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)