Switch custom error stream output to diagnostic (#4846)

This switches most error printing to use diagnostics instead of direct
stream writes, even when not a specific file diagnostic. I'm allowing
empty filenames for this use-case.

This allows a little more specific testing to validate coverage of
output using the diagnostic coverage test. I'm adding a few tests to
cover things that weren't previously tested.

Separately, this also forces a little more standardization in format...
considering how changes like #4568 show effort being spent to _mirror_
diagnostic style, my thought is now to just use diagnostic code where
possible.

Note this also allows incrementally better testing of the language
server; I'm changing the crash fix from #4847 in favor of diagnostic
testing.

---------

Co-authored-by: Chandler Carruth <[email protected]>

Author: jonmeow

toolchain/check/check_unit.cpp

@@ -366,7 +366,7 @@ auto CheckUnit::ProcessNodeIds() -> bool {
     auto converted = unit_and_imports_->unit->node_converter->ConvertLoc(
         node_id, [](DiagnosticLoc, const DiagnosticBase<>&) {});
     converted.loc.FormatLocation(output);
-    output << ": checking " << context_.parse_tree().node_kind(node_id) << "\n";
+    output << "checking " << context_.parse_tree().node_kind(node_id) << "\n";
     // Crash output has a tab indent; try to indent slightly past that.
     converted.loc.FormatSnippet(output, /*indent=*/10);
   });

toolchain/check/context.cpp

@@ -223,7 +223,7 @@ auto Context::DiagnoseDuplicateName(SemIRLoc dup_def, SemIRLoc prev_def)
       .Emit();
 }
 
-auto Context::DiagnosePoisonedName(SemIRLoc loc) -> void {
+auto Context::DiagnosePoisonedName(SemIR::InstId loc) -> void {
   // TODO: Improve the diagnostic to replace NodeId::None with the location
   // where the name was poisoned. See discussion in
   // https://github.com/carbon-language/carbon-lang/pull/4654#discussion_r1876607172

toolchain/check/context.h

@@ -266,7 +266,7 @@ class Context {
   auto DiagnoseDuplicateName(SemIRLoc dup_def, SemIRLoc prev_def) -> void;
 
   // Prints a diagnostic for a poisoned name.
-  auto DiagnosePoisonedName(SemIRLoc loc) -> void;
+  auto DiagnosePoisonedName(SemIR::InstId loc) -> void;
 
   // Prints a diagnostic for a missing name.
   auto DiagnoseNameNotFound(SemIRLoc loc, SemIR::NameId name_id) -> void;

toolchain/check/testdata/basics/builtin_insts.carbon

@@ -2,7 +2,7 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// ARGS: compile --no-prelude-import --phase=check --dump-raw-sem-ir --builtin-sem-ir %s
+// ARGS: --include-diagnostic-kind compile --no-prelude-import --phase=check --dump-raw-sem-ir --builtin-sem-ir %s
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:

toolchain/check/testdata/interop/cpp/no_prelude/fail_fuzzing.carbon

@@ -0,0 +1,17 @@
+// 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
+//
+// ARGS: --include-diagnostic-kind --fuzzing compile --no-prelude-import --phase=check %s
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/no_prelude/fail_fuzzing.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/no_prelude/fail_fuzzing.carbon
+
+// CHECK:STDERR: fail_fuzzing.carbon:[[@LINE+4]]:1: error: `Cpp` import found during fuzzing [CppInteropFuzzing]
+// CHECK:STDERR: import Cpp library "file.h";
+// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR:
+import Cpp library "file.h";

toolchain/codegen/testdata/assembly/basic.carbon

@@ -2,7 +2,7 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// ARGS: compile --no-prelude-import --target=x86_64-unknown-linux-gnu --output=- %s
+// ARGS: --include-diagnostic-kind compile --no-prelude-import --target=x86_64-unknown-linux-gnu --output=- %s
 //
 // To test this file alone, run:
 //   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/codegen/testdata/assembly/basic.carbon

toolchain/codegen/testdata/fail_target_triple.carbon

@@ -2,7 +2,7 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// ARGS: compile --no-prelude-import --target=x86_687-unknown-linux-gnu --output=- %s
+// ARGS: --include-diagnostic-kind compile --no-prelude-import --target=x86_687-unknown-linux-gnu --output=- %s
 // No autoupdate because the message comes from LLVM.
 // To test this file alone, run:
 //   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/codegen/testdata/fail_target_triple.carbon

toolchain/codegen/testdata/objcode/basic.carbon

@@ -2,7 +2,7 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// ARGS: compile --no-prelude-import --target=x86_64-unknown-linux-gnu --output=%t %s
+// ARGS: --include-diagnostic-kind compile --no-prelude-import --target=x86_64-unknown-linux-gnu --output=%t %s
 //
 // TODO: Add a way to write some basic tests for object file outputs.
 // AUTOUPDATE

toolchain/diagnostics/BUILD

@@ -81,6 +81,15 @@ cc_test(
     ],
 )
 
+cc_library(
+    name = "file_diagnostics",
+    hdrs = ["file_diagnostics.h"],
+    deps = [
+        ":diagnostic_emitter",
+        "@llvm-project//llvm:Support",
+    ],
+)
+
 cc_library(
     name = "format_providers",
     srcs = ["format_providers.cpp"],

toolchain/diagnostics/coverage_test.cpp

@@ -24,8 +24,10 @@ constexpr DiagnosticKind UntestedDiagnosticKinds[] = {
     DiagnosticKind::TestDiagnostic,
     DiagnosticKind::TestDiagnosticNote,
 
-    // Driver specific.
-    DiagnosticKind::CppInteropFuzzing,
+    // Diagnosing erroneous install conditions, but test environments are
+    // typically correct.
+    DiagnosticKind::CompilePreludeManifestError,
+    DiagnosticKind::DriverInstallInvalid,
 
     // These diagnose filesystem issues that are hard to unit test.
     DiagnosticKind::ErrorReadingFile,