Diagnose ObjC-style string literals at top level

Both Swift parsers have always recognized `@"foo"` as an attempt to write a string literal by a programmer with Objective-C muscle memory, but some of SwiftParser’s lookahead helpers didn’t recognize that token sequence; instead they interpreted the `@` as the beginning of an attribute and therefore a missing declaration. Correct them.

Author: beccadax

Sources/SwiftParser/Declarations.swift

@@ -99,6 +99,10 @@ extension TokenConsumer {
       if subparser.at(.rightBrace) || subparser.at(.endOfFile) || subparser.at(.poundEndif) {
         return true
       }
+      if subparser.at(.stringQuote) {
+        // `@"abc"` is an invalid Objective-C-style string literal, not a declaration.
+        return false
+      }
     }
 
     let declStartKeyword: DeclarationKeyword?

Sources/SwiftParser/Expressions.swift

@@ -57,6 +57,10 @@ extension TokenConsumer {
         return true
       }
     }
+    if self.at(.atSign) && self.peek(isAt: .stringQuote) {
+      // Invalid Objective-C-style string literal
+      return true
+    }
 
     // 'repeat' is the start of a pack expansion expression.
     if self.at(.keyword(.repeat)) {

Tests/SwiftParserTest/ExpressionTests.swift

@@ -1214,6 +1214,31 @@ final class ExpressionTests: ParserTestCase {
         DiagnosticSpec(message: "invalid escape sequence in literal")
       ]
     )
+
+    // <rdar://problem/19833424> QoI: Bad error message when using Objective-C literals (@"Hello")
+    assertParse(
+      """
+      _ = 1️⃣@"a"
+      """,
+      diagnostics: [
+        DiagnosticSpec(message: "string literals in Swift are not preceded by an '@' sign", fixIts: ["remove '@'"])
+      ],
+      fixedSource: """
+        _ = "a"
+        """
+    )
+
+    assertParse(
+      """
+      1️⃣@"a"
+      """,
+      diagnostics: [
+        DiagnosticSpec(message: "string literals in Swift are not preceded by an '@' sign", fixIts: ["remove '@'"])
+      ],
+      fixedSource: """
+        "a"
+        """
+    )
   }
 
   func testAdjacentRawStringLiterals() {

Tests/SwiftParserTest/translated/ModuleSelectorTests.swift

@@ -1957,9 +1957,9 @@ final class ModuleSelectorTests: ParserTestCase {
       #"_ = Swift::1️⃣@"fnord""#,
       diagnostics: [
         DiagnosticSpec(message: "expected identifier", fixIts: ["insert identifier"]),
-        DiagnosticSpec(message: #"extraneous code '@"fnord"' at top level"#),
+        DiagnosticSpec(message: "string literals in Swift are not preceded by an '@' sign", fixIts: ["remove '@'"]),
       ],
-      fixedSource: #"_ = Swift::<#identifier#>@"fnord""#
+      fixedSource: #"_ = Swift::<#identifier#>"fnord""#
     )
     assertParse(
       #"_ = Swift::1️⃣"fnord""#,