Support modifiers in LSP semantic tokens (#4130)

Ref: https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#semanticTokenModifiers

We may as well populate these if we can while we're at it. Starting with
deprecated and "defaultLibrary", aka builtin symbols, seems useful to
report to clients, which can colorize these differently in the editor to
call out their difference.

Author: stefanvanburen

private/buf/buflsp/server.go

@@ -36,6 +36,9 @@ const (
 	maxSymbolResults = 1000
 )
 
+// The subset of SemanticTokenTypes that we support.
+// Must match the order of [semanticTypeLegend].
+// https://microsoft.github.io/language-server-protocol/specifications/lsp/3.18/specification/#textDocument_semanticTokens
 const (
 	semanticTypeType = iota
 	semanticTypeStruct
@@ -45,15 +48,35 @@ const (
 	semanticTypeInterface
 	semanticTypeMethod
 	semanticTypeDecorator
+	semanticTypeNamespace
+)
+
+// The subset of SemanticTokenModifiers that we support.
+// Must match the order of [semanticModifierLegend].
+// Semantic modifiers are encoded as a bitset, hence the shifted iota.
+// https://microsoft.github.io/language-server-protocol/specifications/lsp/3.18/specification/#textDocument_semanticTokens
+const (
+	semanticModifierDeprecated = 1 << iota
+	semanticModifierDefaultLibrary
 )
 
 var (
-	// These slices must match the order of the indices in the above const block.
+	// These slices must match the order of the indices in the above const blocks.
 	semanticTypeLegend = []string{
-		"type", "struct", "variable", "enum",
-		"enumMember", "interface", "method", "decorator",
+		"type",
+		"struct",
+		"variable",
+		"enum",
+		"enumMember",
+		"interface",
+		"method",
+		"decorator",
+		"namespace",
+	}
+	semanticModifierLegend = []string{
+		"deprecated",
+		"defaultLibrary", // maps to builtin values
 	}
-	semanticModifierLegend = []string{}
 )
 
 // server is an implementation of [protocol.Server].
@@ -442,10 +465,13 @@ func (s *server) SemanticTokensFull(
 	)
 	for _, symbol := range file.symbols {
 		var semanticType uint32
+		var semanticModifier uint32
 		if symbol.isOption {
 			semanticType = semanticTypeDecorator
 		} else {
 			switch symbol.ir.Kind() {
+			case ir.SymbolKindPackage:
+				semanticType = semanticTypeNamespace
 			case ir.SymbolKindMessage:
 				semanticType = semanticTypeStruct
 			case ir.SymbolKindEnum:
@@ -454,6 +480,7 @@ func (s *server) SemanticTokensFull(
 				// For predeclared types, we set semanticType to semanticTypeType
 				if symbol.IsBuiltIn() {
 					semanticType = semanticTypeType
+					semanticModifier += semanticModifierDefaultLibrary
 				} else {
 					semanticType = semanticTypeVariable
 				}
@@ -469,6 +496,9 @@ func (s *server) SemanticTokensFull(
 				continue
 			}
 		}
+		if _, ok := symbol.ir.Deprecated().AsBool(); ok {
+			semanticModifier += semanticModifierDeprecated
+		}
 
 		startLocation := symbol.span.Location(symbol.span.Start, positionalEncoding)
 		endLocation := symbol.span.Location(symbol.span.End, positionalEncoding)
@@ -486,7 +516,7 @@ func (s *server) SemanticTokensFull(
 			if i == startLocation.Line {
 				symbolLen -= uint32(startLocation.Column - 1)
 			}
-			encoded = append(encoded, newLine-prevLine, newCol, symbolLen, semanticType, 0)
+			encoded = append(encoded, newLine-prevLine, newCol, symbolLen, semanticType, semanticModifier)
 			prevLine = newLine
 			if i == startLocation.Line {
 				prevCol = uint32(startLocation.Column - 1)