Remove feature flag for has-async-alt attr

This patch removes the feature flag for @hasAsyncAlternative since it's
already protected by the experimental concurrency flag and will go in
with the concurrency features.

This fixes one of Doug's comments on
swiftlang#36027.

Author: etcwilde

include/swift/AST/DiagnosticsParse.def

@@ -1673,10 +1673,6 @@ ERROR(sil_inst_autodiff_invalid_witness_generic_signature,PointsToFirstBadToken,
       (StringRef, StringRef))
 
 // hasAsyncAlternative
-ERROR(requires_has_async_alternative, none,
-      "'%0' support is required to be explicitly enabled using "
-      "-experimental-has-async-alternative-attribute", (StringRef))
-
 ERROR(has_async_alternative_invalid_name, none,
       "argument of '%0' attribute must be an identifier or full function name",
       (StringRef))

include/swift/Basic/LangOptions.h

@@ -384,9 +384,6 @@ namespace swift {
     ASTVerifierOverrideKind ASTVerifierOverride =
         ASTVerifierOverrideKind::NoOverride;
 
-    /// Allow @hasAsyncAlternative attribute
-    bool EnableExperimentalHasAsyncAlternative = false;
-
     /// Sets the target we are building for and updates platform conditions
     /// to match.
     ///

include/swift/Option/FrontendOptions.td

@@ -793,8 +793,4 @@ def new_driver_path
   : Separate<["-"], "new-driver-path">, MetaVarName<"<path>">,
   HelpText<"Path of the new driver to be used">;
 
-def experimental_has_async_alternative_attribute:
-  Flag<["-"], "experimental-has-async-alternative-attribute">,
-  HelpText<"Allow use of @hasAsyncAlternative">;
-
 } // end let Flags = [FrontendOption, NoDriverOption, HelpHidden]

lib/Frontend/CompilerInvocation.cpp

@@ -713,9 +713,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     }
   }
 
-  Opts.EnableExperimentalHasAsyncAlternative |=
-      Args.hasArg(OPT_experimental_has_async_alternative_attribute);
-
   return HadError || UnsupportedOS || UnsupportedArch;
 }
 

lib/Parse/ParseDecl.cpp

@@ -2694,12 +2694,8 @@ bool Parser::parseNewDeclAttribute(DeclAttributes &Attributes, SourceLoc AtLoc,
       return false;
     }
 
-    if (!DiscardAttribute) {
-      if (Context.LangOpts.EnableExperimentalHasAsyncAlternative)
-        Attributes.add(attr);
-      else
-        diagnose(Loc, diag::requires_has_async_alternative, AttrName);
-    }
+    if (!DiscardAttribute)
+      Attributes.add(attr);
     break;
   }
   }

test/attr/attr_hasasyncalternative.swift

@@ -1,6 +1,6 @@
 // REQUIRES: concurrency
 
-// RUN: %target-typecheck-verify-swift -enable-experimental-concurrency -experimental-has-async-alternative-attribute
+// RUN: %target-typecheck-verify-swift -enable-experimental-concurrency
 
 @hasAsyncAlternative
 func func1() {}