Render diagnostics through the terminal library (#7683)

Diagnostic rendering was part of the data model: `Loc::FormatLocation`
and
`Loc::FormatSnippet` wrote plain text to a stream, so no capability,
color, or
terminal width could reach them.

A diagnostic is now one message with labels, contexts, and location
information
attached, and `Diagnostics::Renderer` draws it into a
`Terminal::Buffer`. The
design is in `toolchain/docs/diagnostics_rendering.md` (#7668),
authoring in
`toolchain/docs/diagnostics.md`.

Before:

```
example.carbon:6:3: error: 1 argument passed to function expecting 0 arguments
  Run0(1);
  ^~~~~~~
example.carbon:3:1: note: calling function declared here
fn Run0() {}
^~~~~~~~~~~
```

After:

```
error: 1 argument passed to function expecting 0 arguments
       ╭─┤ example.carbon:6:3
       ┆
     3 │ fn Run0() {}
       · ─────┬─────
       ·      ╰──┤ calling function declared here
       ┆
->   6 │   Run0(1);
       ·   ───────
       ╰────────────
```

`--no-diagnostic-snippets` gives a compact form, `example.carbon:6:3-9`.
File
tests use it; the rendering tests under `toolchain/diagnostics/testdata`
ask for
snippets.

C++ interop diagnostics are data now rather than text Clang drew:
`SemIR::ClangSourceLocId` names a `clang::SourceRange`, and a Clang
location is
marked as the point it names.

`StreamConsumer` moves to a `:renderer` target that `:emitter` does not
depend
on, keeping `common/terminal` out of `consumer.h`. Capabilities come
from the
file behind the error stream, so tests get plain ASCII.

`renderer_fuzzer` draws diagnostics built from fuzzer bytes with every
capability combination, since a shape the renderer doesn't expect must
never be
why the compiler dies while reporting a problem.

TODOs mark the follow-ups: line weights and crossing lines, wrapping
that keeps
a quoted type whole, and ordering diagnostics that share a byte offset.

Also: `IncorrectExtension`'s hint moves into its message, since its note
had no
location of its own, and `strip_noise` in `check_diagnostics.py` blanks
multi-line `#define`s instead of deleting them.

Assisted-by: Claude Code

toolchain/parse/testdata/function/declaration.carbon

@@ -45,60 +45,44 @@ private override default fn I();
 
 // --- fail_identifier_instead_of_sig.carbon
 
-// CHECK:STDERR: fail_identifier_instead_of_sig.carbon:[[@LINE+4]]:8: error: `fn` declarations must either end with a `;` or have a `{ ... }` block for a definition [ExpectedDeclSemiOrDefinition]
-// CHECK:STDERR: fn foo bar;
-// CHECK:STDERR:        ^~~
+// CHECK:STDERR: fail_identifier_instead_of_sig.carbon:[[@LINE+2]]:8-10: error: `fn` declarations must either end with a `;` or have a `{ ... }` block for a definition [ExpectedDeclSemiOrDefinition]
 // CHECK:STDERR:
 fn foo bar;
 
 // --- fail_missing_implicit_close.carbon
 
-// CHECK:STDERR: fail_missing_implicit_close.carbon:[[@LINE+7]]:7: error: opening symbol without a corresponding closing symbol [UnmatchedOpening]
-// CHECK:STDERR: fn Div[();
-// CHECK:STDERR:       ^
-// CHECK:STDERR: fail_missing_implicit_close.carbon:[[@LINE+4]]:10: note: possibly missing `]` here [PossiblyMissingBracketHere]
-// CHECK:STDERR: fn Div[();
-// CHECK:STDERR:          ^
+// CHECK:STDERR: fail_missing_implicit_close.carbon:[[@LINE+3]]:7: error: opening symbol without a corresponding closing symbol [UnmatchedOpening]
+// CHECK:STDERR: fail_missing_implicit_close.carbon:[[@LINE+2]]:10: note: possibly missing `]` here [PossiblyMissingBracketHere]
 // CHECK:STDERR:
 fn Div[();
 
 // --- fail_missing_name.carbon
 
-// CHECK:STDERR: fail_missing_name.carbon:[[@LINE+4]]:4: error: `fn` introducer should be followed by a name [ExpectedDeclName]
-// CHECK:STDERR: fn ();
-// CHECK:STDERR:    ^
+// CHECK:STDERR: fail_missing_name.carbon:[[@LINE+2]]:4: error: `fn` introducer should be followed by a name [ExpectedDeclName]
 // CHECK:STDERR:
 fn ();
 
 // --- fail_no_sig_or_semi.carbon
 
 fn foo
-// CHECK:STDERR: fail_no_sig_or_semi.carbon:[[@LINE+4]]:1: error: `fn` declarations must either end with a `;` or have a `{ ... }` block for a definition [ExpectedDeclSemiOrDefinition]
-// CHECK:STDERR:
-// CHECK:STDERR: ^
+// CHECK:STDERR: fail_no_sig_or_semi.carbon:[[@LINE+2]]:1: error: `fn` declarations must either end with a `;` or have a `{ ... }` block for a definition [ExpectedDeclSemiOrDefinition]
 // CHECK:STDERR:
 
 // --- fail_only_fn_and_semi.carbon
 
-// CHECK:STDERR: fail_only_fn_and_semi.carbon:[[@LINE+4]]:3: error: `fn` introducer should be followed by a name [ExpectedDeclName]
-// CHECK:STDERR: fn;
-// CHECK:STDERR:   ^
+// CHECK:STDERR: fail_only_fn_and_semi.carbon:[[@LINE+2]]:3: error: `fn` introducer should be followed by a name [ExpectedDeclName]
 // CHECK:STDERR:
 fn;
 
 // --- fail_repeated_fn_and_semi.carbon
 
-// CHECK:STDERR: fail_repeated_fn_and_semi.carbon:[[@LINE+4]]:4: error: `fn` introducer should be followed by a name [ExpectedDeclName]
-// CHECK:STDERR: fn fn;
-// CHECK:STDERR:    ^~
+// CHECK:STDERR: fail_repeated_fn_and_semi.carbon:[[@LINE+2]]:4-5: error: `fn` introducer should be followed by a name [ExpectedDeclName]
 // CHECK:STDERR:
 fn fn;
 
 // --- fail_skip_indented_newline_until_outdent.carbon
 
-  // CHECK:STDERR: fail_skip_indented_newline_until_outdent.carbon:[[@LINE+4]]:6: error: `fn` introducer should be followed by a name [ExpectedDeclName]
-  // CHECK:STDERR:   fn (x,
-  // CHECK:STDERR:      ^
+  // CHECK:STDERR: fail_skip_indented_newline_until_outdent.carbon:[[@LINE+2]]:6: error: `fn` introducer should be followed by a name [ExpectedDeclName]
   // CHECK:STDERR:
   fn (x,
       y,
@@ -107,9 +91,7 @@ fn F();
 
 // --- fail_skip_indented_newline_with_semi.carbon
 
-// CHECK:STDERR: fail_skip_indented_newline_with_semi.carbon:[[@LINE+4]]:4: error: `fn` introducer should be followed by a name [ExpectedDeclName]
-// CHECK:STDERR: fn (x,
-// CHECK:STDERR:    ^
+// CHECK:STDERR: fail_skip_indented_newline_with_semi.carbon:[[@LINE+2]]:4: error: `fn` introducer should be followed by a name [ExpectedDeclName]
 // CHECK:STDERR:
 fn (x,
     y,
@@ -118,9 +100,7 @@ fn F();
 
 // --- fail_skip_indented_newline_without_semi.carbon
 
-// CHECK:STDERR: fail_skip_indented_newline_without_semi.carbon:[[@LINE+4]]:4: error: `fn` introducer should be followed by a name [ExpectedDeclName]
-// CHECK:STDERR: fn (x,
-// CHECK:STDERR:    ^
+// CHECK:STDERR: fail_skip_indented_newline_without_semi.carbon:[[@LINE+2]]:4: error: `fn` introducer should be followed by a name [ExpectedDeclName]
 // CHECK:STDERR:
 fn (x,
     y,
@@ -129,18 +109,14 @@ fn F();
 
 // --- fail_skip_to_newline_without_semi.carbon
 
-// CHECK:STDERR: fail_skip_to_newline_without_semi.carbon:[[@LINE+4]]:4: error: `fn` introducer should be followed by a name [ExpectedDeclName]
-// CHECK:STDERR: fn ()
-// CHECK:STDERR:    ^
+// CHECK:STDERR: fail_skip_to_newline_without_semi.carbon:[[@LINE+2]]:4: error: `fn` introducer should be followed by a name [ExpectedDeclName]
 // CHECK:STDERR:
 fn ()
 fn F();
 
 // --- fail_skip_without_semi_to_curly.carbon
 
-// CHECK:STDERR: fail_skip_without_semi_to_curly.carbon:[[@LINE+4]]:1: error: unrecognized declaration introducer [UnrecognizedDecl]
-// CHECK:STDERR: struct X { fn () }
-// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR: fail_skip_without_semi_to_curly.carbon:[[@LINE+2]]:1-6: error: unrecognized declaration introducer [UnrecognizedDecl]
 // CHECK:STDERR:
 struct X { fn () }
 fn F();
@@ -151,9 +127,7 @@ fn foo(bar);
 
 // --- fail_without_name_and_many_tokens_in_params.carbon
 
-// CHECK:STDERR: fail_without_name_and_many_tokens_in_params.carbon:[[@LINE+4]]:4: error: `fn` introducer should be followed by a name [ExpectedDeclName]
-// CHECK:STDERR: fn (a tokens c d e f g h i j k l m n o p q r s t u v w x y z);
-// CHECK:STDERR:    ^
+// CHECK:STDERR: fail_without_name_and_many_tokens_in_params.carbon:[[@LINE+2]]:4: error: `fn` introducer should be followed by a name [ExpectedDeclName]
 // CHECK:STDERR:
 fn (a tokens c d e f g h i j k l m n o p q r s t u v w x y z);
 
@@ -164,17 +138,13 @@ fn ComplexFormParam(x:? X.Y(Z));
 
 // --- fail_ref_on_form_parameter.carbon
 
-// CHECK:STDERR: fail_ref_on_form_parameter.carbon:[[@LINE+4]]:6: error: `ref` is only allowed on a runtime binding [ExpectedRuntimeBindingPatternAfterRef]
-// CHECK:STDERR: fn F(ref x:? Form);
-// CHECK:STDERR:      ^~~
+// CHECK:STDERR: fail_ref_on_form_parameter.carbon:[[@LINE+2]]:6-8: error: `ref` is only allowed on a runtime binding [ExpectedRuntimeBindingPatternAfterRef]
 // CHECK:STDERR:
 fn F(ref x:? Form);
 
 // --- fail_form_within_var.carbon
 
-// CHECK:STDERR: fail_form_within_var.carbon:[[@LINE+4]]:26: error: found `:?` binding inside `var` pattern [NonRegularBindingInVarDecl]
-// CHECK:STDERR: fn FormInVar(var x:? Form);
-// CHECK:STDERR:                          ^
+// CHECK:STDERR: fail_form_within_var.carbon:[[@LINE+2]]:26: error: found `:?` binding inside `var` pattern [NonRegularBindingInVarDecl]
 // CHECK:STDERR:
 fn FormInVar(var x:? Form);
 
@@ -191,9 +161,7 @@ let a: auto = fn -> ref u32 => 0;
 
 // --- fail_ref_return_missing_type.carbon
 
-// CHECK:STDERR: fail_ref_return_missing_type.carbon:[[@LINE+4]]:22: error: expected expression [ExpectedExpr]
-// CHECK:STDERR: fn RefReturn() -> ref;
-// CHECK:STDERR:                      ^
+// CHECK:STDERR: fail_ref_return_missing_type.carbon:[[@LINE+2]]:22: error: expected expression [ExpectedExpr]
 // CHECK:STDERR:
 fn RefReturn() -> ref;
 
@@ -207,9 +175,7 @@ let b: auto = fn -> var u32 => 0;
 
 // --- fail_val_return_missing_type.carbon
 
-// CHECK:STDERR: fail_val_return_missing_type.carbon:[[@LINE+4]]:22: error: expected expression [ExpectedExpr]
-// CHECK:STDERR: fn ValReturn() -> val;
-// CHECK:STDERR:                      ^
+// CHECK:STDERR: fail_val_return_missing_type.carbon:[[@LINE+2]]:22: error: expected expression [ExpectedExpr]
 // CHECK:STDERR:
 fn ValReturn() -> val;
 

toolchain/parse/testdata/function/definition.carbon

@@ -36,9 +36,7 @@ fn F() {
   // Note: this might become valid depending on the expression syntax. This test
   // shouldn't be taken as a sign it should remain invalid.
   bar
-// CHECK:STDERR: fail_identifier_in_statements.carbon:[[@LINE+4]]:1: error: expected `;` after expression statement [ExpectedExprSemi]
-// CHECK:STDERR: }
-// CHECK:STDERR: ^
+// CHECK:STDERR: fail_identifier_in_statements.carbon:[[@LINE+2]]:1: error: expected `;` after expression statement [ExpectedExprSemi]
 // CHECK:STDERR:
 }
 
@@ -61,25 +59,19 @@ impl T as I {
 
 // --- fail_builtin_identifier.carbon
 
-// CHECK:STDERR: fail_builtin_identifier.carbon:[[@LINE+4]]:18: error: expected builtin function name after `=` [ExpectedBuiltinName]
-// CHECK:STDERR: fn NotString() = banana;
-// CHECK:STDERR:                  ^~~~~~
+// CHECK:STDERR: fail_builtin_identifier.carbon:[[@LINE+2]]:18-23: error: expected builtin function name after `=` [ExpectedBuiltinName]
 // CHECK:STDERR:
 fn NotString() = banana;
 
 // --- fail_builtin_two_strings.carbon
 
-// CHECK:STDERR: fail_builtin_two_strings.carbon:[[@LINE+4]]:32: error: `fn` declarations must end with a `;` [ExpectedDeclSemi]
-// CHECK:STDERR: fn JunkAfterString() = "hello" "world";
-// CHECK:STDERR:                                ^~~~~~~
+// CHECK:STDERR: fail_builtin_two_strings.carbon:[[@LINE+2]]:32-38: error: `fn` declarations must end with a `;` [ExpectedDeclSemi]
 // CHECK:STDERR:
 fn JunkAfterString() = "hello" "world";
 
 // --- fail_builtin_spurious_equals.carbon
 
-// CHECK:STDERR: fail_builtin_spurious_equals.carbon:[[@LINE+4]]:23: error: expected builtin function name after `=` [ExpectedBuiltinName]
-// CHECK:STDERR: fn SpuriousEquals() = { }
-// CHECK:STDERR:                       ^
+// CHECK:STDERR: fail_builtin_spurious_equals.carbon:[[@LINE+2]]:23: error: expected builtin function name after `=` [ExpectedBuiltinName]
 // CHECK:STDERR:
 fn SpuriousEquals() = { }
 fn TestRecoveryFromSpuriousEquals();

toolchain/parse/testdata/function/extern.carbon

@@ -22,21 +22,15 @@ extern library default fn G();
 
 // --- fail_name_missing.carbon
 
-// CHECK:STDERR: fail_name_missing.carbon:[[@LINE+4]]:16: error: expected `default` or a string literal to specify the library name [ExpectedLibraryNameOrDefault]
-// CHECK:STDERR: extern library fn G();
-// CHECK:STDERR:                ^~
+// CHECK:STDERR: fail_name_missing.carbon:[[@LINE+2]]:16-17: error: expected `default` or a string literal to specify the library name [ExpectedLibraryNameOrDefault]
 // CHECK:STDERR:
 extern library fn G();
 
 // --- fail_identifier.carbon
 
-// CHECK:STDERR: fail_identifier.carbon:[[@LINE+8]]:16: error: expected `default` or a string literal to specify the library name [ExpectedLibraryNameOrDefault]
-// CHECK:STDERR: extern library foo fn G();
-// CHECK:STDERR:                ^~~
+// CHECK:STDERR: fail_identifier.carbon:[[@LINE+4]]:16-18: error: expected `default` or a string literal to specify the library name [ExpectedLibraryNameOrDefault]
 // CHECK:STDERR:
-// CHECK:STDERR: fail_identifier.carbon:[[@LINE+4]]:16: error: unrecognized declaration introducer [UnrecognizedDecl]
-// CHECK:STDERR: extern library foo fn G();
-// CHECK:STDERR:                ^~~
+// CHECK:STDERR: fail_identifier.carbon:[[@LINE+2]]:16-18: error: unrecognized declaration introducer [UnrecognizedDecl]
 // CHECK:STDERR:
 extern library foo fn G();
 

toolchain/parse/testdata/function/fail_template_form_param.carbon

@@ -9,9 +9,7 @@
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/function/fail_template_form_param.carbon
 
 // `template` cannot be combined with a `:?` form binding.
-// CHECK:STDERR: fail_template_form_param.carbon:[[@LINE+4]]:6: error: `template` is only allowed on a generic binding [ExpectedGenericBindingPatternAfterTemplate]
-// CHECK:STDERR: fn F(template x:? Form);
-// CHECK:STDERR:      ^~~~~~~~
+// CHECK:STDERR: fail_template_form_param.carbon:[[@LINE+2]]:6-13: error: `template` is only allowed on a generic binding [ExpectedGenericBindingPatternAfterTemplate]
 // CHECK:STDERR:
 fn F(template x:? Form);