mod/modfile: better error message when disallowed field is used

The current error message looks like this:

```
source: conflicting values 1 and {kind!:("self"|"git")} (mismatched types int and struct):
    cuelang.org/go/mod/modfile/schema.cue:24:21
    cuelang.org/go/mod/modfile/schema.cue:65:12
    cuelang.org/go/mod/modfile/schema.cue:122:11
source: conflicting values 2 and 1:
    cuelang.org/go/mod/modfile/schema.cue:24:21
    cuelang.org/go/mod/modfile/schema.cue:24:25
    cuelang.org/go/mod/modfile/schema.cue:55:18
```

The new error message is somewhat improved:

```
instance: invalid module.cue file: source field is not allowed at this language version; need at least v0.9.0-alpha.0
```

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I81481a90fcd5a4478fed8c8b6711b878b37ee461
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1193895
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: rogpeppe

mod/modfile/modfile.go

@@ -40,7 +40,9 @@ import (
 )
 
 //go:embed schema.cue
-var moduleSchemaData []byte
+var moduleSchemaData string
+
+const schemaFile = "cuelang.org/go/mod/modfile/schema.cue"
 
 // File represents the contents of a cue.mod/module.cue file.
 type File struct {
@@ -152,10 +154,14 @@ type schemaInfo struct {
 	EarliestClosedSchemaVersion string               `json:"earliestClosedSchemaVersion"`
 }
 
+// moduleSchemaDo runs f with information about all the schema versions
+// present in schema.cue. It does this within a mutex because it is
+// not currently allowed to use cue.Value concurrently.
+// TODO remove the mutex when https://cuelang.org/issue/2733 is fixed.
 func moduleSchemaDo[T any](f func(*cue.Context, *schemaInfo) (T, error)) (T, error) {
 	moduleSchemaOnce.Do(func() {
 		_cueContext = cuecontext.New()
-		schemav := _cueContext.CompileBytes(moduleSchemaData, cue.Filename("cuelang.org/go/mod/modfile/schema.cue"))
+		schemav := _cueContext.CompileString(moduleSchemaData, cue.Filename(schemaFile))
 		if err := schemav.Decode(&_schemas); err != nil {
 			panic(fmt.Errorf("internal error: invalid CUE module.cue schema: %v", errors.Details(err, nil)))
 		}
@@ -372,10 +378,55 @@ func parse(modfile []byte, filename string, strict bool) (*File, error) {
 }
 
 func newCUEError(err error, filename string) error {
-	// TODO we have some potential to improve error messages here.
+	ps := errors.Positions(err)
+	for _, p := range ps {
+		if errStr := findErrorComment(p); errStr != "" {
+			return fmt.Errorf("invalid module.cue file: %s", errStr)
+		}
+	}
+	// TODO we have more potential to improve error messages here.
 	return err
 }
 
+// findErrorComment finds an error comment in the form
+//
+//	//error: ...
+//
+// before the given position.
+// This works as a kind of poor-man's error primitive
+// so we can customize the error strings when verification
+// fails.
+func findErrorComment(p token.Pos) string {
+	if p.Filename() != schemaFile {
+		return ""
+	}
+	off := p.Offset()
+	source := moduleSchemaData
+	if off > len(source) {
+		return ""
+	}
+	source, _, ok := cutLast(source[:off], "\n")
+	if !ok {
+		return ""
+	}
+	_, errorLine, ok := cutLast(source, "\n")
+	if !ok {
+		return ""
+	}
+	errStr, ok := strings.CutPrefix(errorLine, "//error: ")
+	if !ok {
+		return ""
+	}
+	return errStr
+}
+
+func cutLast(s, sep string) (before, after string, found bool) {
+	if i := strings.LastIndex(s, sep); i >= 0 {
+		return s[:i], s[i+len(sep):], true
+	}
+	return "", s, false
+}
+
 // DepVersions returns the versions of all the modules depended on by the
 // file. The caller should not modify the returned slice.
 //

mod/modfile/modfile_test.go

@@ -139,7 +139,7 @@ module: "foo.com/bar@v0"
 language: version: "v0.8.6"
 source: kind: "git"
 `,
-	wantError: `source: conflicting values 1 and (.|\n)+`,
+	wantError: `invalid module.cue file: source field is not allowed at this language version; need at least v0.9.0-alpha.0`,
 }, {
 	testName: "AmbiguousDefaults",
 	parse:    Parse,

mod/modfile/schema.cue

@@ -18,11 +18,6 @@
 // used instead, which will be rewritten to the canonical form by the cue command tooling.
 // To check against that form,
 
-// Note: we're using 1&2 rather than _|_ because
-// use of _|_ causes the source location of the errors
-// to be lost. See https://github.com/cue-lang/cue/issues/2319.
-let unimplemented = 1 & 2
-
 // versions holds an element for each supported version
 // of the schema. The version key specifies that
 // the schema covers all versions from then until the
@@ -51,8 +46,8 @@ versions: "v0.8.0": {
 	versions["v0.9.0-alpha.0"]
 
 	// The source field was added in v0.9.0, so "remove"
-	// it here by marking it as unimplemented.
-	#File: source?: unimplemented
+	// it here by marking it as an error when used.
+	#File: source?: _errorSourceFieldRequiredVersion
 }
 
 versions: "v0.9.0-alpha.0": {
@@ -131,3 +126,14 @@ versions: "v0.9.0-alpha.0": {
 		// kind!: "self" | "git" | "bzr" | "hg" | "svn"
 	}
 }
+
+// The //error comments are specially recognized by the parsing
+// code so we can avoid opaque conflict errors.
+// TODO use error function when that's available.
+//
+// Note: we're using 1&2 rather than _|_ because
+// use of _|_ causes the source location of the errors
+// to be lost. See https://github.com/cue-lang/cue/issues/2319.
+
+//error: source field is not allowed at this language version; need at least v0.9.0-alpha.0
+let _errorSourceFieldRequiredVersion = 1&2