| // run-pass |
| #![allow(dead_code)] |
| |
| use std::mem; |
| |
| // Get around the limitations of CTFE in today's Rust. |
| const fn choice_u64(c: bool, a: u64, b: u64) -> u64 { |
| (-(c as i64) as u64) & a | (-(!c as i64) as u64) & b |
| } |
| |
| const fn max_usize(a: usize, b: usize) -> usize { |
| choice_u64(a > b, a as u64, b as u64) as usize |
| } |
| |
| const fn align_to(size: usize, align: usize) -> usize { |
| (size + (align - 1)) & !(align - 1) |
| } |
| |
| const fn packed_union_size_of<A, B>() -> usize { |
| max_usize(mem::size_of::<A>(), mem::size_of::<B>()) |
| } |
| |
| const fn union_align_of<A, B>() -> usize { |
| max_usize(mem::align_of::<A>(), mem::align_of::<B>()) |
| } |
| |
| const fn union_size_of<A, B>() -> usize { |
| align_to(packed_union_size_of::<A, B>(), union_align_of::<A, B>()) |
| } |
| |
| macro_rules! fake_union { |
| ($name:ident { $a:ty, $b:ty }) => ( |
| struct $name { |
| _align: ([$a; 0], [$b; 0]), |
| _bytes: [u8; union_size_of::<$a, $b>()] |
| } |
| ) |
| } |
| |
| // Check that we can (poorly) emulate unions by |
| // calling size_of and align_of at compile-time. |
| fake_union!(U { u16, [u8; 3] }); |
| |
| fn test(u: U) { |
| assert_eq!(mem::size_of_val(&u._bytes), 4); |
| } |
| |
| fn main() { |
| assert_eq!(mem::size_of::<U>(), 4); |
| assert_eq!(mem::align_of::<U>(), 2); |
| } |