[Parser] Explicit keyword allow-list for contextual keyword prefixed

Author: rintaro

Sources/SwiftParser/Statements.swift

@@ -938,14 +938,19 @@ extension TokenConsumer {
       return true
 
     case .keyword:
+      // Some lexer-classified keywords can start expressions.
       switch Keyword(next.tokenText) {
-      case .as, .is, .in:
-        // E.g. <word> is <expr>
-        return false
-      default:
-        // Other lexer-classified keywords are identifier-like.
-        // E.g. <word> self
+      case .Any, .Self, .self, .super, .`init`, .true, .false, .nil:
+        return true
+      case .repeat, .try:
+        return true
+      case .if, .switch:
         return true
+      case .do where self.experimentalFeatures.contains(.doExpressions):
+        return true
+
+      default:
+        return false
       }
 
     case .binaryOperator, .equal, .arrow, .infixQuestionMark:

Tests/SwiftParserTest/StatementTests.swift

@@ -609,21 +609,22 @@ final class StatementTests: ParserTestCase {
 
     assertParse(
       """
-      discard 1️⃣case
+      discard1️⃣ 2️⃣case
       """,
       diagnostics: [
         DiagnosticSpec(
           locationMarker: "1️⃣",
-          message: "expected expression in 'discard' statement",
-          fixIts: ["insert expression"]
+          message: "consecutive statements on a line must be separated by newline or ';'",
+          fixIts: ["insert newline", "insert ';'"]
         ),
         DiagnosticSpec(
-          locationMarker: "1️⃣",
+          locationMarker: "2️⃣",
           message: "'case' can only appear inside a 'switch' statement or 'enum' declaration"
         ),
       ],
       fixedSource: """
-        discard <#expression#>case
+        discard
+        case
         """
     )