cmd/cue: suggest cue mod fix on all commands to add language.version

We did this for commands like `cue mod tidy` or `cue mod publish`,
but not the rest of commands which load CUE packages like `cue export`.
It makes sense to make that suggestion now that language.version
is required in all cases, not just when tidying or publishing.

Fixes #3232.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I24bdb96de36f1f4586ddd52fe7ad25a4934a2582
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1199278
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Matthew Sackman <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

cmd/cue/cmd/common.go

@@ -517,7 +517,7 @@ func parseArgs(cmd *Command, args []string, cfg *config) (p *buildPlan, err erro
 
 	for _, b := range builds {
 		if b.Err != nil {
-			return nil, b.Err
+			return nil, suggestModCommand(b.Err)
 		}
 		switch {
 		case !b.User:

cmd/cue/cmd/modtidy.go

@@ -102,7 +102,10 @@ func suggestModCommand(err error) error {
 	notTidyErr := new(modload.ErrModuleNotTidy)
 	switch {
 	case errors.Is(err, modfile.ErrNoLanguageVersion):
-		err = fmt.Errorf("%w; run 'cue mod fix'", err)
+		// TODO(mvdan): note that we cannot use standard Go error wrapping here via %w
+		// as then errors.Print calls errors.Errors which reaches for the first CUE error
+		// via errors.As, skipping over any non-CUE-wrapped errors.
+		err = fmt.Errorf("%v; run 'cue mod fix'", err)
 	case errors.As(err, &notTidyErr):
 		if notTidyErr.Reason == "" {
 			err = fmt.Errorf("module is not tidy, use 'cue mod tidy'")

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

@@ -32,7 +32,7 @@ cd premodules-downstream-direct
 # Downstream consumption via `cue export` fails; when the current module lacks
 # a language.version field, we always nudge the user or developer to add one.
 ! exec cue export
-stderr '^no language version declared in module.cue$'
+stderr '^no language version declared in module.cue; run ''cue mod fix''$'
 
 # 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.