Add diagnostic message for identifier editor placeholder

Author: kimdv

Sources/SwiftParser/Lexer/Cursor.swift

@@ -2058,6 +2058,7 @@ extension Lexer.Cursor {
 extension Lexer.Cursor {
   mutating func tryLexEditorPlaceholder(sourceBufferStart: Lexer.Cursor) -> Lexer.Result {
     assert(self.is(at: "<") && self.is(offset: 1, at: "#"))
+    let start = self
     var ptr = self
     let leftAngleConsumed = ptr.advance(matching: "<")
     let poundConsumed = ptr.advance(matching: "#")
@@ -2072,7 +2073,10 @@ extension Lexer.Cursor {
         let closingAngleConsumed = ptr.advance(matching: ">")
         assert(closingAngleConsumed)
         self = ptr
-        return Lexer.Result(.identifier)
+          return Lexer.Result(
+            .identifier,
+            error: LexingDiagnostic(.invalidIdentifierStartCharacter, position: start)
+          )
       default:
         break
       }

Sources/SwiftParserDiagnostics/LexerDiagnosticMessages.swift

@@ -38,6 +38,7 @@ public extension TokenError {
 
 /// Please order the cases in this enum alphabetically by case name.
 public enum StaticTokenError: String, DiagnosticMessage {
+  case editorPlaceholder = "editor placeholder in source file"
   case expectedBinaryExponentInHexFloatLiteral = "hexadecimal floating point literal must end with an exponent"
   case expectedClosingBraceInUnicodeEscape = #"expected '}' in \u{...} escape sequence"#
   case expectedDigitInFloatLiteral = "expected a digit in floating point exponent"
@@ -132,6 +133,7 @@ public extension SwiftSyntax.TokenDiagnostic {
     }
 
     switch self.kind {
+    case .editorPlaceholder: return StaticTokenError.editorPlaceholder
     case .expectedBinaryExponentInHexFloatLiteral: return StaticTokenError.expectedBinaryExponentInHexFloatLiteral
     case .expectedClosingBraceInUnicodeEscape: return StaticTokenError.expectedClosingBraceInUnicodeEscape
     case .expectedDigitInFloatLiteral: return StaticTokenError.expectedDigitInFloatLiteral

Sources/SwiftSyntax/TokenDiagnostic.swift

@@ -22,6 +22,7 @@ public struct TokenDiagnostic: Hashable {
   public enum Kind {
     // Please order these alphabetically
 
+    case editorPlaceholder
     case expectedBinaryExponentInHexFloatLiteral
     case expectedClosingBraceInUnicodeEscape
     case expectedDigitInFloatLiteral
@@ -94,6 +95,7 @@ public struct TokenDiagnostic: Hashable {
 
   public var severity: Severity {
     switch kind {
+    case .editorPlaceholder: return .error
     case .expectedBinaryExponentInHexFloatLiteral: return .error
     case .expectedClosingBraceInUnicodeEscape: return .error
     case .expectedDigitInFloatLiteral: return .error

Tests/SwiftParserTest/translated/IdentifiersTests.swift

@@ -83,10 +83,10 @@ final class IdentifiersTests: XCTestCase {
     AssertParse(
       """
       // Placeholders are recognized as identifiers but with error.
-      func <#some name#>() {}
+      func 1️⃣<#some name#>() {}
       """,
       diagnostics: [
-        // TODO: (good first issue) Old parser expected error on line 2: editor placeholder in source file
+        DiagnosticSpec(message: "editor placeholder in source file")
       ]
     )
   }