v3: keep V3/C fallback bug-report output off stdout so v -o - stays valid C (PR #28131 review)

send_prepared_c_error_bug_report and submit_v3_compiler_error_bug_report printed
the bug-report banner, the uploaded C/V context, and status lines with println.
For `v -o - source.v`, V1 has already written the generated C to stdout in
builder.cc; appending the report there corrupts the documented `-o -` output for
exactly the programs that needed the compatibility fallback (and the upload
succeeding made it worse, since that path prints the most text).

Emit all report diagnostics on stderr (eprintln), matching the fallback notice,
which was already on stderr. Covers both the generated-C path
(send_prepared_c_error_bug_report + print_c_error_bug_report_context +
print_report_lines) and the V3 internal-error path
(submit_v3_compiler_error_bug_report). Adds an e2e test that a `-o -` fallback
build keeps stdout as pure generated C while the report goes to stderr.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: medvednikov

cmd/v/macos_v3_test.v

@@ -1647,6 +1647,76 @@ fn main() {
 	}
 }
 
+// End-to-end (PR #28131 review): a V3 internal-error fallback for `v -o - source.v` must
+// keep stdout as pure generated C. V1 has already written the C to stdout, so the report
+// banner, its context, and the fallback notice all go to stderr — never stdout — or the
+// documented `-o -` output would be invalid C for exactly the programs that needed the
+// fallback.
+fn test_macos_v3_fallback_report_stays_off_generated_c_stdout() {
+	$if macos {
+		root := os.join_path(os.real_path(os.vtmp_dir()), 'macos_v3_stdout_c_${os.getpid()}')
+		os.rmdir_all(root) or {}
+		os.mkdir_all(root) or { panic(err) }
+		defer {
+			os.rmdir_all(root) or {}
+		}
+		source := os.join_path(root, 'gen.v')
+		os.write_file(source, 'struct Opt[T] {
+	val  T
+	some bool
+}
+
+fn some[T](val T) Opt[T] {
+	return Opt[T]{
+		val:  val
+		some: true
+	}
+}
+
+fn (f Opt[T]) map[U](op fn (T) U) Opt[U] {
+	if f.some {
+		return some[U](op(f.val))
+	}
+	return Opt[U]{}
+}
+
+fn main() {
+	result := some("hello").map(|s| s.len)
+	assert result.some && result.val == 5
+}
+')!
+		mut environment := os.environ()
+		// Exercise a real fallback; clear the job-level no-fallback guard CI may set.
+		environment.delete('V_MACOS_V3_NO_FALLBACK')
+		environment['V_C_ERROR_BUG_REPORT_DISABLED'] = ''
+		// Unroutable endpoint: the send fails fast, exercising the report-diagnostic
+		// output (the notice and "was not sent") without contacting a real server.
+		environment['V_C_ERROR_BUG_REPORT_URL'] = 'http://127.0.0.1:1/bug-report'
+		environment['VFLAGS'] = ''
+		environment['VOSARGS'] = ''
+		mut process := os.new_process(@VEXE)
+		process.set_args(['-gc', 'none', '-o', '-', source])
+		process.set_environment(environment)
+		process.set_redirect_stdio()
+		process.run()
+		process.wait()
+		stdout := process.stdout_slurp()
+		stderr := process.stderr_slurp()
+		exit_code := process.code
+		process.close()
+		assert exit_code == 0, stdout + stderr
+		// The fallback happened and its diagnostics went to stderr.
+		assert stderr.contains('the experimental V3 compiler could not build this program'), stderr
+		assert stderr.contains('bug report was not sent'), stderr
+		// stdout is the generated C only — valid C, with no report text of any kind.
+		assert stdout.contains('typedef') || stdout.contains('#define'), 'stdout is not generated C'
+		for leaked in ['could not build this program', 'compiler bug report',
+			'bug report was not sent', 'so this can be fixed', 'opt out of these automatic'] {
+			assert !stdout.contains(leaked), 'report text leaked onto `-o -` stdout: `${leaked}`'
+		}
+	}
+}
+
 fn clear_macos_v3_report_env() {
 	for suffix in ['PRESENT', 'KIND', 'CCOMPILER', 'COUTPUT', 'TAG', 'VFILE', 'VSOURCE'] {
 		os.unsetenv('V_MACOS_V3_REPORT_${suffix}')

vlib/v/builder/c_error_report.v

@@ -175,16 +175,19 @@ fn (mut v Builder) send_prepared_c_error_bug_report(raw_report CErrorBugReport,
 		}
 		return
 	}
-	println('================== C compiler bug report ==============')
+	// Report diagnostics go to stderr, never stdout: with `v -o - source.v` the generated
+	// C is already on stdout, so appending this banner there would corrupt the documented
+	// `-o -` output for exactly the programs that needed the fallback.
+	eprintln('================== C compiler bug report ==============')
 	if is_v3_fallback {
 		print_v3_fallback_notice(report_url, true, report_includes_v_source(report))
 	}
 	if tool_output != '' {
-		println(tool_output)
+		eprintln(tool_output)
 	}
-	println('V ${report.v_version}, ${report.target_os}/${report.arch}, cc: ${report.ccompiler}, build options: ${report.build_options}')
+	eprintln('V ${report.v_version}, ${report.target_os}/${report.arch}, cc: ${report.ccompiler}, build options: ${report.build_options}')
 	print_c_error_bug_report_context(report)
-	println('='.repeat('================== C compiler bug report =============='.len))
+	eprintln('='.repeat('================== C compiler bug report =============='.len))
 }
 
 // submit_external_c_error_bug_report submits C diagnostics and generated source produced by
@@ -474,13 +477,16 @@ fn (mut v Builder) submit_v3_compiler_error_bug_report(v3_stage string, v3_outpu
 		print_v3_fallback_notice('', false, false)
 		return
 	}
-	println('================== V3 compiler bug report ==============')
+	// Report diagnostics go to stderr, never stdout: with `v -o - source.v` the generated
+	// C is already on stdout, so appending this banner there would corrupt the documented
+	// `-o -` output for exactly the programs that needed the fallback.
+	eprintln('================== V3 compiler bug report ==============')
 	print_v3_fallback_notice(report_url, true, report.v_source != '')
 	if tool_output != '' {
-		println(tool_output)
+		eprintln(tool_output)
 	}
-	println('V ${report.v_version}, ${report.target_os}/${report.arch}, build options: ${report.build_options}')
-	println('='.repeat('================== V3 compiler bug report =============='.len))
+	eprintln('V ${report.v_version}, ${report.target_os}/${report.arch}, build options: ${report.build_options}')
+	eprintln('='.repeat('================== V3 compiler bug report =============='.len))
 }
 
 // v3_report_v_source returns the bounded V source snippet uploaded for an internal
@@ -1242,25 +1248,27 @@ fn truncated_report_text(text string, max_bytes int) string {
 	return text[..head_bytes] + c_error_bug_report_truncation_notice + text[text.len - tail_bytes..]
 }
 
+// print_c_error_bug_report_context prints the uploaded C/V context to stderr — never
+// stdout — so it cannot corrupt a `v -o - source.v` generated-C stream.
 fn print_c_error_bug_report_context(report CErrorBugReport) {
-	println('Generated C lines sent from ${report.c_file}:${report.c_line}:')
+	eprintln('Generated C lines sent from ${report.c_file}:${report.c_line}:')
 	print_report_lines(report.c_context, report.c_line)
 	if report.v_file != '' {
-		println('Corresponding V lines sent from ${report.v_file}:${report.v_line}:')
+		eprintln('Corresponding V lines sent from ${report.v_file}:${report.v_line}:')
 		print_report_lines(report.v_context, report.v_line)
 	} else {
-		println('Corresponding V lines sent: no V source mapping was available.')
+		eprintln('Corresponding V lines sent: no V source mapping was available.')
 	}
 }
 
 fn print_report_lines(lines []CErrorReportLine, center int) {
 	if lines.len == 0 {
-		println('  (no source lines available)')
+		eprintln('  (no source lines available)')
 		return
 	}
 	for line in lines {
 		prefix := if line.line == center { '>' } else { ' ' }
-		println('${prefix} ${line.line:6} | ${line.text}')
+		eprintln('${prefix} ${line.line:6} | ${line.text}')
 	}
 }