cmd/cue: add mod fix command

This wires up the `modfile.FixLegacy` function to a new
`cue mod fix` command.

Fixes #3137

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: Idbb071493c4596feabf3b2c6c9ee8a53847d6127
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194780
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cmd/cue/cmd/mod.go

@@ -45,6 +45,7 @@ See also:
 	}
 
 	cmd.AddCommand(newModEditCmd(c))
+	cmd.AddCommand(newModFixCmd(c))
 	cmd.AddCommand(newModGetCmd(c))
 	cmd.AddCommand(newModInitCmd(c))
 	cmd.AddCommand(newModRegistryCmd(c))

cmd/cue/cmd/modfix.go

@@ -0,0 +1,80 @@
+// Copyright 2024 The CUE Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package cmd
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"cuelang.org/go/mod/modfile"
+	"github.com/spf13/cobra"
+)
+
+func newModFixCmd(c *Command) *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "fix",
+		Short: "fix a legacy cue.mod/module.cue file",
+		Long: `WARNING: THIS COMMAND IS EXPERIMENTAL.
+
+Fix provides a way to migrate from a legacy module.cue file
+to the new standard syntax. It
+
+- adds a language.version field
+- moves unrecognized fields into the custom.legacy field
+- adds a major version to the module path
+
+If there is no module path, it chooses an arbitrary path (test.example@v0).
+
+If the module.cue file is already compatible with the new syntax,
+it is just formatted without making any other changes.
+
+Note: you must enable the modules experiment with:
+	export CUE_EXPERIMENT=modules
+for this command to work.
+`,
+		RunE: mkRunE(c, runModFix),
+		Args: cobra.ExactArgs(0),
+	}
+	return cmd
+}
+
+func runModFix(cmd *Command, args []string) error {
+	modRoot, err := findModuleRoot()
+	if err != nil {
+		return err
+	}
+	modPath := filepath.Join(modRoot, "cue.mod", "module.cue")
+	data, err := os.ReadFile(modPath)
+	if err != nil {
+		return err
+	}
+	mf, err := modfile.FixLegacy(data, modPath)
+	if err != nil {
+		return err
+	}
+	newData, err := mf.Format()
+	if err != nil {
+		return fmt.Errorf("internal error: invalid module.cue file generated: %v", err)
+	}
+	if bytes.Equal(newData, data) {
+		return nil
+	}
+	if err := os.WriteFile(modPath, newData, 0o666); err != nil {
+		return err
+	}
+	return nil
+}

cmd/cue/cmd/testdata/script/modfix_initial.txtar

@@ -0,0 +1,23 @@
+# This test checks that the legacy module.cue fixing code
+# is wired up correctly. More comprehensive tests for
+# this functionality are inside the modfile package.
+
+exec cue mod fix
+cmp cue.mod/module.cue want-module
+
+exec cue mod fix
+cmp cue.mod/module.cue want-module
+
+-- cue.mod/module.cue --
+module: "foo.com"
+foo: "bar"
+-- want-module --
+module: "foo.com@v0"
+language: {
+	version: "v0.9.0"
+}
+custom: {
+	legacy: {
+		foo: "bar"
+	}
+}