tests/ui/transmutability/unions/should_pad_variants.rs

   1 //! The variants of a union must be padded with uninit bytes such that they have
   2 //! the same length (in bytes).
   3
   4 #![crate_type = "lib"]
   5 #![feature(transmutability)]
   6 #![allow(dead_code)]
   7
   8 mod assert {
   9     use std::mem::{Assume, BikeshedIntrinsicFrom};
  10
  11     pub fn is_transmutable<Src, Dst, Context>()
  12     where
  13         Dst: BikeshedIntrinsicFrom<Src, Context, {
  14             Assume::ALIGNMENT
  15                 .and(Assume::LIFETIMES)
  16                 .and(Assume::SAFETY)
  17                 .and(Assume::VALIDITY)
  18         }>
  19     {}
  20 }
  21
  22 #[derive(Clone, Copy)]
  23 #[repr(C)] struct Zst;
  24
  25 #[derive(Clone, Copy)]
  26 #[repr(u8)] enum V0 { V = 0 }
  27
  28 #[derive(Clone, Copy)]
  29 #[repr(u8)] enum V2 { V = 2 }
  30
  31 #[repr(C)]
  32 union Lopsided {
  33     smol: Zst,
  34     lorg: V0,
  35 }
  36
  37 #[repr(C)] struct Src(V0, Zst, V2);
  38 #[repr(C)] struct Dst(V0, Lopsided, V2);
  39
  40 fn should_pad_variants() {
  41     struct Context;
  42     // If the implementation (incorrectly) fails to pad `Lopsided::smol` with
  43     // an uninitialized byte, this transmutation might be (wrongly) accepted:
  44     assert::is_transmutable::<Src, Dst, Context>(); //~ ERROR cannot be safely transmuted
  45 }