cgen: restore old behaviour for struct Name{field &C.FILE};i:=Name{};unsafe{i.free()} (fix #25325) (#25327)

Author: spytheman

vlib/v/gen/c/auto_free_methods.v

@@ -62,7 +62,7 @@ fn (mut g Gen) gen_free_method(typ ast.Type) string {
 
 	match mut sym.info {
 		ast.Struct {
-			g.gen_free_for_struct(objtyp, sym.info, styp, fn_name)
+			g.gen_free_for_struct(objtyp, sym.info, styp, fn_name, sym.is_builtin())
 		}
 		ast.Array {
 			g.gen_free_for_array(sym.info, styp, fn_name)
@@ -105,7 +105,11 @@ fn (mut g Gen) gen_free_for_interface(sym ast.TypeSymbol, info ast.Interface, st
 	fn_builder.writeln('}')
 }
 
-fn (mut g Gen) gen_free_for_struct(typ ast.Type, info ast.Struct, styp string, fn_name string) {
+fn (mut g Gen) gen_free_for_struct(typ ast.Type, info ast.Struct, styp string, ofn_name string, sym_is_builtin bool) {
+	mut fn_name := ofn_name
+	if sym_is_builtin {
+		fn_name = 'builtin__${fn_name}'
+	}
 	g.definitions.writeln('${g.static_non_parallel}void ${fn_name}(${styp}* it);')
 	mut fn_builder := strings.new_builder(128)
 	defer {

vlib/v/gen/c/testdata/free_for_struct_with_c_fields.c.must_have

@@ -0,0 +1,5 @@
+static void main__Abc_free(main__Abc* it);
+static void builtin__FILE_free(FILE* it);
+static void builtin__FILE_free(FILE* it) {
+static void main__Abc_free(main__Abc* it) {
+builtin__FILE_free(&(it->myfile));

vlib/v/gen/c/testdata/free_for_struct_with_c_fields.out

@@ -0,0 +1,3 @@
+Abc{
+    myfile: &nil
+}

vlib/v/gen/c/testdata/free_for_struct_with_c_fields.vv

@@ -0,0 +1,10 @@
+struct Abc {
+	myfile &C.FILE = unsafe { nil }
+}
+
+@[manualfree]
+fn main() {
+	a := Abc{}
+	println(a)
+	unsafe { a.free() }
+}