tests/ui/layout/unsafe-cell-hides-niche.rs

   1 // For rust-lang/rust#68303: the contents of `UnsafeCell<T>` cannot
   2 // participate in the niche-optimization for enum discriminants. This
   3 // test checks that an `Option<UnsafeCell<NonZeroU32>>` has the same
   4 // size in memory as an `Option<UnsafeCell<u32>>` (namely, 8 bytes).
   5
   6 // check-pass
   7 // compile-flags: --crate-type=lib
   8 // only-x86
   9
  10 #![feature(repr_simd)]
  11
  12 use std::cell::{UnsafeCell, RefCell, Cell};
  13 use std::mem::size_of;
  14 use std::num::NonZeroU32 as N32;
  15 use std::sync::{Mutex, RwLock};
  16
  17 struct Wrapper<T>(#[allow(unused_tuple_struct_fields)] T);
  18
  19 #[repr(transparent)]
  20 struct Transparent<T>(#[allow(unused_tuple_struct_fields)] T);
  21
  22 struct NoNiche<T>(UnsafeCell<T>);
  23
  24 struct Size<const S: usize>;
  25
  26 macro_rules! check_sizes {
  27     (check_one_specific_size: $ty:ty, $size:expr) => {
  28         const _: Size::<{$size}> = Size::<{size_of::<$ty>()}>;
  29     };
  30     // Any tests run on `UnsafeCell` must be the same for `Cell`
  31     (UnsafeCell<$ty:ty>: $size:expr => $optioned_size:expr) => {
  32         check_sizes!(Cell<$ty>: $size => $optioned_size);
  33         check_sizes!(@actual_check: UnsafeCell<$ty>: $size => $optioned_size);
  34     };
  35     ($ty:ty: $size:expr => $optioned_size:expr) => {
  36         check_sizes!(@actual_check: $ty: $size => $optioned_size);
  37     };
  38     // This branch does the actual checking logic, the `@actual_check` prefix is here to distinguish
  39     // it from other branches and not accidentally match any.
  40     (@actual_check: $ty:ty: $size:expr => $optioned_size:expr) => {
  41         check_sizes!(check_one_specific_size: $ty, $size);
  42         check_sizes!(check_one_specific_size: Option<$ty>, $optioned_size);
  43         check_sizes!(check_no_niche_opt: $size != $optioned_size, $ty);
  44     };
  45     // only check that there is no niche (size goes up when wrapped in an option),
  46     // don't check actual sizes
  47     ($ty:ty) => {
  48         check_sizes!(check_no_niche_opt: true, $ty);
  49     };
  50     (check_no_niche_opt: $no_niche_opt:expr, $ty:ty) => {
  51         const _: () = if $no_niche_opt { assert!(size_of::<$ty>() < size_of::<Option<$ty>>()); };
  52     };
  53 }
  54
  55 const PTR_SIZE: usize = std::mem::size_of::<*const ()>();
  56
  57 check_sizes!(Wrapper<u32>:     4 => 8);
  58 check_sizes!(Wrapper<N32>:     4 => 4); // (✓ niche opt)
  59 check_sizes!(Transparent<u32>: 4 => 8);
  60 check_sizes!(Transparent<N32>: 4 => 4); // (✓ niche opt)
  61 check_sizes!(NoNiche<u32>:     4 => 8);
  62 check_sizes!(NoNiche<N32>:     4 => 8);
  63
  64 check_sizes!(UnsafeCell<u32>:  4 => 8);
  65 check_sizes!(UnsafeCell<N32>:  4 => 8);
  66
  67 check_sizes!(UnsafeCell<&()>: PTR_SIZE => PTR_SIZE * 2);
  68 check_sizes!(   RefCell<&()>: PTR_SIZE * 2 => PTR_SIZE * 3);
  69
  70 check_sizes!(RwLock<&()>);
  71 check_sizes!(Mutex<&()>);
  72
  73 check_sizes!(UnsafeCell<&[i32]>: PTR_SIZE * 2 => PTR_SIZE * 3);
  74 check_sizes!(UnsafeCell<(&(), &())>: PTR_SIZE * 2 => PTR_SIZE * 3);
  75
  76 trait Trait {}
  77 check_sizes!(UnsafeCell<&dyn Trait>: PTR_SIZE * 2 => PTR_SIZE * 3);
  78
  79 #[repr(simd)]
  80 pub struct Vec4<T>([T; 4]);
  81
  82 check_sizes!(UnsafeCell<Vec4<N32>>: 16 => 32);