mod/modfile: add support for "source" field

This adds experimental support for the source field
as described in this proposal:
https://github.com/cue-lang/proposal/blob/main/designs/modules.v3/3017-module-files.md

For #3017.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: If6d440be9b63c33800b41881b9c87f5a00a5d17f
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1192905
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

mod/modfile/modfile.go

@@ -45,13 +45,20 @@ var moduleSchemaData []byte
 type File struct {
 	Module   string          `json:"module"`
 	Language *Language       `json:"language,omitempty"`
+	Source   *Source         `json:"source,omitempty"`
 	Deps     map[string]*Dep `json:"deps,omitempty"`
 	versions []module.Version
 	// defaultMajorVersions maps from module base path to the
 	// major version default for that path.
 	defaultMajorVersions map[string]string
 }
 
+// Source represents how to transform from a module's
+// source to its actual contents.
+type Source struct {
+	Kind string `json:"kind"`
+}
+
 // Format returns a formatted representation of f
 // in CUE syntax.
 func (f *File) Format() ([]byte, error) {

mod/modfile/modfile_test.go

@@ -78,6 +78,63 @@ deps: "other.com/something@v0": v: "v0.2.3"
 		"foo.com/bar": "v0",
 		"example.com": "v1",
 	},
+}, {
+	testName: "WithSource",
+	parse:    Parse,
+	data: `
+language: version: "v0.4.3"
+module: "foo.com/bar@v0"
+source: kind: "git"
+`,
+	want: &File{
+		Language: &Language{
+			Version: "v0.4.3",
+		},
+		Module: "foo.com/bar@v0",
+		Source: &Source{
+			Kind: "git",
+		},
+	},
+	wantDefaults: map[string]string{
+		"foo.com/bar": "v0",
+	},
+}, {
+	testName: "WithExplicitSource",
+	parse:    Parse,
+	data: `
+language: version: "v0.4.3"
+module: "foo.com/bar@v0"
+source: kind: "self"
+`,
+	want: &File{
+		Language: &Language{
+			Version: "v0.4.3",
+		},
+		Module: "foo.com/bar@v0",
+		Source: &Source{
+			Kind: "self",
+		},
+	},
+	wantDefaults: map[string]string{
+		"foo.com/bar": "v0",
+	},
+}, {
+	testName: "WithUnknownSourceKind",
+	parse:    Parse,
+	data: `
+language: version: "v0.4.3"
+module: "foo.com/bar@v0"
+source: kind: "bad"
+`,
+	wantError: `source.kind: 2 errors in empty disjunction:
+source.kind: conflicting values "git" and "bad":
+    cuelang.org/go/mod/modfile/schema.cue:45:11
+    cuelang.org/go/mod/modfile/schema.cue:166:18
+    module.cue:4:15
+source.kind: conflicting values "self" and "bad":
+    cuelang.org/go/mod/modfile/schema.cue:45:11
+    cuelang.org/go/mod/modfile/schema.cue:166:9
+    module.cue:4:15`,
 }, {
 	testName: "AmbiguousDefaults",
 	parse:    Parse,

mod/modfile/schema.cue

@@ -40,6 +40,10 @@ let unimplemented = 1&2
 	// module indicates the module's path.
 	module?: #Module | ""
 
+	// source holds information about the source of the files within the
+	// module. This field is mandatory at publish time.
+	source?: #Source
+
 	// version indicates the language version used by the code
 	// in this module - the minimum version of CUE required
 	// to evaluate the code in this module. When a later version of CUE
@@ -151,3 +155,16 @@ let unimplemented = 1&2
 	// TODO require the CUE version?
 	// language!: version!: _
 }
+
+// #Source describes a source of truth for a module's content.
+#Source: {
+	// kind specifies the kind of source.
+	//
+	// The special value "none" signifies that the module that is
+	// standalone, associated with no particular source other than
+	// the contents of the module itself.
+	kind!: "self" | "git"
+
+	// TODO support for other VCSs:
+	// kind!: "self" | "git" | "bzr" | "hg" | "svn"
+}