Add a regression test

src/test/run-pass/foreign-truncated-arguments.rs

+// Copyright 2016 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.
+
+// compile-flags: -O
+// Regression test for https://github.com/rust-lang/rust/issues/33868
+
+#[repr(C)]
+pub struct S {
+    a: u32,
+    b: f32,
+    c: u32
+}
+
+#[no_mangle]
+#[inline(never)]
+pub extern "C" fn test(s: S) -> u32 {
+    s.c
+}
+
+fn main() {
+    assert_eq!(test(S{a: 0, b: 0.0, c: 42}), 42);
+}