61 lines
1.7 KiB
Rust
61 lines
1.7 KiB
Rust
// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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#![feature(const_fn)]
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use std::mem;
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// Get around the limitations of CTFE in today's Rust.
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const fn choice_u64(c: bool, a: u64, b: u64) -> u64 {
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(-(c as i64) as u64) & a | (-(!c as i64) as u64) & b
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}
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const fn max_usize(a: usize, b: usize) -> usize {
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choice_u64(a > b, a as u64, b as u64) as usize
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}
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const fn align_to(size: usize, align: usize) -> usize {
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(size + (align - 1)) & !(align - 1)
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}
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const fn packed_union_size_of<A, B>() -> usize {
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max_usize(mem::size_of::<A>(), mem::size_of::<B>())
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}
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const fn union_align_of<A, B>() -> usize {
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max_usize(mem::align_of::<A>(), mem::align_of::<B>())
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}
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const fn union_size_of<A, B>() -> usize {
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align_to(packed_union_size_of::<A, B>(), union_align_of::<A, B>())
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}
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macro_rules! fake_union {
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($name:ident { $a:ty, $b:ty }) => (
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struct $name {
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_align: ([$a; 0], [$b; 0]),
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_bytes: [u8; union_size_of::<$a, $b>()]
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}
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)
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}
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// Check that we can (poorly) emulate unions by
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// calling size_of and align_of at compile-time.
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fake_union!(U { u16, [u8; 3] });
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fn test(u: U) {
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assert_eq!(mem::size_of_val(&u._bytes), 4);
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}
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fn main() {
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assert_eq!(mem::size_of::<U>(), 4);
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assert_eq!(mem::align_of::<U>(), 2);
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}
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