Fix having multiple reprs on the same type.

This bug has applied to master for an indefinite period of time and is orthogonal to univariant layout optimization.

src/test/run-pass/multiple-reprs.rs

@@ -0,0 +1,44 @@
+// Copyright 2012 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.
+
+
+use std::mem::size_of;
+
+// The two enums that follow are designed so that bugs trigger layout optimization.
+// Specifically, if either of the following reprs used here is not detected by the compiler,
+// then the sizes will be wrong.
+
+#[repr(C, u8)]
+enum E1 {
+    A(u8, u16, u8),
+    B(u8, u16, u8)
+}
+
+#[repr(u8, C)]
+enum E2 {
+    A(u8, u16, u8),
+    B(u8, u16, u8)
+}
+
+// From pr 37429
+pub const SIZEOF_QUERY:      usize = 21;
+
+#[repr(C,packed)]
+pub struct p0f_api_query {
+    pub magic: u32,
+    pub addr_type: u8,
+    pub addr: [u8; 16],
+    
+}
+pub fn main() {
+    assert_eq!(size_of::<E1>(), 6);
+    assert_eq!(size_of::<E2>(), 6);
+    assert_eq!(size_of::<p0f_api_query>(), 21);
+}

src/test/run-pass/type-sizes.rs

@@ -26,7 +26,7 @@ enum e2 {
     a(u32), b
 }
 
-#[repr(u8)]
+#[repr(C, u8)]
 enum e3 {
     a([u16; 0], u8), b
 }