[extern] Give @_extern(c) functions @convention(c) type

This ensures that when we reference them, we get an appropriately-typed
function value. SILGen can then thunk to whatever function type is
needed.

Author: DougGregor

lib/SILGen/SILGenExpr.cpp

@@ -996,6 +996,9 @@ emitRValueForDecl(SILLocation loc, ConcreteDeclRef declRef, Type ncRefType,
   // If the referenced decl isn't a VarDecl, it should be a constant of some
   // sort.
   SILDeclRef silDeclRef(decl);
+  if (ExternAttr::find(decl->getAttrs(), ExternKind::C))
+    silDeclRef = silDeclRef.asForeign();
+
   assert(silDeclRef.getParameterListCount() == 1);
   auto substType = cast<AnyFunctionType>(refType);
   auto typeContext = getFunctionTypeInfo(substType);

lib/Sema/TypeCheckDecl.cpp

@@ -2565,6 +2565,11 @@ InterfaceTypeRequest::evaluate(Evaluator &eval, ValueDecl *D) const {
           infoBuilder = infoBuilder.withSendingResult();
       }
 
+      if (ExternAttr::find(D->getAttrs(), ExternKind::C)) {
+        infoBuilder = infoBuilder.withRepresentation(
+          FunctionTypeRepresentation::CFunctionPointer);
+      }
+
       // Lifetime dependencies only apply to the outer function type.
       if (!hasSelf && lifetimeDependenceInfo.has_value()) {
         infoBuilder =

test/SILGen/extern_c.swift

@@ -27,6 +27,10 @@ func withoutCName() -> Int
 @_extern(c, "default_arg")
 func defaultArg(_ x: Int = 42)
 
+func callMe(body: (Int) -> Int) -> Int {
+  return body(17)
+}
+
 func main() {
   // CHECK-DAG: [[F1:%.+]] = function_ref @$s8extern_c9withCNameyS2iFTo : $@convention(c) (Int) -> Int
   // CHECK-DAG: [[F2:%.+]] = function_ref @$s8extern_c12takeCFuncPtryyS2iXCFTo : $@convention(c) (@convention(c) (Int) -> Int) -> ()
@@ -46,6 +50,14 @@ func main() {
   // CHECK-DAG: [[F7:%.+]] = function_ref @$s8extern_c10defaultArgyySiFTo : $@convention(c) (Int) -> ()
   // CHECK-DAG: apply [[F7]]([[DEFAULT_V]]) : $@convention(c) (Int) -> ()
   defaultArg()
+
+  // CHECK-DAG: [[CREF:%[0-9]+]] = function_ref @$s8extern_c16publicVisibilityyS2iFTo : $@convention(c) (Int) -> Int
+  // CHECK-DAG: [[THUNK:%[0-9]+]] = function_ref @$sS2iIetCyd_S2iIegyd_TR : $@convention(thin) (Int, @convention(c) (Int) -> Int) -> Int
+  // CHECK-DAG: [[APPLIED:%[0-9]+]] = partial_apply [callee_guaranteed] [[THUNK]]([[CREF]]) : $@convention(thin) (Int, @convention(c) (Int) -> Int) -> Int
+  // CHECK-DAG: [[NOESCAPE_APPLIED:%[0-9]+]] = convert_escape_to_noescape [not_guaranteed] [[APPLIED]] to $@noescape @callee_guaranteed (Int) -> Int
+  // CHECK-DAG: [[CALL_ME:%[0-9]+]] = function_ref @$s8extern_c6callMe4bodyS3iXE_tF
+  // CHECK-DAG: apply [[CALL_ME]]([[NOESCAPE_APPLIED]]) : $@convention(thin) (@guaranteed @noescape @callee_guaranteed (Int) -> Int) -> Int
+  _ = callMe(body: publicVisibility)
 }
 
 main()