Correctly set return type attributes on foreign function declarations

The ArgType type gives us a generic way to specify an attribute for a
type to ensure ABI conformance for foreign functions. But the code that
actually sets the argument attributes in the function declaration
only sets the attribute for the return type when the type is indirect.

Since LLVMAddAttribute() doesn't allow to set attributes on the return
type, we have to use LLVMAddFunctionAttribute() instead.

This didn't cause problems yet, because currently only some indirect
types require attributes to be set.

src/librustc/middle/trans/foreign.rs

@@ -934,22 +934,17 @@ pub fn lltype_for_foreign_fn(ccx: &CrateContext, ty: ty::t) -> Type {
 
 fn add_argument_attributes(tys: &ForeignTypes,
                            llfn: ValueRef) {
-    let mut i = 0;
-
-    if tys.fn_ty.ret_ty.is_indirect() {
-        match tys.fn_ty.ret_ty.attr {
-            Some(attr) => {
-                let llarg = get_param(llfn, i);
-                unsafe {
-                    llvm::LLVMAddAttribute(llarg, attr as c_uint);
-                }
-            }
-            None => {}
-        }
+    let mut i = if tys.fn_ty.ret_ty.is_indirect() { 1 } else { 0 };
 
-        i += 1;
+    match tys.fn_ty.ret_ty.attr {
+        Some(attr) => unsafe {
+            llvm::LLVMAddFunctionAttribute(llfn, i as c_uint, attr as u64);
+        },
+        None => {}
     }
 
+    i += 1;
+
     for &arg_ty in tys.fn_ty.arg_tys.iter() {
         if arg_ty.is_ignore() {
             continue;
@@ -958,12 +953,9 @@ fn add_argument_attributes(tys: &ForeignTypes,
         if arg_ty.pad.is_some() { i += 1; }
 
         match arg_ty.attr {
-            Some(attr) => {
-                let llarg = get_param(llfn, i);
-                unsafe {
-                    llvm::LLVMAddAttribute(llarg, attr as c_uint);
-                }
-            }
+            Some(attr) => unsafe {
+                llvm::LLVMAddFunctionAttribute(llfn, i as c_uint, attr as u64);
+            },
             None => ()
         }