cue/load: deprecate Config.StdRoot

This field has not had the intended effect ever since
commit 96ef9aa,
and it seems dubious in principle anyway, so deprecate it.

Also factor out the "is standard library" functionality
to its own function and add a TODO in passing.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: Idf055a8139393908acdaf35820a005d8241ec4a0
Dispatch-Trailer: {"type":"trybot","CL":1185361,"patchset":4,"ref":"refs/changes/61/1185361/4","targetBranch":"master"}

cue/load/config.go

@@ -252,7 +252,7 @@ type Config struct {
 	DataFiles bool
 
 	// StdRoot specifies an alternative directory for standard libraries.
-	// This is mostly used for bootstrapping.
+	// Deprecated: this has no effect.
 	StdRoot string
 
 	// ParseFile is called to read and parse each file when preparing a
@@ -310,6 +310,7 @@ type fsPath string
 
 func addImportQualifier(pkg importPath, name string) (importPath, errors.Error) {
 	if name != "" {
+		// TODO use module.ParseImportPath
 		s := string(pkg)
 		if i := strings.LastIndexByte(s, '/'); i >= 0 {
 			s = s[i+1:]

cue/load/import.go

@@ -213,13 +213,8 @@ func (l *loader) _loadFunc(pos token.Pos, path string) *build.Instance {
 		return l.cfg.newErrInstance(errors.Newf(pos, "relative import paths not allowed (%q)", path))
 	}
 
-	// is it a builtin?
-	if strings.IndexByte(strings.Split(path, "/")[0], '.') == -1 {
-		if l.cfg.StdRoot != "" {
-			p := l.newInstance(pos, impPath)
-			_ = l.importPkg(pos, p)
-			return p
-		}
+	if isStdlibPackage(path) {
+		// It looks like a builtin.
 		return nil
 	}
 
@@ -408,3 +403,10 @@ func absPathForSourceLoc(loc module.SourceLoc) (string, error) {
 	}
 	return filepath.Join(osPath, loc.Dir), nil
 }
+
+// isStdlibPackage reports whether pkgPath looks like
+// an import from the standard library.
+func isStdlibPackage(pkgPath string) bool {
+	firstElem, _, _ := strings.Cut(pkgPath, "/")
+	return strings.IndexByte(firstElem, '.') == -1
+}