Disable the ability to use '$' as an identifier harder

When in Swift 3 Compatibility Mode we now acceptable a standalone
'$' as an identifier.  In all other cases this is now disallowed
and must be surrounded by backticks.

include/swift/AST/DiagnosticsParse.def

@@ -994,7 +994,9 @@ ERROR(expected_dollar_numeric,none,
 ERROR(dollar_numeric_too_large,none,
       "numeric value following '$' is too large", ())
 ERROR(numeric_literal_numeric_member,none,
-   "expected named member of numeric literal", ())
+      "expected named member of numeric literal", ())
+ERROR(standalone_dollar_identifier,none,
+     "'$' is not an identifier; use backticks to escape it", ())
 
 ERROR(anon_closure_arg_not_in_closure,none,
       "anonymous closure argument not contained in a closure", ())

lib/Basic/StringExtras.cpp

@@ -28,7 +28,8 @@ using namespace swift;
 using namespace camel_case;
 
 bool swift::canBeArgumentLabel(StringRef identifier) {
-  if (identifier == "var" || identifier == "let" || identifier == "inout")
+  if (identifier == "var" || identifier == "let" || identifier == "inout" ||
+      identifier == "$")
     return false;
 
   return true;

lib/Parse/Lexer.cpp

@@ -474,7 +474,7 @@ static bool isValidIdentifierContinuationCodePoint(uint32_t c) {
 static bool isValidIdentifierStartCodePoint(uint32_t c) {
   if (!isValidIdentifierContinuationCodePoint(c))
     return false;
-  if (c < 0x80 && (isDigit(c) || c == '$'))
+  if (c < 0x80 && isDigit(c))
     return false;
 
   // N1518: Recommendations for extended identifier characters for C and C++
@@ -819,10 +819,15 @@ void Lexer::lexDollarIdent() {
     }
   }
 
-  // It's always an error to see a standalone $
   if (CurPtr == tokStart + 1) {
-    diagnose(tokStart, diag::expected_dollar_numeric);
+    // It is always an error to see a standalone '$' when not in Swift 3
-    return formToken(tok::unknown, tokStart);
+    // compatibility mode.
+    if (!LangOpts.isSwiftVersion3()) {
+      // Offer to replace '$' with '`$`'.
+      diagnose(tokStart, diag::standalone_dollar_identifier)
+        .fixItReplaceChars(getSourceLoc(tokStart), getSourceLoc(CurPtr), "`$`");
+    }
+    return formToken(tok::identifier, tokStart);
   }
 
   // We reserve $nonNumeric for persistent bindings in the debugger.

test/Compatibility/dollar_identifier.swift

@@ -0,0 +1,23 @@
+// RUN: %target-parse-verify-swift -swift-version 3
+
+// Dollar is allowed as an identifier head in Swift 3.
+
+func dollarVar() {
+  var $ : Int = 42 // No error
+  $ += 1
+  print($)
+}
+func dollarLet() {
+  let $ = 42 // No error
+  print($)
+}
+func dollarClass() {
+  class $ {} // No error
+}
+func dollarEnum() {
+  enum $ {} // No error
+}
+func dollarStruct() {
+  struct $ {} // No error
+}
+

test/Parse/dollar_identifier.swift

@@ -1,29 +1,56 @@
-// RUN: %target-parse-verify-swift
+// RUN: %target-parse-verify-swift -swift-version 4
 
-// SR-1661: Dollar was accidentally allowed as an identifier and identifier head.
+// SR-1661: Dollar was accidentally allowed as an identifier in Swift 3.
+// SE-0144: Reject this behavior in the future.
 
 func dollarVar() {
-  var $ : Int = 42 // expected-error {{expected numeric value following '$'}} expected-error {{expected pattern}}
+  var $ : Int = 42 // expected-error {{'$' is not an identifier; use backticks to escape it}} {{7-8=`$`}}
+  $ += 1 // expected-error {{'$' is not an identifier; use backticks to escape it}} {{3-4=`$`}}
+  print($) // expected-error {{'$' is not an identifier; use backticks to escape it}} {{9-10=`$`}}
 }
 func dollarLet() {
-  let $ = 42 // expected-error {{expected numeric value following '$'}} expected-error {{expected pattern}} 
+  let $ = 42 // expected-error {{'$' is not an identifier; use backticks to escape it}} {{7-8=`$`}}
+  print($) // expected-error {{'$' is not an identifier; use backticks to escape it}} {{9-10=`$`}}
 }
 func dollarClass() {
-  class $ {} // expected-error {{expected numeric value following '$'}} 
+  class $ {} // expected-error {{'$' is not an identifier; use backticks to escape it}} {{9-10=`$`}}
-             // expected-error@-1 {{expression resolves to an unused function}}
-             // expected-error@-2 {{expected identifier in class declaration}}
-             // expected-error@-3 {{braced block of statements is an unused closure}}
 }
 func dollarEnum() {
-  enum $ {} // expected-error {{expected numeric value following '$'}} 
+  enum $ {} // expected-error {{'$' is not an identifier; use backticks to escape it}} {{8-9=`$`}}
-            // expected-error@-1 {{expected identifier in enum declaration}}
-            // expected-error@-2 {{expression resolves to an unused function}}
-            // expected-error@-3 {{braced block of statements is an unused closure}}
 }
 func dollarStruct() {
-  struct $ {} // expected-error {{expected numeric value following '$'}}
+  struct $ {} // expected-error {{'$' is not an identifier; use backticks to escape it}} {{10-11=`$`}}
-              // expected-error@-1 {{expected identifier in struct declaration}}
-              // expected-error@-2 {{braced block of statements is an unused closure}}
-              // expected-error@-3 {{expression resolves to an unused function}}
 }
 
+func dollarFunc() {
+  func $($ dollarParam: Int) {}
+  // expected-error@-1 {{'$' is not an identifier; use backticks to escape it}} {{8-9=`$`}}
+  // expected-error@-2 {{'$' is not an identifier; use backticks to escape it}} {{10-11=`$`}}
+  $($: 24)
+  // expected-error@-1 {{'$' is not an identifier; use backticks to escape it}} {{3-4=`$`}}
+  // expected-error@-2 {{'$' is not an identifier; use backticks to escape it}} {{5-6=`$`}}
+}
+
+func escapedDollarVar() {
+  var `$` : Int = 42 // no error
+  `$` += 1
+  print(`$`)
+}
+func escapedDollarLet() {
+  let `$` = 42 // no error
+  print(`$`)
+}
+func escapedDollarClass() {
+  class `$` {} // no error
+}
+func escapedDollarEnum() {
+  enum `$` {} // no error
+}
+func escapedDollarStruct() {
+  struct `$` {} // no error
+}
+
+func escapedDollarFunc() {
+  func `$`(`$`: Int) {} // no error
+  `$`(`$`: 25) // no error
+}