rustc_trans: take into account primitives larger than 8 bytes.

src/test/run-pass/issue-38763.rs

@@ -0,0 +1,21 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(i128_type)]
+
+#[repr(C)]
+pub struct Foo(i128);
+
+#[no_mangle]
+pub extern "C" fn foo(x: Foo) -> Foo { x }
+
+fn main() {
+    foo(Foo(1));
+}