Don't treat a # as starting a declaration.

The recent change to rely on the general macro-expansion expression nodes
for `#line` et al broke return statements because we were classifying the
expression as a declaration.

Author: DougGregor

Sources/SwiftParser/Declarations.swift

@@ -174,12 +174,16 @@ extension Parser {
       return RawDeclSyntax(directive)
     case (.poundWarningKeyword, _)?, (.poundErrorKeyword, _)?:
       return self.parsePoundDiagnosticDeclaration()
-    case (.pound, _)?:
-      return RawDeclSyntax(self.parseMacroExpansionDeclaration())
     case nil:
       break
     }
 
+    if (self.at(.pound)) {
+      // FIXME: If we can have attributes for macro expansions, handle this
+      // via DeclarationStart.
+      return RawDeclSyntax(self.parseMacroExpansionDeclaration())
+    }
+
     let attrs = DeclAttributes(
       attributes: self.parseAttributeList(),
       modifiers: self.parseModifierList())

Sources/SwiftParser/RawTokenKindSubset.swift

@@ -574,14 +574,12 @@ enum PoundDeclarationStart: RawTokenKindSubset {
   case poundIfKeyword
   case poundWarningKeyword
   case poundErrorKeyword
-  case pound
 
   init?(lexeme: Lexer.Lexeme) {
     switch lexeme.tokenKind {
     case .poundIfKeyword: self = .poundIfKeyword
     case .poundWarningKeyword: self = .poundWarningKeyword
     case .poundErrorKeyword: self = .poundErrorKeyword
-    case .pound: self = .pound
     default: return nil
     }
   }
@@ -591,7 +589,6 @@ enum PoundDeclarationStart: RawTokenKindSubset {
     case .poundIfKeyword: return .poundIfKeyword
     case .poundWarningKeyword: return .poundWarningKeyword
     case .poundErrorKeyword: return .poundErrorKeyword
-    case .pound: return .pound
     }
   }
 }

Tests/SwiftParserTest/Expressions.swift

@@ -238,6 +238,10 @@ final class ExpressionTests: XCTestCase {
       __COLUMN__
       __FUNCTION__
       __DSO_HANDLE__
+
+      func f() {
+        return #function
+      }
       """
     )
   }