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/check/testdata/interop/cpp/function/import/struct.carbon

@@ -25,57 +25,29 @@ auto foo2(ForwardDeclaredStruct) -> void;
 
 library "[[@TEST_NAME]]";
 
-// CHECK:STDERR: fail_import_decl_value_param_type.carbon:[[@LINE+8]]:10: in file included here [InCppInclude]
-// CHECK:STDERR: ./decl_value_param_type.h:4:6: error: argument type 'ForwardDeclaredStruct' is incomplete [CppInteropParseError]
-// CHECK:STDERR:     4 | auto foo1(ForwardDeclaredStruct) -> void;
-// CHECK:STDERR:       |      ^~~~
-// CHECK:STDERR: fail_import_decl_value_param_type.carbon:[[@LINE+4]]:10: in file included here [InCppInclude]
+// CHECK:STDERR: included from: fail_import_decl_value_param_type.carbon:[[@LINE+3]]:10 [InCppInclude]
+// CHECK:STDERR: ./decl_value_param_type.h:4:6-9: error: argument type 'ForwardDeclaredStruct' is incomplete [CppInteropParseError]
 // CHECK:STDERR: ./decl_value_param_type.h:2:8: note: forward declaration of 'ForwardDeclaredStruct' [CppInteropParseNote]
-// CHECK:STDERR:     2 | struct ForwardDeclaredStruct;
-// CHECK:STDERR:       |        ^
 import Cpp library "decl_value_param_type.h";
 
 fn F(p: Cpp.ForwardDeclaredStruct*) {
-  // CHECK:STDERR: fail_import_decl_value_param_type.carbon:[[@LINE+12]]:3: note: in thunk for C++ function used here [InCppThunk]
-  // CHECK:STDERR:   Cpp.foo1(*p);
-  // CHECK:STDERR:   ^~~~~~~~~~~~
+  // CHECK:STDERR: fail_import_decl_value_param_type.carbon:[[@LINE+5]]:3-14: note: in thunk for C++ function used here [InCppThunk]
   // CHECK:STDERR:
-  // CHECK:STDERR: fail_import_decl_value_param_type.carbon:[[@LINE-7]]:10: in file included here [InCppInclude]
-  // CHECK:STDERR: ./decl_value_param_type.h:5:6: error: argument type 'ForwardDeclaredStruct' is incomplete [CppInteropParseError]
-  // CHECK:STDERR:     5 | auto foo2(ForwardDeclaredStruct) -> void;
-  // CHECK:STDERR:       |      ^~~~
-  // CHECK:STDERR: fail_import_decl_value_param_type.carbon:[[@LINE-11]]:10: in file included here [InCppInclude]
+  // CHECK:STDERR: included from: fail_import_decl_value_param_type.carbon:[[@LINE-5]]:10 [InCppInclude]
+  // CHECK:STDERR: ./decl_value_param_type.h:5:6-9: error: argument type 'ForwardDeclaredStruct' is incomplete [CppInteropParseError]
   // CHECK:STDERR: ./decl_value_param_type.h:2:8: note: forward declaration of 'ForwardDeclaredStruct' [CppInteropParseNote]
-  // CHECK:STDERR:     2 | struct ForwardDeclaredStruct;
-  // CHECK:STDERR:       |        ^
   Cpp.foo1(*p);
-  // CHECK:STDERR: fail_import_decl_value_param_type.carbon:[[@LINE+28]]:3: note: in thunk for C++ function used here [InCppThunk]
-  // CHECK:STDERR:   Cpp.foo2(*p);
-  // CHECK:STDERR:   ^~~~~~~~~~~~
+  // CHECK:STDERR: fail_import_decl_value_param_type.carbon:[[@LINE+12]]:3-14: note: in thunk for C++ function used here [InCppThunk]
   // CHECK:STDERR:
-  // CHECK:STDERR: fail_import_decl_value_param_type.carbon:[[@LINE-5]]:12: error: forming value of incomplete type `Cpp.ForwardDeclaredStruct` [IncompleteTypeInValueConversion]
-  // CHECK:STDERR:   Cpp.foo1(*p);
-  // CHECK:STDERR:            ^~
-  // CHECK:STDERR: fail_import_decl_value_param_type.carbon:[[@LINE-23]]:10: in file included here [InCppInclude]
+  // CHECK:STDERR: fail_import_decl_value_param_type.carbon:[[@LINE-3]]:12-13: error: forming value of incomplete type `Cpp.ForwardDeclaredStruct` [IncompleteTypeInValueConversion]
+  // CHECK:STDERR: included from: fail_import_decl_value_param_type.carbon:[[@LINE-12]]:10 [InCppInclude]
   // CHECK:STDERR: ./decl_value_param_type.h:2:8: note: class was forward declared here [ClassForwardDeclaredHere]
-  // CHECK:STDERR: struct ForwardDeclaredStruct;
-  // CHECK:STDERR:        ^
-  // CHECK:STDERR: fail_import_decl_value_param_type.carbon:[[@LINE-27]]:10: in file included here [InCppInclude]
   // CHECK:STDERR: ./decl_value_param_type.h:4:32: note: initializing function parameter [InCallToFunctionParam]
-  // CHECK:STDERR: auto foo1(ForwardDeclaredStruct) -> void;
-  // CHECK:STDERR:                                ^
   // CHECK:STDERR:
-  // CHECK:STDERR: fail_import_decl_value_param_type.carbon:[[@LINE+12]]:12: error: forming value of incomplete type `Cpp.ForwardDeclaredStruct` [IncompleteTypeInValueConversion]
-  // CHECK:STDERR:   Cpp.foo2(*p);
-  // CHECK:STDERR:            ^~
-  // CHECK:STDERR: fail_import_decl_value_param_type.carbon:[[@LINE-35]]:10: in file included here [InCppInclude]
+  // CHECK:STDERR: fail_import_decl_value_param_type.carbon:[[@LINE+5]]:12-13: error: forming value of incomplete type `Cpp.ForwardDeclaredStruct` [IncompleteTypeInValueConversion]
+  // CHECK:STDERR: included from: fail_import_decl_value_param_type.carbon:[[@LINE-17]]:10 [InCppInclude]
   // CHECK:STDERR: ./decl_value_param_type.h:2:8: note: class was forward declared here [ClassForwardDeclaredHere]
-  // CHECK:STDERR: struct ForwardDeclaredStruct;
-  // CHECK:STDERR:        ^
-  // CHECK:STDERR: fail_import_decl_value_param_type.carbon:[[@LINE-39]]:10: in file included here [InCppInclude]
   // CHECK:STDERR: ./decl_value_param_type.h:5:32: note: initializing function parameter [InCallToFunctionParam]
-  // CHECK:STDERR: auto foo2(ForwardDeclaredStruct) -> void;
-  // CHECK:STDERR:                                ^
   // CHECK:STDERR:
   Cpp.foo2(*p);
 }
@@ -118,25 +90,15 @@ auto foo_non_copyable(NonCopyableStruct) -> void;
 
 library "[[@TEST_NAME]]";
 
-// CHECK:STDERR: fail_import_non_copyable_param_type.carbon:[[@LINE+12]]:10: in file included here [InCppInclude]
-// CHECK:STDERR: ./non_copyable_param_type.h:6:6: error: call to deleted constructor of 'NonCopyableStruct' [CppInteropParseError]
-// CHECK:STDERR:     6 | auto foo_non_copyable(NonCopyableStruct) -> void;
-// CHECK:STDERR:       |      ^~~~~~~~~~~~~~~~
-// CHECK:STDERR: fail_import_non_copyable_param_type.carbon:[[@LINE+8]]:10: in file included here [InCppInclude]
+// CHECK:STDERR: included from: fail_import_non_copyable_param_type.carbon:[[@LINE+4]]:10 [InCppInclude]
+// CHECK:STDERR: ./non_copyable_param_type.h:6:6-21: error: call to deleted constructor of 'NonCopyableStruct' [CppInteropParseError]
 // CHECK:STDERR: ./non_copyable_param_type.h:3:3: note: 'NonCopyableStruct' has been explicitly marked deleted here [CppInteropParseNote]
-// CHECK:STDERR:     3 |   NonCopyableStruct(const NonCopyableStruct&) = delete;
-// CHECK:STDERR:       |   ^
-// CHECK:STDERR: fail_import_non_copyable_param_type.carbon:[[@LINE+4]]:10: in file included here [InCppInclude]
 // CHECK:STDERR: ./non_copyable_param_type.h:6:40: note: passing argument to parameter here [CppInteropParseNote]
-// CHECK:STDERR:     6 | auto foo_non_copyable(NonCopyableStruct) -> void;
-// CHECK:STDERR:       |                                        ^
 import Cpp library "non_copyable_param_type.h";
 
 fn F(s: Cpp.NonCopyableStruct) {
   //@dump-sem-ir-begin
-  // CHECK:STDERR: fail_import_non_copyable_param_type.carbon:[[@LINE+4]]:3: note: in thunk for C++ function used here [InCppThunk]
-  // CHECK:STDERR:   Cpp.foo_non_copyable(s);
-  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR: fail_import_non_copyable_param_type.carbon:[[@LINE+2]]:3-25: note: in thunk for C++ function used here [InCppThunk]
   // CHECK:STDERR:
   Cpp.foo_non_copyable(s);
   //@dump-sem-ir-end
@@ -321,13 +283,9 @@ library "[[@TEST_NAME]]";
 import Cpp library "incomplete.h";
 
 fn F() {
-  // CHECK:STDERR: fail_import_incomplete.carbon:[[@LINE+8]]:3: error: member access into incomplete class `Cpp.IncompleteStruct` [QualifiedExprInIncompleteClassScope]
-  // CHECK:STDERR:   Cpp.IncompleteStruct.foo();
-  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~~~~~~~
-  // CHECK:STDERR: fail_import_incomplete.carbon:[[@LINE-6]]:10: in file included here [InCppInclude]
+  // CHECK:STDERR: fail_import_incomplete.carbon:[[@LINE+4]]:3-26: error: member access into incomplete class `Cpp.IncompleteStruct` [QualifiedExprInIncompleteClassScope]
+  // CHECK:STDERR: included from: fail_import_incomplete.carbon:[[@LINE-4]]:10 [InCppInclude]
   // CHECK:STDERR: ./incomplete.h:2:8: note: class was forward declared here [ClassForwardDeclaredHere]
-  // CHECK:STDERR: struct IncompleteStruct;
-  // CHECK:STDERR:        ^
   // CHECK:STDERR:
   Cpp.IncompleteStruct.foo();
 }
@@ -391,36 +349,19 @@ auto foo_decl_value_return() -> DeclValueReturnStruct;
 library "[[@TEST_NAME]]";
 
 // TODO: We should only produce one error here.
-// CHECK:STDERR: fail_import_decl_value_return_type.carbon:[[@LINE+12]]:10: in file included here [InCppInclude]
-// CHECK:STDERR: ./decl_value_return_type.h:4:6: error: calling 'foo_decl_value_return' with incomplete return type 'DeclValueReturnStruct' [CppInteropParseError]
-// CHECK:STDERR:     4 | auto foo_decl_value_return() -> DeclValueReturnStruct;
-// CHECK:STDERR:       |      ^~~~~~~~~~~~~~~~~~~~~
-// CHECK:STDERR: fail_import_decl_value_return_type.carbon:[[@LINE+8]]:10: in file included here [InCppInclude]
+// CHECK:STDERR: included from: fail_import_decl_value_return_type.carbon:[[@LINE+4]]:10 [InCppInclude]
+// CHECK:STDERR: ./decl_value_return_type.h:4:6-26: error: calling 'foo_decl_value_return' with incomplete return type 'DeclValueReturnStruct' [CppInteropParseError]
 // CHECK:STDERR: ./decl_value_return_type.h:4:6: note: 'foo_decl_value_return' declared here [CppInteropParseNote]
-// CHECK:STDERR:     4 | auto foo_decl_value_return() -> DeclValueReturnStruct;
-// CHECK:STDERR:       |      ^
-// CHECK:STDERR: fail_import_decl_value_return_type.carbon:[[@LINE+4]]:10: in file included here [InCppInclude]
 // CHECK:STDERR: ./decl_value_return_type.h:2:8: note: forward declaration of 'DeclValueReturnStruct' [CppInteropParseNote]
-// CHECK:STDERR:     2 | struct DeclValueReturnStruct;
-// CHECK:STDERR:       |        ^
 import Cpp library "decl_value_return_type.h";
 
 fn F() {
-  // CHECK:STDERR: fail_import_decl_value_return_type.carbon:[[@LINE+16]]:3: note: in thunk for C++ function used here [InCppThunk]
-  // CHECK:STDERR:   Cpp.foo_decl_value_return();
-  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR: fail_import_decl_value_return_type.carbon:[[@LINE+7]]:3-29: note: in thunk for C++ function used here [InCppThunk]
   // CHECK:STDERR:
-  // CHECK:STDERR: fail_import_decl_value_return_type.carbon:[[@LINE+12]]:3: error: function returns incomplete type `Cpp.DeclValueReturnStruct` [IncompleteTypeInFunctionReturnType]
-  // CHECK:STDERR:   Cpp.foo_decl_value_return();
-  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~~~~~~~~~~
-  // CHECK:STDERR: fail_import_decl_value_return_type.carbon:[[@LINE-10]]:10: in file included here [InCppInclude]
+  // CHECK:STDERR: fail_import_decl_value_return_type.carbon:[[@LINE+5]]:3-29: error: function returns incomplete type `Cpp.DeclValueReturnStruct` [IncompleteTypeInFunctionReturnType]
+  // CHECK:STDERR: included from: fail_import_decl_value_return_type.carbon:[[@LINE-6]]:10 [InCppInclude]
   // CHECK:STDERR: ./decl_value_return_type.h:2:8: note: class was forward declared here [ClassForwardDeclaredHere]
-  // CHECK:STDERR: struct DeclValueReturnStruct;
-  // CHECK:STDERR:        ^
-  // CHECK:STDERR: fail_import_decl_value_return_type.carbon:[[@LINE-14]]:10: in file included here [InCppInclude]
   // CHECK:STDERR: ./decl_value_return_type.h:4:33: note: return type declared here [IncompleteReturnTypeHere]
-  // CHECK:STDERR: auto foo_decl_value_return() -> DeclValueReturnStruct;
-  // CHECK:STDERR:                                 ^
   // CHECK:STDERR:
   Cpp.foo_decl_value_return();
 }
@@ -559,7 +500,7 @@ fn F() {
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @F(%s.param: %NonCopyableStruct) {
 // CHECK:STDOUT: !entry:
-// CHECK:STDOUT:   %Cpp.ref.loc24: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:   %Cpp.ref.loc14: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
 // CHECK:STDOUT:   %foo_non_copyable.ref: %foo_non_copyable.cpp_overload_set.type = name_ref foo_non_copyable, imports.%foo_non_copyable.cpp_overload_set.value [concrete = constants.%foo_non_copyable.cpp_overload_set.value]
 // CHECK:STDOUT:   %s.ref: %NonCopyableStruct = name_ref s, %s
 // CHECK:STDOUT:   %foo_non_copyable.call: init %empty_tuple.type = call imports.%foo_non_copyable.decl(%s.ref)