Add test to verify that argument size mismatch throws UB

rust-version

@@ -1 +1 @@
-14b15521c52549ebbb113173b4abecd124b5a823
+e83f7563495dbe2629b0cbc738afb0808c4482e1

tests/compile-fail/shim_arg_size.rs

@@ -0,0 +1,21 @@
+#![feature(rustc_private)]
+
+extern crate libc;
+
+// error-pattern: scalar size mismatch
+fn main() {
+    extern "C" {
+        // Use the wrong type(ie. not the pointer width) for the `size`
+        // argument.
+        #[cfg(target_pointer_width="64")]
+        fn malloc(size: u32) -> *mut std::ffi::c_void;
+
+        #[cfg(target_pointer_width="32")]
+        fn malloc(size: u16) -> *mut std::ffi::c_void;
+    }
+
+    unsafe {
+        let p1 = malloc(42);
+        libc::free(p1);
+    };
+}