cmd/cue: suggest cue mod fix when language.version is missing

That is, if a user tries to use `mod tidy`, `mod get`, or `mod publish`
on a module without a `language.version` field, which is now mandatory,
suggest what the user should run to resolve that issue.

Note that we don't apply this change to non-mod commands such as
`cue export` because, as the TODOs explain, those scenarios should work
for the sake of backwards compatibility as a downstream user.

We also don't try to make this be consistent for all `cue mod` commands
as it's not clear that all of them should require `language.version`.
For example, `mod init` creates a module, `mod fix` can add the field,
and we should arguably teach `mod edit` to add or set the field too.
For now, add the suggestion to the three `cue mod` commands which are
most likely to be used when developing or publishing a CUE module.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I11cef96900b2a5984ad6c07abbd705108c2c2759
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1195784
Reviewed-by: Paul Jolly <paul@myitcv.io>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

cmd/cue/cmd/modget.go

@@ -77,7 +77,7 @@ func runModGet(cmd *Command, args []string) error {
 	}
 	mf, err := modload.UpdateVersions(ctx, os.DirFS(modRoot), ".", reg, args)
 	if err != nil {
-		return err
+		return suggestModCommand(err)
 	}
 	// TODO check whether it's changed or not.
 	data, err := mf.Format()

cmd/cue/cmd/modpublish.go

@@ -122,7 +122,7 @@ func runModUpload(cmd *Command, args []string) error {
 		return err
 	}
 	if err := modload.CheckTidy(ctx, os.DirFS(modRoot), ".", reg); err != nil {
-		return suggestModTidy(err)
+		return suggestModCommand(err)
 	}
 
 	modPath := filepath.Join(modRoot, "cue.mod/module.cue")

cmd/cue/cmd/modtidy.go

@@ -24,6 +24,7 @@ import (
 	"github.com/spf13/cobra"
 
 	"cuelang.org/go/internal/mod/modload"
+	"cuelang.org/go/mod/modfile"
 )
 
 func newModTidyCmd(c *Command) *cobra.Command {
@@ -67,11 +68,11 @@ func runModTidy(cmd *Command, args []string) error {
 	}
 	if flagCheck.Bool(cmd) {
 		err := modload.CheckTidy(ctx, os.DirFS(modRoot), ".", reg)
-		return suggestModTidy(err)
+		return suggestModCommand(err)
 	}
 	mf, err := modload.Tidy(ctx, os.DirFS(modRoot), ".", reg)
 	if err != nil {
-		return err
+		return suggestModCommand(err)
 	}
 	data, err := mf.Format()
 	if err != nil {
@@ -93,11 +94,16 @@ func runModTidy(cmd *Command, args []string) error {
 	return nil
 }
 
-// suggestModTidy rewrites [modload.ErrModuleNotTidy] errors
-// so that they suggest running `cue mod tidy` to the user.
-func suggestModTidy(err error) error {
+// suggestModCommand rewrites a non-nil error to suggest to the user
+// what command they could use to fix a problem.
+// [modload.ErrModuleNotTidy] suggests running `cue mod tidy`,
+// and [modfile.ErrNoLanguageVersion] suggests running `cue mod fix`.
+func suggestModCommand(err error) error {
 	notTidyErr := new(modload.ErrModuleNotTidy)
-	if errors.As(err, &notTidyErr) {
+	switch {
+	case errors.Is(err, modfile.ErrNoLanguageVersion):
+		err = fmt.Errorf("%w; run 'cue mod fix'", err)
+	case errors.As(err, &notTidyErr):
 		if notTidyErr.Reason == "" {
 			err = fmt.Errorf("module is not tidy, use 'cue mod tidy'")
 		} else {

cmd/cue/cmd/testdata/script/modpublish_no_source.txtar

@@ -49,6 +49,6 @@ module: "test.example/05@v1"
 language: version: "v0.5.0"
 
 -- none/both.stderr --
-no language version declared in module.cue
+no language version declared in module.cue; run 'cue mod fix'
 -- none/cue.mod/module.cue --
 module: "test.example/none@v1"

cmd/cue/cmd/testdata/script/modtidy_with_version.txtar

@@ -5,7 +5,7 @@
 cmp stderr want-stderr
 
 -- want-stderr --
-no language version declared in module.cue
+no language version declared in module.cue; run 'cue mod fix'
 -- cue.mod/module.cue --
 module: "main.org@v0"
 

cmd/cue/cmd/testdata/script/module_compatibility_backwards.txtar

@@ -37,11 +37,11 @@ stderr '^no language version declared in module.cue$'
 # Upstream development works once `cue mod fix` adds a language.version field.
 # TODO(mvdan): we don't have anything like `cue mod edit -langversion` if the user wants an older version.
 ! exec cue mod tidy --check
-stderr '^no language version declared in module.cue$'
+stderr '^no language version declared in module.cue; run ''cue mod fix''$'
 ! exec cue mod get some.dependency
-stderr '^no language version declared in module.cue$'
+stderr '^no language version declared in module.cue; run ''cue mod fix''$'
 ! exec cue mod publish v0.0.2
-stderr '^no language version declared in module.cue$'
+stderr '^no language version declared in module.cue; run ''cue mod fix''$'
 exec cue mod fix
 cmp cue.mod/module.cue ${WORK}/premodules-module.cue.fixed
 exec cue export

mod/modfile/modfile.go

@@ -220,11 +220,11 @@ var schemaVersionLimits = sync.OnceValue(func() [2]string {
 	return limits
 })
 
-// Parse verifies that the module file has correct syntax.
+// Parse verifies that the module file has correct syntax
+// and follows the schema following the required language.version field.
 // The file name is used for error messages.
 // All dependencies must be specified correctly: with major
-// versions in the module paths and canonical dependency
-// versions.
+// versions in the module paths and canonical dependency versions.
 func Parse(modfile []byte, filename string) (*File, error) {
 	return parse(modfile, filename, true)
 }
@@ -349,8 +349,7 @@ func parse(modfile []byte, filename string, strict bool) (*File, error) {
 		return nil, errors.Wrapf(err, token.NoPos, "cannot determine language version")
 	}
 	if base.Language.Version == "" {
-		// TODO is something different we could do here?
-		return nil, fmt.Errorf("no language version declared in module.cue")
+		return nil, ErrNoLanguageVersion
 	}
 	if !semver.IsValid(base.Language.Version) {
 		return nil, fmt.Errorf("language version %q in module.cue is not valid semantic version", base.Language.Version)
@@ -466,6 +465,10 @@ func parse(modfile []byte, filename string, strict bool) (*File, error) {
 	return mf, nil
 }
 
+// ErrNoLanguageVersion is returned by [Parse] and [ParseNonStrict]
+// when a cue.mod/module.cue file lacks the `language.version` field.
+var ErrNoLanguageVersion = fmt.Errorf("no language version declared in module.cue")
+
 func parseDataOnlyCUE(ctx *cue.Context, cueData []byte, filename string) (*ast.File, error) {
 	dec := encoding.NewDecoder(ctx, &build.File{
 		Filename:       filename,