mirror of https://github.com/rust-lang/rust
53 lines
1.6 KiB
Rust
53 lines
1.6 KiB
Rust
//@ run-pass
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#![allow(dead_code)]
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// Tests that unions aren't subject to unsafe non-zero/niche-filling optimizations.
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//
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// For example, if a union `U` can contain both a `&T` and a `*const T`, there's definitely no
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// bit-value that an `Option<U>` could reuse as `None`; this test makes sure that isn't done.
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//
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// Secondly, this tests the status quo (not a guarantee; subject to change!) to not apply such
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// optimizations to types containing unions even if they're theoretically possible. (discussion:
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// https://github.com/rust-lang/rust/issues/36394)
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//
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// Notably this nails down part of the behavior that `MaybeUninit` assumes: that an
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// `Option<MaybeUninit<&u8>>` does not take advantage of non-zero optimization, and thus is a safe
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// construct.
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use std::mem::{size_of, transmute};
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union U1<A: Copy> {
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a: A,
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}
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union U2<A: Copy, B: Copy> {
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a: A,
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b: B,
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}
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// Option<E> uses a value other than 0 and 1 as None
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#[derive(Clone,Copy)]
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enum E {
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A = 0,
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B = 1,
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}
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fn main() {
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// Unions do not participate in niche-filling/non-zero optimization...
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assert!(size_of::<Option<U2<&u8, u8>>>() > size_of::<U2<&u8, u8>>());
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assert!(size_of::<Option<U2<&u8, ()>>>() > size_of::<U2<&u8, ()>>());
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assert!(size_of::<Option<U2<u8, E>>>() > size_of::<U2<u8, E>>());
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// ...even when theoretically possible:
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assert!(size_of::<Option<U1<&u8>>>() > size_of::<U1<&u8>>());
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assert!(size_of::<Option<U2<&u8, &u8>>>() > size_of::<U2<&u8, &u8>>());
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// The unused bits of the () variant can have any value.
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let zeroed: U2<&u8, ()> = unsafe { transmute(std::ptr::null::<u8>()) };
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if let None = Some(zeroed) {
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panic!()
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}
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}
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