auto merge of #20060 : Aatch/rust/enum-repr, r=alexcrichton

The previous behaviour of using the smallest type possible caused LLVM
to treat padding too conservatively, causing poor codegen. This commit
changes the behaviour to use an alignment-sized integer as the
discriminant. This keeps types the same size, but helps LLVM understand
the data structure a little better, resulting in better codegen.

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

@@ -18,6 +18,17 @@ struct w {a: int, b: ()}
 struct x {a: int, b: (), c: ()}
 struct y {x: int}
 
+enum e1 {
+    a(u8, u32), b(u32), c
+}
+enum e2 {
+    a(u32), b
+}
+
+enum e3 {
+    a([u16, ..0], u8), b
+}
+
 pub fn main() {
     assert_eq!(size_of::<u8>(), 1 as uint);
     assert_eq!(size_of::<u32>(), 4 as uint);
@@ -34,4 +45,11 @@ pub fn main() {
     assert_eq!(size_of::<w>(), size_of::<int>());
     assert_eq!(size_of::<x>(), size_of::<int>());
     assert_eq!(size_of::<int>(), size_of::<y>());
+
+    // Make sure enum types are the appropriate size, mostly
+    // around ensuring alignment is handled properly
+
+    assert_eq!(size_of::<e1>(), 8 as uint);
+    assert_eq!(size_of::<e2>(), 8 as uint);
+    assert_eq!(size_of::<e3>(), 4 as uint);
 }