismatch.carbon
+++ b/explorer/testdata/struct/fail_field_access_mismatch.carbon
@@ -7,10 +7,10 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/struct/fail_field_access_mismatch.carbon:15: struct {.x: i32, .y: i32} does not have a field named z
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/struct/fail_field_access_mismatch.carbon:[[@LINE+1]]: struct {.x: i32, .y: i32} does not have a field named z
   return {.x = 1, .y = 2}.z - 1;
 }
diff --git a/explorer/testdata/tuple/fail_equality_type.carbon b/explorer/testdata/tuple/fail_equality_type.carbon
index f11a8d0094df..c7433af81297 100644
--- a/explorer/testdata/tuple/fail_equality_type.carbon
+++ b/explorer/testdata/tuple/fail_equality_type.carbon
@@ -4,15 +4,15 @@
 //
 // RUN: %{not} %{explorer} %s 2>&1 2>&1 | %{FileCheck} %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/tuple/fail_equality_type.carbon:16: type error in ==
-// CHECK: expected: (i32, i32)
-// CHECK: actual: (i32)
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var t1: (i32, i32) = (5, 2);
   var t2: (i32,) = (5,);
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/tuple/fail_equality_type.carbon:[[@LINE+3]]: type error in ==
+  // CHECK: expected: (i32, i32)
+  // CHECK: actual: (i32)
   if (t1 == t2) {
     return 1;
   } else {
diff --git a/explorer/testdata/tuple/fail_index.carbon b/explorer/testdata/tuple/fail_index.carbon
index c548f1f3e370..fa0ef77faf77 100644
--- a/explorer/testdata/tuple/fail_index.carbon
+++ b/explorer/testdata/tuple/fail_index.carbon
@@ -7,11 +7,11 @@
 // RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
 // RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
 // AUTOUPDATE: %{explorer} %s
-// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/tuple/fail_index.carbon:16: index 2 is out of range for type (i32, i32)
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var x: auto = (0, 1);
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/tuple/fail_index.carbon:[[@LINE+1]]: index 2 is out of range for type (i32, i32)
   return x[2];
 }
diff --git a/explorer/update_checks.py b/explorer/update_checks.py
index ae378ef41cca..baf69f5be793 100755
--- a/explorer/update_checks.py
+++ b/explorer/update_checks.py
@@ -10,9 +10,11 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 from concurrent import futures
 import os
+import re
 import subprocess
 import sys
-from typing import Set
+from abc import ABC, abstractmethod
+from typing import Any, Dict, List, Optional, Set
 
 _BIN = "./bazel-bin/explorer/explorer"
 _TESTDATA = "explorer/testdata"
@@ -23,6 +25,9 @@ _AUTOUPDATE_MARKER = "// AUTOUPDATE: "
 # Indicates no autoupdate is requested.
 _NOAUTOUPDATE_MARKER = "// NOAUTOUPDATE"
 
+# A regexp matching lines that contain line number references.
+_LINE_NUMBER_RE = r"(COMPILATION ERROR: [^:]*:)([1-9][0-9]*)(:.*)"
+
 
 def _get_tests() -> Set[str]:
     """Get the list of tests from the filesystem."""
@@ -39,6 +44,114 @@ def _get_tests() -> Set[str]:
     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 CheckLine(Line):
+    """A `// CHECK:` line generated from the test output."""
+
+    def __init__(self) -> None:
+        self.indent = ""
+
+    @staticmethod
+    def escape(s: str) -> str:
+        """Escape any FileCheck special characters in `s`."""
+        return s.replace("{{", "{{[{][{]}}").replace("[[", "{{[[][[]}}")
+
+    def print_before_line(self, line: int) -> bool:
+        """Determine if we'd prefer to print this CHECK before line `line`."""
+        return True
+
+
+class SimpleCheckLine(CheckLine):
+    """A `// CHECK:` line that checks for an exact string."""
+
+    def __init__(self, expected: str) -> None:
+        super().__init__()
+        self.expected = expected
+
+    def format(self, **kwargs: Any) -> str:
+        if self.expected:
+            return f"{self.indent}// CHECK: {self.expected}\n"
+        else:
+            return f"{self.indent}// CHECK-EMPTY\n"
+
+
+class CheckLineWithLineNumber(CheckLine):
+    """A `// CHECK:` line where the expected output includes a line number.
+
+    Such result lines need to 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, before: str, line_number: int, after: str) -> None:
+        super().__init__()
+        self.before = before
+        self.line_number = line_number
+        self.after = after
+
+    def format(
+        self, *, output_line_number: int, line_number_remap: Dict[int, int]
+    ) -> str:
+        delta = line_number_remap[self.line_number] - output_line_number
+        # We use `:+d` here to produce `LINE-n` or `LINE+n` as appropriate.
+        return (
+            f"{self.indent}// CHECK: {self.before}[[@LINE{delta:+d}]]"
+            + f"{self.after}\n"
+        )
+
+    def print_before_line(self, line: int) -> bool:
+        return line >= self.line_number
+
+
+def _make_check_line(out_line: str) -> CheckLine:
+    """Given a line of output, determine what CHECK line to produce."""
+    out_line = out_line.rstrip()
+    match = re.match(_LINE_NUMBER_RE, out_line)
+    if match:
+        # Convert from 1-based line numbers to 0-based indexes.
+        diagnostic_line_number = int(match[2]) - 1
+        return CheckLineWithLineNumber(
+            match[1], diagnostic_line_number, match[3]
+        )
+    else:
+        return SimpleCheckLine(out_line)
+
+
+def _should_produce_check_line(
+    check_line: CheckLine,
+    orig_line: Optional[OriginalLine],
+    autoupdate_index: int,
+) -> bool:
+    """Determine whether it's time to produce a given CHECK line."""
+    if not orig_line:
+        # If there's no original line, we have no choice.
+        return True
+    if orig_line.line_number <= autoupdate_index:
+        # Don't put any CHECK lines before the AUTOUPDATE line.
+        return False
+    return check_line.print_before_line(orig_line.line_number)
+
+
 def _update_check_once(test: str) -> bool:
     """Updates the CHECK: lines for `test` by running explorer.
 
@@ -48,13 +161,9 @@ def _update_check_once(test: str) -> bool:
         orig_lines = f.readlines()
 
     # Remove old OUT.
-    lines_without_check = [
-        x for x in orig_lines if not x.startswith("// CHECK")
-    ]
-    num_orig_check_lines = len(orig_lines) - len(lines_without_check)
     autoupdate_index = None
     noautoupdate_index = None
-    for line_index, line in enumerate(lines_without_check):
+    for line_index, line in enumerate(orig_lines):
         if line.startswith(_AUTOUPDATE_MARKER):
             autoupdate_index = line_index
             autoupdate_cmd = line[len(_AUTOUPDATE_MARKER) :]
@@ -91,31 +200,68 @@ def _update_check_once(test: str) -> bool:
     # when used.
     # TODO: Maybe revisit and see if lit can be convinced to give a
     # root-relative path.
-    out = out.replace(test, "{{.*}}/%s" % test)
+    out = CheckLine.escape(out).replace(test, "{{.*}}/%s" % test)
     out_lines = out.splitlines()
 
+    orig_line_iter = iter(
+        OriginalLine(i, line) for i, line in enumerate(orig_lines)
+    )
+    check_line_iter = iter(_make_check_line(out_line) for out_line in out_lines)
+    next_orig_line: Optional[OriginalLine] = next(orig_line_iter, None)
+    next_check_line: Optional[CheckLine] = next(check_line_iter, None)
+
+    # Interleave the original lines and the CHECK: lines into a list of
+    # `result_lines`.
+    result_lines: List[Line] = []
+    # Mapping from `orig_lines` indexes to `result_lines` indexes.
+    line_number_remap: Dict[int, int] = {}
+    while next_orig_line or next_check_line:
+        if next_check_line and _should_produce_check_line(
+            next_check_line, next_orig_line, autoupdate_index
+        ):
+            # Indent the CHECK: line to match the next original line.
+            if next_orig_line:
+                match = re.match(" *", next_orig_line.text)
+                if match:
+                    next_check_line.indent = match[0]
+            result_lines.append(next_check_line)
+            next_check_line = next(check_line_iter, None)
+        else:
+            assert next_orig_line, "no lines left"
+            # Include this original line if it isn't a CHECK: line.
+            if not re.match(" *// CHECK", next_orig_line.text):
+                line_number_remap[next_orig_line.line_number] = len(
+                    result_lines
+                )
+                result_lines.append(next_orig_line)
+            next_orig_line = next(orig_line_iter, None)
+
+    # 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 open(test, "w") as f:
-        f.writelines(lines_without_check[: autoupdate_index + 1])
-        for line in out_lines:
-            line = line.rstrip()
-            if line:
-                f.write("// CHECK: %s\n" % line)
-            else:
-                f.write("// CHECK-EMPTY:\n")
-        f.writelines(lines_without_check[autoupdate_index + 1 :])
-
-    # Compares the number of CHECK: lines originally with the number added.
-    return num_orig_check_lines != len(out_lines)
+        f.writelines(formatted_result_lines)
+    return True
 
 
 def _update_check(test: str) -> None:
     """Wraps CHECK: updates for test files."""
-    if _update_check_once(test):
-        # If the number of output lines changes, run again because output can be
-        # line-specific. However, output should stabilize quickly.
-        if _update_check_once(test):
-            raise ValueError("The output of %s kept changing" % test)
+    # If the number of output lines changes, run again because output can be
+    # line-specific. However, output should stabilize quickly.
+    if (
+        _update_check_once(test)
+        and _update_check_once(test)
+        and _update_check_once(test)
+    ):
+        raise ValueError("The output of %s kept changing" % test)
     print(".", end="", flush=True)