mod/modfile: use fixed language version for Fix

CUE currently uses the latest known language version when fixing legacy
module files.

This is probably incorrect, at least in the future, because the CUE
version used by such legacy modules is likely to be an older version and
so use older language features that might be deprecated or altered by a
newer language version.

To fix that, so we can always use a specific version (v0.9.0) when
fixing such files.

Fixes #3222.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: Ie525e069548c9b8714e72b058e2f8322cb94f5dd
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1197598
Reviewed-by: Paul Jolly <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: rogpeppe

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

@@ -14,7 +14,7 @@ foo: "bar"
 -- want-module --
 module: "foo.com"
 language: {
-	version: "v0.10.0"
+	version: "v0.9.0"
 }
 custom: {
 	legacy: {

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

@@ -99,7 +99,7 @@ module: "premodules.example"
 -- premodules-module.cue.fixed --
 module: "premodules.example"
 language: {
-	version: "v0.10.0"
+	version: "v0.9.0"
 }
 -- premodules/cue.mod/pkg/premodules.example/root.cue --
 package root

mod/modfile/modfile.go

@@ -350,7 +350,12 @@ func FixLegacy(modfile []byte, filename string) (*File, error) {
 	f = &File{
 		Module: mpath,
 		Language: &Language{
-			Version: cueversion.LanguageVersion(),
+			// If there's a legacy module file, the CUE code
+			// is unlikely to be using new language features,
+			// so keep the language version fixed rather than
+			// using cueversion.LanguageVersion().
+			// See https://cuelang.org/issue/3222.
+			Version: "v0.9.0",
 		},
 		Custom: custom,
 	}

mod/modfile/modfile_test.go

@@ -361,7 +361,7 @@ module: "foo.com/bar"
 `,
 	want: &File{
 		Module:   "foo.com/bar",
-		Language: &Language{Version: "v0.10.0"},
+		Language: &Language{Version: "v0.9.0"},
 	},
 	wantDefaults: map[string]string{
 		"foo.com/bar": "v0",
@@ -373,7 +373,7 @@ module: "foo.com/bar"
 `,
 	want: &File{
 		Module:   "test.example",
-		Language: &Language{Version: "v0.10.0"},
+		Language: &Language{Version: "v0.9.0"},
 	},
 	wantDefaults: map[string]string{
 		"test.example": "v0",
@@ -386,7 +386,7 @@ module: ""
 `,
 	want: &File{
 		Module:   "test.example",
-		Language: &Language{Version: "v0.10.0"},
+		Language: &Language{Version: "v0.9.0"},
 	},
 	wantDefaults: map[string]string{
 		"test.example": "v0",
@@ -401,7 +401,7 @@ other: field: 123
 `,
 	want: &File{
 		Module:   "foo.com",
-		Language: &Language{Version: "v0.10.0"},
+		Language: &Language{Version: "v0.9.0"},
 		Custom: map[string]map[string]any{
 			"legacy": {
 				"some":  true,