[Concurrency] Emit thunk for synchronous -> async function conversion.

Author: DougGregor

lib/SIL/IR/TypeLowering.cpp

@@ -3014,6 +3014,10 @@ TypeConverter::checkForABIDifferences(SILModule &M,
   // trivial.
   if (auto fnTy1 = type1.getAs<SILFunctionType>()) {
     if (auto fnTy2 = type2.getAs<SILFunctionType>()) {
+      // Async/synchronous conversions always need a thunk.
+      if (fnTy1->isAsync() != fnTy2->isAsync())
+        return ABIDifference::NeedsThunk;
+
       // @convention(block) is a single retainable pointer so optionality
       // change is allowed.
       if (optionalityChange)

test/SILGen/async_conversion.swift

@@ -0,0 +1,13 @@
+// RUN: %target-swift-frontend -emit-silgen %s -module-name test -swift-version 5 -enable-experimental-concurrency | %FileCheck %s
+// REQUIRES: concurrency
+
+func f(_: Int, _: String) -> String? { nil }
+
+// CHECK-LABEL: sil hidden [ossa] @$s4testAAyyF : $@convention(thin) () -> () {
+func test() {
+  // CHECK: [[F:%.*]] = function_ref @$s4test1fySSSgSi_SStF : $@convention(thin) (Int, @guaranteed String) -> @owned Optional<String>
+  // CHECK: [[THICK_F:%.*]] = thin_to_thick_function [[F]] : $@convention(thin) (Int, @guaranteed String) -> @owned Optional<String> to $@callee_guaranteed (Int, @guaranteed String) -> @owned Optional<String>
+  // CHECK: [[THUNK:%.*]] = function_ref @$sSiS2SSgIegygo_SiSSAAIegHygo_TR : $@convention(thin) @async (Int, @guaranteed String, @guaranteed @callee_guaranteed (Int, @guaranteed String) -> @owned Optional<String>) -> @owned Optional<String>
+  // CHECK: partial_apply [callee_guaranteed] [[THUNK]]([[THICK_F]]) : $@convention(thin) @async (Int, @guaranteed String, @guaranteed @callee_guaranteed (Int, @guaranteed String) -> @owned Optional<String>) -> @owned Optional<String>
+  let _: (Int, String) async -> String? = f
+}