[Diagnostics] For errors that are downgraded to warnings via

warnUntilSwiftVersion, specify when the diagnostic will become an
error in the message.

Author: hborla

include/swift/AST/DiagnosticsCommon.def

@@ -44,6 +44,10 @@ NOTE(previous_decldef,none,
 NOTE(brace_stmt_suggest_do,none,
      "did you mean to use a 'do' statement?", ())
 
+WARNING(error_in_future_swift_version,none,
+        "%0; this is an error in Swift %1",
+        (DiagnosticInfo *, unsigned))
+
 // Generic disambiguation
 NOTE(while_parsing_as_left_angle_bracket,none,
      "while parsing this '<' as a type parameter bracket", ())

lib/AST/DiagnosticEngine.cpp

@@ -16,6 +16,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "swift/AST/DiagnosticEngine.h"
+#include "swift/AST/DiagnosticsCommon.h"
 #include "swift/AST/ASTContext.h"
 #include "swift/AST/ASTPrinter.h"
 #include "swift/AST/Decl.h"
@@ -321,8 +322,10 @@ InFlightDiagnostic::limitBehavior(DiagnosticBehavior limit) {
 
 InFlightDiagnostic &
 InFlightDiagnostic::warnUntilSwiftVersion(unsigned majorVersion) {
-  if (!Engine->languageVersion.isVersionAtLeast(majorVersion))
-    this->limitBehavior(DiagnosticBehavior::Warning);
+  if (!Engine->languageVersion.isVersionAtLeast(majorVersion)) {
+    limitBehavior(DiagnosticBehavior::Warning)
+      .wrapIn(diag::error_in_future_swift_version, majorVersion);
+  }
 
   return *this;
 }