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carbon-lang/toolchain/check/testdata/interop/cpp/basics/cpp_diagnostics.carbon
T
Chandler Carruth f0848b1f5e Trailing comments (#7441) 
Carbon currently requires a comment to be the only non-whitespace on its
line. A `//` comment that follows other content on a line, called a
_trailing comment_, is a lexer error. This proposal removes that
restriction, allowing a comment to follow other content on a line.
Everything else about comments is unchanged: a comment still begins with
`//`, still requires whitespace after the `//`, and still runs to the
end of the line. Carbon continues to provide only line comments; no
block or intra-line comments are added.

Three observations motivate the change. First, trailing comments are
well suited to short _annotations_ attached to a specific entity or
value on a line. Second, the lexer design now makes it trivial to lex
trailing comments, and in fact requires extra logic and potentially cost
to reject them. Third, C++ code routinely uses trailing comments, so
allowing them lets Carbon carry the layout of migrated code over
directly, rather than reworking each comment to read well in a different
structure.

Implementation notes (beyond the proposal's design):

Keeping trailing comments cheap to lex required a few supporting
changes, all of which keep the cost off the lexer's hot path:

- The lexer already dispatches `//` to comment lexing wherever it
appears, so classifying a comment as trailing is a single O(1) check of
whether the `//` is the line's first non-whitespace (`start + indent`).
The hot comment path is otherwise unchanged.

- That check relies on each line's recorded indentation being its real
leading whitespace. Multi-line string literals previously recorded the
column where the literal opened for the lines they span; they now record
the true (closing-delimiter) indentation instead.

- Parser error recovery (`SkipPastLikelyEnd`) had relied on that
opening-column indentation to keep tokens following a multi-line string
literal attached to the same construct. It now reconstructs that
relationship directly by consulting the line on which the literal
opened, including when other tokens follow the closing delimiter (such
as `''' + "more"`). This is on the cold recovery path.

- `CommentData` records the trailing bit in the high bit of its length
field, keeping it at 8 bytes.

Assisted-by: Claude Code
2026-07-04 06:42:02 +00:00

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// 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
//
// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/none.carbon
//
// AUTOUPDATE
// TIP: To test this file alone, run:
// TIP: bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/basics/cpp_diagnostics.carbon
// TIP: To dump output, run:
// TIP: bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/basics/cpp_diagnostics.carbon
// ============================================================================
// One error
// ============================================================================
// --- one_error.h
#error "error1"
// --- fail_import_cpp_file_with_one_error.carbon
library "[[@TEST_NAME]]";
// CHECK:STDERR: fail_import_cpp_file_with_one_error.carbon:[[@LINE+5]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./one_error.h:2:2: error: "error1" [CppInteropParseError]
// CHECK:STDERR: 2 | #error "error1"
// CHECK:STDERR: | ^
// CHECK:STDERR:
import Cpp library "one_error.h";
// ============================================================================
// Multiple errors
// ============================================================================
// --- multiple_errors.h
#error "error1"
#error "error2"
// --- fail_import_cpp_file_with_multiple_errors.carbon
library "[[@TEST_NAME]]";
// CHECK:STDERR: fail_import_cpp_file_with_multiple_errors.carbon:[[@LINE+10]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./multiple_errors.h:2:2: error: "error1" [CppInteropParseError]
// CHECK:STDERR: 2 | #error "error1"
// CHECK:STDERR: | ^
// CHECK:STDERR:
// CHECK:STDERR: fail_import_cpp_file_with_multiple_errors.carbon:[[@LINE+5]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./multiple_errors.h:3:2: error: "error2" [CppInteropParseError]
// CHECK:STDERR: 3 | #error "error2"
// CHECK:STDERR: | ^
// CHECK:STDERR:
import Cpp library "multiple_errors.h";
// ============================================================================
// One warning
// ============================================================================
// --- one_warning.h
#warning "warning1"
// --- import_cpp_file_with_one_warning.carbon
library "[[@TEST_NAME]]";
//@dump-sem-ir-begin
// CHECK:STDERR: import_cpp_file_with_one_warning.carbon:[[@LINE+5]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./one_warning.h:2:2: warning: "warning1" [CppInteropParseWarning]
// CHECK:STDERR: 2 | #warning "warning1"
// CHECK:STDERR: | ^
// CHECK:STDERR:
import Cpp library "one_warning.h";
//@dump-sem-ir-end
// ============================================================================
// Multiple warnings
// ============================================================================
// --- multiple_warnings.h
#warning "warning1"
#warning "warning2"
#warning "warning3"
// --- import_cpp_file_with_multiple_warnings.carbon
library "[[@TEST_NAME]]";
//@dump-sem-ir-begin
// CHECK:STDERR: import_cpp_file_with_multiple_warnings.carbon:[[@LINE+15]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./multiple_warnings.h:2:2: warning: "warning1" [CppInteropParseWarning]
// CHECK:STDERR: 2 | #warning "warning1"
// CHECK:STDERR: | ^
// CHECK:STDERR:
// CHECK:STDERR: import_cpp_file_with_multiple_warnings.carbon:[[@LINE+10]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./multiple_warnings.h:3:2: warning: "warning2" [CppInteropParseWarning]
// CHECK:STDERR: 3 | #warning "warning2"
// CHECK:STDERR: | ^
// CHECK:STDERR:
// CHECK:STDERR: import_cpp_file_with_multiple_warnings.carbon:[[@LINE+5]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./multiple_warnings.h:4:2: warning: "warning3" [CppInteropParseWarning]
// CHECK:STDERR: 4 | #warning "warning3"
// CHECK:STDERR: | ^
// CHECK:STDERR:
import Cpp library "multiple_warnings.h";
//@dump-sem-ir-end
// ============================================================================
// One error and one warning
// ============================================================================
// --- one_error_and_one_warning.h
#error "error1"
#warning "warning1"
// --- fail_import_cpp_file_with_one_error_and_one_warning.carbon
library "[[@TEST_NAME]]";
// CHECK:STDERR: fail_import_cpp_file_with_one_error_and_one_warning.carbon:[[@LINE+10]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./one_error_and_one_warning.h:2:2: error: "error1" [CppInteropParseError]
// CHECK:STDERR: 2 | #error "error1"
// CHECK:STDERR: | ^
// CHECK:STDERR:
// CHECK:STDERR: fail_import_cpp_file_with_one_error_and_one_warning.carbon:[[@LINE+5]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./one_error_and_one_warning.h:3:2: warning: "warning1" [CppInteropParseWarning]
// CHECK:STDERR: 3 | #warning "warning1"
// CHECK:STDERR: | ^
// CHECK:STDERR:
import Cpp library "one_error_and_one_warning.h";
// ============================================================================
// Multiple errors and multiple warnings
// ============================================================================
// --- multiple_errors_and_multiple_warnings.h
#error "error1"
#error "error2"
#warning "warning1"
#warning "warning2"
#warning "warning3"
// --- fail_import_cpp_file_with_multiple_errors_and_multiple_warnings.carbon
library "[[@TEST_NAME]]";
// CHECK:STDERR: fail_import_cpp_file_with_multiple_errors_and_multiple_warnings.carbon:[[@LINE+25]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./multiple_errors_and_multiple_warnings.h:2:2: error: "error1" [CppInteropParseError]
// CHECK:STDERR: 2 | #error "error1"
// CHECK:STDERR: | ^
// CHECK:STDERR:
// CHECK:STDERR: fail_import_cpp_file_with_multiple_errors_and_multiple_warnings.carbon:[[@LINE+20]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./multiple_errors_and_multiple_warnings.h:3:2: error: "error2" [CppInteropParseError]
// CHECK:STDERR: 3 | #error "error2"
// CHECK:STDERR: | ^
// CHECK:STDERR:
// CHECK:STDERR: fail_import_cpp_file_with_multiple_errors_and_multiple_warnings.carbon:[[@LINE+15]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./multiple_errors_and_multiple_warnings.h:4:2: warning: "warning1" [CppInteropParseWarning]
// CHECK:STDERR: 4 | #warning "warning1"
// CHECK:STDERR: | ^
// CHECK:STDERR:
// CHECK:STDERR: fail_import_cpp_file_with_multiple_errors_and_multiple_warnings.carbon:[[@LINE+10]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./multiple_errors_and_multiple_warnings.h:5:2: warning: "warning2" [CppInteropParseWarning]
// CHECK:STDERR: 5 | #warning "warning2"
// CHECK:STDERR: | ^
// CHECK:STDERR:
// CHECK:STDERR: fail_import_cpp_file_with_multiple_errors_and_multiple_warnings.carbon:[[@LINE+5]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./multiple_errors_and_multiple_warnings.h:6:2: warning: "warning3" [CppInteropParseWarning]
// CHECK:STDERR: 6 | #warning "warning3"
// CHECK:STDERR: | ^
// CHECK:STDERR:
import Cpp library "multiple_errors_and_multiple_warnings.h";
// ============================================================================
// Multiple files with warnings
// ============================================================================
// --- import_multiple_cpp_files_with_warnings.carbon
library "[[@TEST_NAME]]";
//@dump-sem-ir-begin
// CHECK:STDERR: import_multiple_cpp_files_with_warnings.carbon:[[@LINE+5]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./one_warning.h:2:2: warning: "warning1" [CppInteropParseWarning]
// CHECK:STDERR: 2 | #warning "warning1"
// CHECK:STDERR: | ^
// CHECK:STDERR:
import Cpp library "one_warning.h";
// CHECK:STDERR: import_multiple_cpp_files_with_warnings.carbon:[[@LINE+15]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./multiple_warnings.h:2:2: warning: "warning1" [CppInteropParseWarning]
// CHECK:STDERR: 2 | #warning "warning1"
// CHECK:STDERR: | ^
// CHECK:STDERR:
// CHECK:STDERR: import_multiple_cpp_files_with_warnings.carbon:[[@LINE+10]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./multiple_warnings.h:3:2: warning: "warning2" [CppInteropParseWarning]
// CHECK:STDERR: 3 | #warning "warning2"
// CHECK:STDERR: | ^
// CHECK:STDERR:
// CHECK:STDERR: import_multiple_cpp_files_with_warnings.carbon:[[@LINE+5]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./multiple_warnings.h:4:2: warning: "warning3" [CppInteropParseWarning]
// CHECK:STDERR: 4 | #warning "warning3"
// CHECK:STDERR: | ^
// CHECK:STDERR:
import Cpp library "multiple_warnings.h";
//@dump-sem-ir-end
// ============================================================================
// Multiple files with errors and warnings
// ============================================================================
// --- fail_import_multiple_cpp_files_with_errors_and_warnings.carbon
library "[[@TEST_NAME]]";
// CHECK:STDERR: fail_import_multiple_cpp_files_with_errors_and_warnings.carbon:[[@LINE+10]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./one_error_and_one_warning.h:2:2: error: "error1" [CppInteropParseError]
// CHECK:STDERR: 2 | #error "error1"
// CHECK:STDERR: | ^
// CHECK:STDERR:
// CHECK:STDERR: fail_import_multiple_cpp_files_with_errors_and_warnings.carbon:[[@LINE+5]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./one_error_and_one_warning.h:3:2: warning: "warning1" [CppInteropParseWarning]
// CHECK:STDERR: 3 | #warning "warning1"
// CHECK:STDERR: | ^
// CHECK:STDERR:
import Cpp library "one_error_and_one_warning.h";
// CHECK:STDERR: fail_import_multiple_cpp_files_with_errors_and_warnings.carbon:[[@LINE+25]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./multiple_errors_and_multiple_warnings.h:2:2: error: "error1" [CppInteropParseError]
// CHECK:STDERR: 2 | #error "error1"
// CHECK:STDERR: | ^
// CHECK:STDERR:
// CHECK:STDERR: fail_import_multiple_cpp_files_with_errors_and_warnings.carbon:[[@LINE+20]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./multiple_errors_and_multiple_warnings.h:3:2: error: "error2" [CppInteropParseError]
// CHECK:STDERR: 3 | #error "error2"
// CHECK:STDERR: | ^
// CHECK:STDERR:
// CHECK:STDERR: fail_import_multiple_cpp_files_with_errors_and_warnings.carbon:[[@LINE+15]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./multiple_errors_and_multiple_warnings.h:4:2: warning: "warning1" [CppInteropParseWarning]
// CHECK:STDERR: 4 | #warning "warning1"
// CHECK:STDERR: | ^
// CHECK:STDERR:
// CHECK:STDERR: fail_import_multiple_cpp_files_with_errors_and_warnings.carbon:[[@LINE+10]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./multiple_errors_and_multiple_warnings.h:5:2: warning: "warning2" [CppInteropParseWarning]
// CHECK:STDERR: 5 | #warning "warning2"
// CHECK:STDERR: | ^
// CHECK:STDERR:
// CHECK:STDERR: fail_import_multiple_cpp_files_with_errors_and_warnings.carbon:[[@LINE+5]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./multiple_errors_and_multiple_warnings.h:6:2: warning: "warning3" [CppInteropParseWarning]
// CHECK:STDERR: 6 | #warning "warning3"
// CHECK:STDERR: | ^
// CHECK:STDERR:
import Cpp library "multiple_errors_and_multiple_warnings.h";
// ============================================================================
// Indirect error
// ============================================================================
// --- indirect_error.h
#include "one_error.h"
// --- fail_import_indirect_error.carbon
library "[[@TEST_NAME]]";
// CHECK:STDERR: fail_import_indirect_error.carbon:[[@LINE+6]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./indirect_error.h:2:10: in file included here [InCppInclude]
// CHECK:STDERR: ./one_error.h:2:2: error: "error1" [CppInteropParseError]
// CHECK:STDERR: 2 | #error "error1"
// CHECK:STDERR: | ^
// CHECK:STDERR:
import Cpp library "indirect_error.h";
// ============================================================================
// Indirect warning
// ============================================================================
// --- indirect_warning.h
#include "one_warning.h"
// --- import_indirect_warning.carbon
library "[[@TEST_NAME]]";
//@dump-sem-ir-begin
// CHECK:STDERR: import_indirect_warning.carbon:[[@LINE+6]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./indirect_warning.h:2:10: in file included here [InCppInclude]
// CHECK:STDERR: ./one_warning.h:2:2: warning: "warning1" [CppInteropParseWarning]
// CHECK:STDERR: 2 | #warning "warning1"
// CHECK:STDERR: | ^
// CHECK:STDERR:
import Cpp library "indirect_warning.h";
//@dump-sem-ir-end
// ============================================================================
// Lexer error before import
// ============================================================================
// --- fail_import_cpp_library_lexer_error.carbon
library "[[@TEST_NAME]]";
//!Lexer error: whitespace is required after the comment introducer.
// TODO: Move this warning to be after the lexer error.
// CHECK:STDERR: fail_import_cpp_library_lexer_error.carbon:[[@LINE+9]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./one_warning.h:2:2: warning: "warning1" [CppInteropParseWarning]
// CHECK:STDERR: 2 | #warning "warning1"
// CHECK:STDERR: | ^
// CHECK:STDERR:
// CHECK:STDERR: fail_import_cpp_library_lexer_error.carbon:[[@LINE-8]]:3: error: whitespace is required after '//' [NoWhitespaceAfterCommentIntroducer]
// CHECK:STDERR: //!Lexer error: whitespace is required after the comment introducer.
// CHECK:STDERR: ^
// CHECK:STDERR:
import Cpp library "one_warning.h";
// ============================================================================
// Diagnostic with fix-it hints
// ============================================================================
// --- fix_it_hints.h
double score = 0.1
// --- fail_import_fix_it_hints.carbon
library "[[@TEST_NAME]]";
// CHECK:STDERR: fail_import_fix_it_hints.carbon:[[@LINE+6]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./fix_it_hints.h:2:19: error: expected ';' after top level declarator [CppInteropParseError]
// CHECK:STDERR: 2 | double score = 0.1
// CHECK:STDERR: | ^
// CHECK:STDERR: | ;
// CHECK:STDERR:
import Cpp library "fix_it_hints.h";
fn F() {
// CHECK:STDERR: fail_import_fix_it_hints.carbon:[[@LINE+4]]:3: error: member name `foo` not found in `Cpp` [MemberNameNotFoundInInstScope]
// CHECK:STDERR: Cpp.foo();
// CHECK:STDERR: ^~~~~~~
// CHECK:STDERR:
Cpp.foo();
}
// ============================================================================
// Diagnostic with notes
// ============================================================================
// --- with_notes.h
void foobar(int);
inline void call_foobar() {
foobar(1, 2);
}
// --- fail_with_notes.carbon
library "[[@TEST_NAME]]";
// CHECK:STDERR: fail_with_notes.carbon:[[@LINE+9]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./with_notes.h:5:3: error: no matching function for call to 'foobar' [CppInteropParseError]
// CHECK:STDERR: 5 | foobar(1, 2);
// CHECK:STDERR: | ^~~~~~
// CHECK:STDERR: fail_with_notes.carbon:[[@LINE+5]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./with_notes.h:2:6: note: candidate function not viable: requires 1 argument, but 2 were provided [CppInteropParseNote]
// CHECK:STDERR: 2 | void foobar(int);
// CHECK:STDERR: | ^ ~~~
// CHECK:STDERR:
import Cpp library "with_notes.h";
fn F() {
Cpp.call_foobar();
}
// ============================================================================
// Context stacks
// ============================================================================
// --- indirect_include.h
#define FOO BAR
#define BAZ void f(error);
FOO
// --- direct_include.h
#define BAR BAZ
#include "indirect_include.h"
// --- fail_use_context_stack.carbon
library "[[@TEST_NAME]]";
// CHECK:STDERR: fail_use_context_stack.carbon:[[@LINE+15]]:10: in file included here [InCppInclude]
// CHECK:STDERR: ./direct_include.h:4:10: in file included here [InCppInclude]
// CHECK:STDERR: ./indirect_include.h:2:13: in expansion of macro defined here [InCppMacroExpansion]
// CHECK:STDERR: ./direct_include.h:2:13: in expansion of macro defined here [InCppMacroExpansion]
// CHECK:STDERR: ./indirect_include.h:3:20: in expansion of macro defined here [InCppMacroExpansion]
// CHECK:STDERR: ./indirect_include.h:5:1: error: unknown type name 'error' [CppInteropParseError]
// CHECK:STDERR: 5 | FOO
// CHECK:STDERR: | ^
// CHECK:STDERR: 2 | #define FOO BAR
// CHECK:STDERR: | ^
// CHECK:STDERR: 2 | #define BAR BAZ
// CHECK:STDERR: | ^
// CHECK:STDERR: 3 | #define BAZ void f(error);
// CHECK:STDERR: | ^
// CHECK:STDERR:
import Cpp library "direct_include.h";
// ============================================================================
// Diagnostic location in inline code.
// ============================================================================
// --- fail_loc_in_inline_simple.carbon
library "[[@TEST_NAME]]";
// CHECK:STDERR: fail_loc_in_inline_simple.carbon:[[@LINE+4]]:9: error: use of undeclared identifier 'banana' [CppInteropParseError]
// CHECK:STDERR: 8 | int n = banana;
// CHECK:STDERR: | ^~~~~~
// CHECK:STDERR:
import Cpp inline "int n = banana;";
// --- fail_loc_in_inline_block.carbon
import Cpp inline '''c++
void f(const int n) {
// CHECK:STDERR: fail_loc_in_inline_block.carbon:[[@LINE+7]]:3: error: cannot assign to variable 'n' with const-qualified type 'const int' [CppInteropParseError]
// CHECK:STDERR: 11 | ++n;
// CHECK:STDERR: | ^ ~
// CHECK:STDERR: fail_loc_in_inline_block.carbon:[[@LINE-4]]:18: note: variable 'n' declared const here [CppInteropParseNote]
// CHECK:STDERR: 3 | void f(const int n) {
// CHECK:STDERR: | ~~~~~~~~~~^
// CHECK:STDERR:
++n;
}
''';
// CHECK:STDOUT: --- import_cpp_file_with_one_warning.carbon
// CHECK:STDOUT:
// CHECK:STDOUT: file {
// CHECK:STDOUT: %Cpp.import_cpp = import_cpp {
// CHECK:STDOUT: import Cpp "one_warning.h"
// CHECK:STDOUT: }
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: --- import_cpp_file_with_multiple_warnings.carbon
// CHECK:STDOUT:
// CHECK:STDOUT: file {
// CHECK:STDOUT: %Cpp.import_cpp = import_cpp {
// CHECK:STDOUT: import Cpp "multiple_warnings.h"
// CHECK:STDOUT: }
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: --- import_multiple_cpp_files_with_warnings.carbon
// CHECK:STDOUT:
// CHECK:STDOUT: file {
// CHECK:STDOUT: %Cpp.import_cpp = import_cpp {
// CHECK:STDOUT: import Cpp "one_warning.h"
// CHECK:STDOUT: import Cpp "multiple_warnings.h"
// CHECK:STDOUT: }
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: --- import_indirect_warning.carbon
// CHECK:STDOUT:
// CHECK:STDOUT: file {
// CHECK:STDOUT: %Cpp.import_cpp = import_cpp {
// CHECK:STDOUT: import Cpp "indirect_warning.h"
// CHECK:STDOUT: }
// CHECK:STDOUT: }
// CHECK:STDOUT:
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