mod/modfile: make it an error to have an empty module path

We are relying on this behavior in the cmd/cue tests but
had no test case for it in mod/modfile. There doesn't
seem any good rationale for allowing an empty module
path, so return an error in that case.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: If078c3716bc3300f1f1fc94f7931bbdd5f2c7eb1
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1207984
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>

Author: rogpeppe

cmd/cue/cmd/testdata/script/modget_bad_modulepath.txtar

@@ -2,9 +2,7 @@
 # path includes the path to the offending file (issue 3259)
 
 ! exec cue mod get example.com/foo
-cmp stderr want-stderr
--- want-stderr --
-cue.mod/module.cue: invalid module path: path "" has no major version
+stderr 'empty module path in.*module.cue'
 -- cue.mod/module.cue --
 module: ""
 language: version: "v0.9.2"

cmd/cue/cmd/testdata/script/registry_with_empty_module_path.txtar

@@ -1,9 +1,5 @@
-exec cue export x.cue
-cmp stdout want-stdout
--- want-stdout --
-{
-    "a": 1
-}
+! exec cue export x.cue
+stderr 'empty module path in .*module\.cue'
 -- cue.mod/module.cue --
 module: ""
 language: version: "v0.8.0"

mod/modfile/modfile.go

@@ -451,6 +451,8 @@ func parse(modfile []byte, filename string, strict bool) (*File, error) {
 		}
 		// There's no main module major version: default to v0.
 		mainMajor = "v0"
+	} else {
+		return nil, fmt.Errorf("empty module path in %q", filename)
 	}
 	if mf.Language != nil {
 		vers := mf.Language.Version
@@ -462,9 +464,10 @@ func parse(modfile []byte, filename string, strict bool) (*File, error) {
 		}
 	}
 	var versions []module.Version
-	// The main module is always the default for its own major version.
-	defaultMajorVersions := map[string]string{
-		mainPath: mainMajor,
+	defaultMajorVersions := make(map[string]string)
+	if mainPath != "" {
+		// The main module is always the default for its own major version.
+		defaultMajorVersions[mainPath] = mainMajor
 	}
 	// Check that major versions match dependency versions.
 	for m, dep := range mf.Deps {

mod/modfile/modfile_test.go

@@ -300,6 +300,11 @@ _foo: "blah.example"
 	parse:    ParseLegacy,
 	data:     `module: ""`,
 	want:     &File{},
+}, {
+	testName:  "NonLegacyEmptyModule",
+	parse:     Parse,
+	data:      `module: "", language: version: "v0.8.0"`,
+	wantError: `empty module path in "module.cue"`,
 }, {
 	testName: "ReferencesNotAllowed#1",
 	parse:    Parse,