Move CHECK lines in tests next to the line that caused the output. (#1224)

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

This is made awkward by a couple of things:

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

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

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

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

Co-authored-by: Jon Meow <jperkins@google.com>

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;

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;

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() {

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 {

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);
 }

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; }
   }

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; }
   }

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
 }

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;
 }