cmd/cgo: add C declaration parameter unused attribute

Fixes #71225

Change-Id: I3e60fdf632f2aa0e63b24225f13e4ace49906925
Reviewed-on: https://go-review.googlesource.com/c/go/+/642196
Reviewed-by: David Chase <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: ianlancetaylor

src/cmd/cgo/out.go

@@ -1094,7 +1094,7 @@ func (p *Package) writeExports(fgo2, fm, fgcc, fgcch io.Writer) {
 		// This unpacks the argument struct above and calls the Go function.
 		fmt.Fprintf(fgo2, "func _cgoexp%s_%s(a *%s) {\n", cPrefix, exp.ExpName, gotype)
 
-		fmt.Fprintf(fm, "void _cgoexp%s_%s(void* p){}\n", cPrefix, exp.ExpName)
+		fmt.Fprintf(fm, "void _cgoexp%s_%s(void* p __attribute__((unused))){}\n", cPrefix, exp.ExpName)
 
 		fmt.Fprintf(fgo2, "\t")
 

test/fixedbugs/issue71225.go

@@ -0,0 +1,19 @@
+// build
+
+//go:build cgo
+
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+// #cgo CFLAGS: -Werror -Wunused-parameter
+import "C"
+
+func main() {
+}
+
+//export Fn
+func Fn() {
+}