gopls/internal/golang: add semantic tokens for control labels

Fixes golang/go#65494

Change-Id: Id044cd12a5c48bb3d5dc8f91657febdc431811b4
Reviewed-on: https://go-review.googlesource.com/c/tools/+/562244
Reviewed-by: Peter Weinberger <pjw@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

gopls/internal/golang/semtok.go

@@ -231,7 +231,9 @@ func (tv *tokenVisitor) inspect(n ast.Node) (descend bool) {
 	case *ast.BlockStmt:
 	case *ast.BranchStmt:
 		tv.token(n.TokPos, len(n.Tok.String()), semtok.TokKeyword, nil)
-		// There's no semantic encoding for labels
+		if n.Label != nil {
+			tv.token(n.Label.Pos(), len(n.Label.Name), semtok.TokLabel, nil)
+		}
 	case *ast.CallExpr:
 		if n.Ellipsis.IsValid() {
 			tv.token(n.Ellipsis, len("..."), semtok.TokOperator, nil)
@@ -303,6 +305,7 @@ func (tv *tokenVisitor) inspect(n ast.Node) (descend bool) {
 		tv.token(n.Interface, len("interface"), semtok.TokKeyword, nil)
 	case *ast.KeyValueExpr:
 	case *ast.LabeledStmt:
+		tv.token(n.Label.Pos(), len(n.Label.Name), semtok.TokLabel, []string{"definition"})
 	case *ast.MapType:
 		tv.token(n.Map, len("map"), semtok.TokKeyword, nil)
 	case *ast.ParenExpr:
@@ -411,7 +414,7 @@ func (tv *tokenVisitor) ident(id *ast.Ident) {
 	case *types.Func:
 		emit(semtok.TokFunction)
 	case *types.Label:
-		// nothing to map it to
+		// Labels are reliably covered by the syntax traversal.
 	case *types.Nil:
 		// nil is a predeclared identifier
 		emit(semtok.TokVariable, "readonly", "defaultLibrary")
@@ -571,8 +574,14 @@ func (tv *tokenVisitor) unkIdent(id *ast.Ident) (semtok.TokenType, []string) {
 			return semtok.TokMethod, def
 		}
 		return semtok.TokVariable, nil
-	case *ast.LabeledStmt, *ast.BranchStmt:
-		// nothing to report
+	case *ast.LabeledStmt:
+		if id == parent.Label {
+			return semtok.TokLabel, def
+		}
+	case *ast.BranchStmt:
+		if id == parent.Label {
+			return semtok.TokLabel, nil
+		}
 	case *ast.CompositeLit:
 		if parent.Type == id {
 			return semtok.TokType, nil
@@ -604,7 +613,17 @@ func isDeprecated(n *ast.CommentGroup) bool {
 
 // definitionFor handles a defining identifier.
 func (tv *tokenVisitor) definitionFor(id *ast.Ident, obj types.Object) (semtok.TokenType, []string) {
-	// PJW: obj == types.Label? probably a nothing
+	// The definition of a types.Label cannot be found by
+	// ascending the syntax tree, and doing so will reach the
+	// FuncDecl, causing us to misinterpret the label as a
+	// parameter (#65494).
+	//
+	// However, labels are reliably covered by the syntax
+	// traversal, so we don't need to use type information.
+	if is[*types.Label](obj) {
+		return "", nil
+	}
+
 	// PJW: look into replacing these syntactic tests with types more generally
 	modifiers := []string{"definition"}
 	for i := len(tv.stack) - 1; i >= 0; i-- {

gopls/internal/protocol/semantic.go

@@ -8,7 +8,7 @@ package protocol
 
 import "fmt"
 
-// SemanticTypes to use in case there is no client, as in the command line, or tests
+// SemanticTypes to use in case there is no client, as in the command line, or tests.
 func SemanticTypes() []string {
 	return semanticTypes[:]
 }

gopls/internal/protocol/semtok/semtok.go

@@ -18,6 +18,9 @@ type Token struct {
 type TokenType string
 
 const (
+	// These are the tokens defined by LSP 3.17, but a client is
+	// free to send its own set; any tokens that the server emits
+	// that are not in this set are simply not encoded in the bitfield.
 	TokNamespace TokenType = "namespace"
 	TokType      TokenType = "type"
 	TokInterface TokenType = "interface"
@@ -32,6 +35,10 @@ const (
 	TokNumber    TokenType = "number"
 	TokOperator  TokenType = "operator"
 	TokMacro     TokenType = "macro" // for templates
+
+	// not part of LSP 3.17 (even though JS has labels)
+	// https://github.com/microsoft/vscode-languageserver-node/issues/1422
+	TokLabel TokenType = "label"
 )
 
 // Encode returns the LSP encoding of a sequence of tokens.

gopls/internal/test/integration/fake/editor.go

@@ -337,6 +337,8 @@ func clientCapabilities(cfg EditorConfig) (protocol.ClientCapabilities, error) {
 		"struct", "typeParameter", "parameter", "variable", "property", "enumMember",
 		"event", "function", "method", "macro", "keyword", "modifier", "comment",
 		"string", "number", "regexp", "operator",
+		// Additional types supported by this client:
+		"label",
 	}
 	capabilities.TextDocument.SemanticTokens.TokenModifiers = []string{
 		"declaration", "definition", "readonly", "static",

gopls/internal/test/marker/testdata/token/range.txt

@@ -17,3 +17,13 @@ func F() { //@token("F", "function", "definition")
 	_ = x //@token("x", "variable", "")
 	_ = F //@token("F", "function", "")
 }
+
+func _() {
+	// A goto's label cannot be found by ascending the syntax tree.
+	goto loop //@ token("goto", "keyword", ""), token("loop", "label", "")
+
+loop: //@token("loop", "label", "definition")
+	for {
+		continue loop //@ token("continue", "keyword", ""), token("loop", "label", "")
+	}
+}