Restore compatibility with Go 1.18

Also test compatibility of fetch_repo and gazelle.

BUILD.bazel

@@ -1,6 +1,6 @@
 load("@bazel_gazelle_is_bazel_module//:defs.bzl", "GAZELLE_IS_BAZEL_MODULE")
 load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
-load("@io_bazel_rules_go//go:def.bzl", "nogo")
+load("@io_bazel_rules_go//go:def.bzl", "go_cross_binary", "nogo")
 load("//:def.bzl", "gazelle", "gazelle_binary")
 
 # gazelle:prefix github.com/bazelbuild/bazel-gazelle
@@ -38,6 +38,12 @@ gazelle_binary(
     ],
 )
 
+go_cross_binary(
+    name = "gazelle_local_go1.18",
+    sdk_version = "1.18",
+    target = ":gazelle_local",
+)
+
 nogo(
     name = "nogo",
     vet = True,

MODULE.bazel

@@ -58,6 +58,10 @@ bazel_dep(name = "bazel_skylib_gazelle_plugin", version = "1.4.1", dev_dependenc
 bazel_dep(name = "stardoc", version = "0.6.2", dev_dependency = True, repo_name = "io_bazel_stardoc")
 
 go_sdk_dev = use_extension("@io_bazel_rules_go//go:extensions.bzl", "go_sdk", dev_dependency = True)
+go_sdk_dev.download(version = "1.22.4")
+
+# Used by compatibility tests, keep as low as possible.
+go_sdk_dev.download(version = "1.18.10")
 
 # Known to exist since it is instantiated by rules_go itself.
 use_repo(

cmd/fetch_repo/BUILD.bazel

@@ -1,10 +1,11 @@
-load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_library", "go_test")
+load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_cross_binary", "go_library", "go_test")
 
 go_library(
     name = "fetch_repo_lib",
     srcs = [
         "clean.go",
         "copy_tree.go",
+        "errorscompat.go",
         "fetch_repo.go",
         "go_mod_download.go",
         "module.go",
@@ -25,6 +26,13 @@ go_binary(
     visibility = ["//visibility:public"],
 )
 
+# Verify that fetch_repo builds with Go 1.18.
+go_cross_binary(
+    name = "fetch_repo_go1.18",
+    sdk_version = "1.18",
+    target = ":fetch_repo",
+)
+
 go_test(
     name = "fetch_repo_test",
     srcs = ["fetch_repo_test.go"],

cmd/fetch_repo/errorscompat.go

@@ -0,0 +1,61 @@
+// Taken from
+// https://github.com/golang/go/blob/20b79fd5775c39061d949569743912ad5e58b0e7/src/errors/join.go
+// TODO: Remove when Go 1.20 is the minimum supported version.
+
+// Copyright 2022 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
+
+// Join returns an error that wraps the given errors.
+// Any nil error values are discarded.
+// Join returns nil if every value in errs is nil.
+// The error formats as the concatenation of the strings obtained
+// by calling the Error method of each element of errs, with a newline
+// between each string.
+//
+// A non-nil error returned by Join implements the Unwrap() []error method.
+func Join(errs ...error) error {
+	n := 0
+	for _, err := range errs {
+		if err != nil {
+			n++
+		}
+	}
+	if n == 0 {
+		return nil
+	}
+	e := &joinError{
+		errs: make([]error, 0, n),
+	}
+	for _, err := range errs {
+		if err != nil {
+			e.errs = append(e.errs, err)
+		}
+	}
+	return e
+}
+
+type joinError struct {
+	errs []error
+}
+
+func (e *joinError) Error() string {
+	// Since Join returns nil if every value in errs is nil,
+	// e.errs cannot be empty.
+	if len(e.errs) == 1 {
+		return e.errs[0].Error()
+	}
+
+	b := []byte(e.errs[0].Error())
+	for _, err := range e.errs[1:] {
+		b = append(b, '\n')
+		b = append(b, err.Error()...)
+	}
+	return string(b)
+}
+
+func (e *joinError) Unwrap() []error {
+	return e.errs
+}

cmd/fetch_repo/go_mod_download.go

@@ -61,7 +61,7 @@ func runGoModDownload(dl *GoModDownloadResult, dest string, importpath string, v
 		return fmt.Errorf("go mod download output format: `%s %s`: parsing JSON: %q error: %w", cmd.Path, strings.Join(cmd.Args, " "), buf.String(), err)
 	}
 	if dl.Error != "" {
-		return errors.Join(errors.New(dl.Error), dlErr)
+		return Join(errors.New(dl.Error), dlErr)
 	}
 	if dlErr != nil {
 		return dlErr

internal/go_repository_tools_srcs.bzl

@@ -10,6 +10,7 @@ GO_REPOSITORY_TOOLS_SRCS = [
     Label("//cmd/fetch_repo:BUILD.bazel"),
     Label("//cmd/fetch_repo:clean.go"),
     Label("//cmd/fetch_repo:copy_tree.go"),
+    Label("//cmd/fetch_repo:errorscompat.go"),
     Label("//cmd/fetch_repo:fetch_repo.go"),
     Label("//cmd/fetch_repo:go_mod_download.go"),
     Label("//cmd/fetch_repo:module.go"),