cgen: interface method fix

Author: medvednikov

vlib/v/gen/c/fn.v

@@ -671,7 +671,13 @@ fn (mut g Gen) is_used_by_main(node ast.FnDecl) bool {
 				if receiver_type !in impl_types {
 					continue
 				}
-				if isym.has_method(node.name) || isym.has_method_with_generic_parent(node.name) {
+				mut interface_requires_method := false
+				if interface_method := isym.find_method(node.name) {
+					interface_requires_method = interface_method.no_body
+				} else if interface_method := isym.find_method_with_generic_parent(node.name) {
+					interface_requires_method = interface_method.no_body
+				}
+				if interface_requires_method {
 					is_used_by_main = true
 					break
 				}

vlib/v/tests/skip_unused/generic_fn_instantiation_pruning_test.v

@@ -35,3 +35,56 @@ fn test_skip_unused_prunes_unused_generic_fn_instantiations() {
 	assert res.output.contains('VV_LOC Map_u8_u32 main__new_T_u32(void)')
 	assert !res.output.contains('VV_LOC Map_u8_int main__new_T_int(void)')
 }
+
+fn test_skip_unused_does_not_emit_impl_methods_for_interface_extensions() {
+	tmp_dir := os.join_path(os.vtmp_dir(), 'v_skip_unused_interface_extension_collision')
+	os.mkdir_all(tmp_dir) or { panic(err) }
+	defer {
+		os.rmdir_all(tmp_dir) or {}
+	}
+	source_path := os.join_path(tmp_dir, 'interface_extension_collision.v')
+	source := [
+		'module main',
+		'',
+		'import crypto.internal.subtle',
+		'',
+		'interface Elem {}',
+		'',
+		'struct Thing {}',
+		'',
+		'struct Holder {',
+		'\titems []Elem',
+		'}',
+		'',
+		'fn (el Elem) equal(_ Elem) bool {',
+		'\treturn true',
+		'}',
+		'',
+		'fn (t Thing) equal(_ Thing) bool {',
+		'\treturn subtle.constant_time_compare([u8(1)], [u8(1)]) == 1',
+		'}',
+		'',
+		'fn (h Holder) ok() bool {',
+		'\tfor i, item in h.items {',
+		'\t\tfor j, obj in h.items {',
+		'\t\t\tif i == j {',
+		'\t\t\t\treturn item.equal(obj)',
+		'\t\t\t}',
+		'\t\t}',
+		'\t}',
+		'\treturn false',
+		'}',
+		'',
+		'fn main() {',
+		'\th := Holder{items: [Elem(Thing{}), Elem(Thing{})]}',
+		'\tprintln(h.ok())',
+		'}',
+	].join('\n')
+	os.write_file(source_path, source) or { panic(err) }
+	res := os.execute('${os.quoted_path(vexe)} -o - ${os.quoted_path(source_path)}')
+	if res.exit_code != 0 {
+		panic(res.output)
+	}
+	assert res.output.contains('VV_LOC bool main__Elem_equal(')
+	assert !res.output.contains('main__Thing_equal(')
+}