[Parse] Add fix-it for 'rethrows' in function type

Author: rintaro

include/swift/AST/DiagnosticsParse.def

@@ -620,7 +620,8 @@ ERROR(expected_type_function_result,PointsToFirstBadToken,
 ERROR(generic_non_function,PointsToFirstBadToken,
       "only syntactic function types can be generic", ())
 ERROR(rethrowing_function_type,none,
-      "only function declarations may be marked 'rethrows'", ())
+      "only function declarations may be marked 'rethrows'; "
+      "did you mean 'throws'?", ())
 ERROR(throws_in_wrong_position,none,
       "'throws' may only occur before '->'", ())
 ERROR(rethrows_in_wrong_position,none,

lib/Parse/ParseExpr.cpp

@@ -2200,7 +2200,8 @@ parseClosureSignatureIfPresent(SmallVectorImpl<CaptureListEntry> &captureList,
       throwsLoc = consumeToken();
     } else if (Tok.is(tok::kw_rethrows)) {
       throwsLoc = consumeToken();
-      diagnose(throwsLoc, diag::rethrowing_function_type);
+      diagnose(throwsLoc, diag::rethrowing_function_type)
+        .fixItReplace(throwsLoc, "throws");
     }
 
     // Parse the optional explicit return type.

lib/Parse/ParseType.cpp

@@ -189,12 +189,12 @@ ParserResult<TypeRepr> Parser::parseType(Diag<> MessageID,
   SourceLoc throwsLoc;
   if (Tok.isAny(tok::kw_throws, tok::kw_rethrows, tok::kw_throw) &&
       peekToken().is(tok::arrow)) {
-    if (Tok.is(tok::kw_rethrows)) {
+    if (Tok.isNot(tok::kw_throws)) {
       // 'rethrows' is only allowed on function declarations for now.
-      diagnose(Tok.getLoc(), diag::rethrowing_function_type);
-    } else if (Tok.is(tok::kw_throw)) {
-      // 'throw' is probably a typo for 'throws'
-      diagnose(Tok.getLoc(), diag::throw_in_function_type)
+      // 'throw' is probably a typo for 'throws'.
+      Diag<> DiagID = Tok.is(tok::kw_rethrows) ?
+        diag::rethrowing_function_type : diag::throw_in_function_type;
+      diagnose(Tok.getLoc(), DiagID)
         .fixItReplace(Tok.getLoc(), "throws");
     }
     throwsLoc = consumeToken();

test/decl/func/rethrows.swift

@@ -3,9 +3,9 @@
 /** Basics *******************************************************************/
 
 // Function types can't be rethrows right now.
-let r1 = {() rethrows -> Int in 0} // expected-error {{only function declarations may be marked 'rethrows'}}
-let r2 : () rethrows -> Int = { 0 } // expected-error {{only function declarations may be marked 'rethrows'}}
-let r3 : Optional<() rethrows -> ()> = nil // expected-error {{only function declarations may be marked 'rethrows'}}
+let r1 = {() rethrows -> Int in 0} // expected-error {{only function declarations may be marked 'rethrows'; did you mean 'throws'?}} {{14-22=throws}}
+let r2 : () rethrows -> Int = { 0 } // expected-error {{only function declarations may be marked 'rethrows'; did you mean 'throws'?}} {{13-21=throws}}
+let r3 : Optional<() rethrows -> ()> = nil // expected-error {{only function declarations may be marked 'rethrows'; did you mean 'throws'?}} {{22-30=throws}}
 
 func f1(_ f: () throws -> ()) rethrows { try f() }
 func f2(_ f: () -> ()) rethrows { f() } // expected-error {{'rethrows' function must take a throwing function argument}}