mod/modfile: check canonical language version

There are a couple of ways that non-canonical version
could find its way into a parser module file. This
CL makes sure that a version is always present and canonical
as long as the `language` field is present.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I39663d73bf2d73146d9d0533bfa5cd81611ff974
Dispatch-Trailer: {"type":"trybot","CL":1178014,"patchset":1,"ref":"refs/changes/14/1178014/1","targetBranch":"master"}

mod/modfile/modfile.go

@@ -207,9 +207,13 @@ func parse(modfile []byte, filename string, strict bool) (*File, error) {
 		mf.Module += "@v0"
 	}
 	if mf.Language != nil {
-		if vers := mf.Language.Version; vers != "" && !semver.IsValid(vers) {
+		vers := mf.Language.Version
+		if !semver.IsValid(vers) {
 			return nil, fmt.Errorf("language version %q in %s is not well formed", vers, filename)
 		}
+		if semver.Canonical(vers) != vers {
+			return nil, fmt.Errorf("language version %v in %s is not canonical", vers, filename)
+		}
 	}
 	var versions []module.Version
 	// The main module is always the default for its own major version.

mod/modfile/modfile_test.go

@@ -122,6 +122,21 @@ module: "foo.com/bar@v0"
 language: version: "vblah"
 module: "foo.com/bar@v0"`,
 	wantError: `language version "vblah" in module.cue is not well formed`,
+}, {
+	testName: "EmptyLanguageVersion",
+	parse:    Parse,
+	data: `
+language: {}
+module: "foo.com/bar@v0"`,
+	wantError: `language version "" in module.cue is not well formed`,
+}, {
+	testName: "NonCanonicalLanguageVersion",
+	parse:    Parse,
+	data: `
+module: "foo.com/bar@v0"
+language: version: "v0.8"
+`,
+	wantError: `language version v0.8 in module.cue is not canonical`,
 }, {
 	testName: "InvalidDepVersion",
 	parse:    Parse,