mod/modfile: add custom data field

This provides a place to put data that users might currently
be storing at the top level of a module.cue file.

We define a two level map to make it easy for people to "do the
right thing" and avoid namespace conflicts and/or ambiguities
between different tools using the same space. The top level
defines the namespace for a set of custom data; the next
level defines attributes within that namespace. This approach
also makes it easier to define validation schemas for
custom data.

The "legacy" namespace is defined as a place to put existing
top level attributes so that there's a default place for an automated
migration tool to put them.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I2ceb5161406bce584e3fd0e947adb43233f2fdce
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194670
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

mod/modfile/modfile.go

@@ -48,10 +48,11 @@ const schemaFile = "cuelang.org/go/mod/modfile/schema.cue"
 
 // File represents the contents of a cue.mod/module.cue file.
 type File struct {
-	Module   string          `json:"module"`
-	Language *Language       `json:"language,omitempty"`
-	Source   *Source         `json:"source,omitempty"`
-	Deps     map[string]*Dep `json:"deps,omitempty"`
+	Module   string                    `json:"module"`
+	Language *Language                 `json:"language,omitempty"`
+	Source   *Source                   `json:"source,omitempty"`
+	Deps     map[string]*Dep           `json:"deps,omitempty"`
+	Custom   map[string]map[string]any `json:"custom,omitempty"`
 	versions []module.Version
 	// defaultMajorVersions maps from module base path to the
 	// major version default for that path.

mod/modfile/modfile_test.go

@@ -276,7 +276,7 @@ deps: "example.com": v: "v1.2.3"
 	data: `
 module: "foo.com/bar"
 something: 4
-cue: lang: "xxx"
+language: version: "xxx"
 `,
 	want: &File{
 		Module: "foo.com/bar",
@@ -296,32 +296,52 @@ _foo: "blah.example"
 	data: `
 module: "foo.com/bar"
 _foo: "v0.9.0"
-cue: lang: _foo
+language: version: _foo
 `,
 	wantError: `invalid module.cue file syntax: references not allowed in data mode:
-    module.cue:4:12`,
+    module.cue:4:20`,
 }, {
 	testName: "ReferencesNotAllowed#2",
 	parse:    Parse,
 	data: `
 module: "foo.com/bar"
 let foo = "v0.9.0"
-cue: lang: foo
+language: version: foo
 `,
 	wantError: `invalid module.cue file syntax: references not allowed in data mode:
     module.cue:3:1
 invalid module.cue file syntax: references not allowed in data mode:
-    module.cue:4:12`,
+    module.cue:4:20`,
 }, {
 	testName: "DefinitionsNotAllowed",
 	parse:    Parse,
 	data: `
 module: "foo.com/bar"
 #x: "v0.9.0"
-cue: lang: "v0.9.0"
+language: version: "v0.9.0"
 `,
 	wantError: `invalid module.cue file syntax: definitions not allowed in data mode:
     module.cue:3:1`,
+}, {
+	testName: "CustomData",
+	parse:    Parse,
+	data: `
+module: "foo.com/bar@v0"
+language: version: "v0.9.0"
+custom: "somewhere.com": foo: true
+`,
+	want: &File{
+		Module:   "foo.com/bar@v0",
+		Language: &Language{Version: "v0.9.0"},
+		Custom: map[string]map[string]any{
+			"somewhere.com": {
+				"foo": true,
+			},
+		},
+	},
+	wantDefaults: map[string]string{
+		"foo.com/bar": "v0",
+	},
 }}
 
 func TestParse(t *testing.T) {
@@ -334,7 +354,7 @@ func TestParse(t *testing.T) {
 				return
 			}
 			qt.Assert(t, qt.IsNil(err), qt.Commentf("details: %v", strings.TrimSuffix(errors.Details(err, nil), "\n")))
-			qt.Assert(t, qt.CmpEquals(f, test.want, cmpopts.IgnoreUnexported(File{})))
+			qt.Assert(t, fileEquals(f, test.want))
 			qt.Assert(t, qt.DeepEquals(f.DepVersions(), test.wantVersions))
 			qt.Assert(t, qt.DeepEquals(f.DefaultMajorVersions(), test.wantDefaults))
 		})
@@ -433,7 +453,7 @@ language: {
 		// Check that it round-trips.
 		f, err := Parse(data, "")
 		qt.Assert(t, qt.IsNil(err))
-		qt.Assert(t, qt.CmpEquals(f, test.file, cmpopts.IgnoreUnexported(File{}), cmpopts.EquateEmpty()))
+		qt.Assert(t, fileEquals(f, test.file))
 	})
 }
 
@@ -452,3 +472,12 @@ func parseVersions(vs ...string) []module.Version {
 	}
 	return vvs
 }
+
+// fileEquals returns a checker that checks whether two File instances
+// are equal.
+func fileEquals(got, want *File) qt.Checker {
+	return qt.CmpEquals(got, want,
+		cmpopts.IgnoreUnexported(File{}),
+		cmpopts.EquateEmpty(),
+	)
+}

mod/modfile/schema.cue

@@ -73,6 +73,13 @@ versions: "v0.9.0-alpha.0": {
 		// deps holds dependency information for modules, keyed by module path.
 		deps?: [#Module]: #Dep
 
+		// custom holds arbitrary data intended for use by
+		// third-party tools. Each field at the top level
+		// represents a tooling namespace, conventionally
+		// a module or domain name. Data migrated from legacy
+		// module.cue files is placed in the "legacy" namespace.
+		custom?: [#Module | "legacy"]: [_]: _
+
 		#Dep: {
 			// v indicates the minimum required version of the module.
 			// This can be null if the version is unknown and the module