Drop @Concurrent from parameter to @asyncHandler entry point.

The async handler code doesn't really run concurrently with where it is
launched, despite the implementation currently using `runDetached`
internally. Drop the @Concurrent and bitcase it back on when needed.

Author: DougGregor

stdlib/public/Concurrency/Task.swift

@@ -281,8 +281,11 @@ extension Task {
 
 }
 
-public func _runAsyncHandler(operation: @concurrent @escaping () async -> ()) {
-  _ = Task.runDetached(operation: operation)
+public func _runAsyncHandler(operation: @escaping () async -> ()) {
+  typealias ConcurrentFunctionType = @concurrent () async -> ()
+  _ = Task.runDetached(
+    operation: unsafeBitCast(operation, to: ConcurrentFunctionType.self)
+  )
 }
 
 // ==== Voluntary Suspension -----------------------------------------------------

test/SILGen/async_handler.swift

@@ -8,7 +8,7 @@ func take<T>(_ t: T) async {
 // CHECK-LABEL: sil [ossa] @$s4test13simpleHandleryySiF : $@convention(thin) (Int) -> () {
 // CHECK:   [[BODYFN:%[0-9]+]] = function_ref @$s4test13simpleHandleryySiYF : $@convention(thin) @async (Int) -> ()
 // CHECK:   [[FN:%[0-9]+]] = partial_apply [callee_guaranteed] [[BODYFN]](%0) : $@convention(thin) @async (Int) -> ()
-// CHECK:   [[INTRINSIC:%[0-9]+]] = function_ref @$s12_Concurrency16_runAsyncHandler9operationyyyYJc_tF : $@convention(thin) (@guaranteed @async @callee_guaranteed () -> ()) -> ()
+// CHECK:   [[INTRINSIC:%[0-9]+]] = function_ref @$s12_Concurrency16_runAsyncHandler9operationyyyYc_tF : $@convention(thin) (@guaranteed @async @callee_guaranteed () -> ()) -> ()
 // CHECK:   {{.*}} = apply [[INTRINSIC]]([[FN]]) : $@convention(thin) (@guaranteed @async @callee_guaranteed () -> ()) -> ()
 // CHECK:   destroy_value [[FN]] : $@async @callee_guaranteed () -> ()
 // CHECK: } // end sil function '$s4test13simpleHandleryySiF'
@@ -21,7 +21,7 @@ public func simpleHandler(_ i: Int) {
 // CHECK:   [[COPY:%[0-9]+]] = copy_value %0 : $String
 // CHECK:   [[BODYFN:%[0-9]+]] = function_ref @$s4test20nonTrivialArgHandleryySSYF : $@convention(thin) @async (@guaranteed String) -> ()
 // CHECK:   [[FN:%[0-9]+]] = partial_apply [callee_guaranteed] [[BODYFN]]([[COPY]]) : $@convention(thin) @async (@guaranteed String) -> ()
-// CHECK:   [[INTRINSIC:%[0-9]+]] = function_ref @$s12_Concurrency16_runAsyncHandler9operationyyyYJc_tF : $@convention(thin) (@guaranteed @async @callee_guaranteed () -> ()) -> ()
+// CHECK:   [[INTRINSIC:%[0-9]+]] = function_ref @$s12_Concurrency16_runAsyncHandler9operationyyyYc_tF : $@convention(thin) (@guaranteed @async @callee_guaranteed () -> ()) -> ()
 // CHECK:   {{.*}} = apply [[INTRINSIC]]([[FN]]) : $@convention(thin) (@guaranteed @async @callee_guaranteed () -> ()) -> ()
 // CHECK:   destroy_value [[FN]] : $@async @callee_guaranteed () -> ()
 // CHECK: } // end sil function '$s4test20nonTrivialArgHandleryySSF'
@@ -35,7 +35,7 @@ public func nonTrivialArgHandler(_ s: String) {
 // CHECK:   copy_addr %0 to [initialization] [[TMP]] : $*T
 // CHECK:   [[BODYFN:%[0-9]+]] = function_ref @$s4test14genericHandleryyxYlF : $@convention(thin) @async <τ_0_0> (@in_guaranteed τ_0_0) -> ()
 // CHECK:   [[FN:%[0-9]+]] = partial_apply [callee_guaranteed] [[BODYFN]]<T>([[TMP]]) : $@convention(thin) @async <τ_0_0> (@in_guaranteed τ_0_0) -> ()
-// CHECK:   [[INTRINSIC:%[0-9]+]] = function_ref @$s12_Concurrency16_runAsyncHandler9operationyyyYJc_tF : $@convention(thin) (@guaranteed @async @callee_guaranteed () -> ()) -> ()
+// CHECK:   [[INTRINSIC:%[0-9]+]] = function_ref @$s12_Concurrency16_runAsyncHandler9operationyyyYc_tF : $@convention(thin) (@guaranteed @async @callee_guaranteed () -> ()) -> ()
 // CHECK:   {{.*}} = apply [[INTRINSIC]]([[FN]]) : $@convention(thin) (@guaranteed @async @callee_guaranteed () -> ()) -> ()
 // CHECK:   destroy_value [[FN]] : $@async @callee_guaranteed () -> ()
 // CHECK: } // end sil function '$s4test14genericHandleryyxlF'
@@ -48,7 +48,7 @@ public struct Mystruct {
   // CHECK-LABEL: sil [ossa] @$s4test8MystructV13memberHandleryySiF : $@convention(method) (Int, Mystruct) -> () {
   // CHECK:   [[BODYFN:%[0-9]+]] = function_ref @$s4test8MystructV13memberHandleryySiYF : $@convention(method) @async (Int, Mystruct) -> ()
   // CHECK:   [[FN:%[0-9]+]] = partial_apply [callee_guaranteed] [[BODYFN]](%0, %1) : $@convention(method) @async (Int, Mystruct) -> ()
-  // CHECK:   [[INTRINSIC:%[0-9]+]] = function_ref @$s12_Concurrency16_runAsyncHandler9operationyyyYJc_tF : $@convention(thin) (@guaranteed @async @callee_guaranteed () -> ()) -> ()
+  // CHECK:   [[INTRINSIC:%[0-9]+]] = function_ref @$s12_Concurrency16_runAsyncHandler9operationyyyYc_tF : $@convention(thin) (@guaranteed @async @callee_guaranteed () -> ()) -> ()
   // CHECK:   {{.*}} = apply [[INTRINSIC]]([[FN]]) : $@convention(thin) (@guaranteed @async @callee_guaranteed () -> ()) -> ()
   // CHECK:   destroy_value [[FN]] : $@async @callee_guaranteed () -> ()
   // CHECK: } // end sil function '$s4test8MystructV13memberHandleryySiF'