src/tools/miri/tests/pass/coercions.rs

#![feature(coerce_unsized, unsize)]

use std::marker::Unsize;
use std::ops::CoerceUnsized;

fn identity_coercion(x: &(dyn Fn(u32) -> u32 + Send)) -> &dyn Fn(u32) -> u32 {
    x
}
fn fn_coercions(f: &fn(u32) -> u32) -> (unsafe fn(u32) -> u32, &(dyn Fn(u32) -> u32 + Send)) {
    (*f, f)
}

fn simple_array_coercion(x: &[u8; 3]) -> &[u8] {
    x
}

fn square(a: u32) -> u32 {
    a * a
}

#[derive(PartialEq, Eq)]
struct PtrWrapper<'a, T: 'a + ?Sized>(u32, u32, (), &'a T);
impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<PtrWrapper<'a, U>> for PtrWrapper<'a, T> {}

struct TrivPtrWrapper<'a, T: 'a + ?Sized>(&'a T);
impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<TrivPtrWrapper<'a, U>>
    for TrivPtrWrapper<'a, T>
{
}

fn coerce_ptr_wrapper(p: PtrWrapper<[u8; 3]>) -> PtrWrapper<[u8]> {
    p
}

fn coerce_triv_ptr_wrapper(p: TrivPtrWrapper<[u8; 3]>) -> TrivPtrWrapper<[u8]> {
    p
}

fn coerce_fat_ptr_wrapper(
    p: PtrWrapper<dyn Fn(u32) -> u32 + Send>,
) -> PtrWrapper<dyn Fn(u32) -> u32> {
    p
}

fn coerce_ptr_wrapper_poly<'a, T, Trait: ?Sized>(p: PtrWrapper<'a, T>) -> PtrWrapper<'a, Trait>
where
    PtrWrapper<'a, T>: CoerceUnsized<PtrWrapper<'a, Trait>>,
{
    p
}

fn main() {
    let a = [0, 1, 2];
    let square_local: fn(u32) -> u32 = square;
    let (f, g) = fn_coercions(&square_local);
    // cannot use `square as *const ()` because we can't know whether the compiler duplicates
    // functions, so two function pointers are only equal if they result from the same function
    // to function pointer cast
    assert_eq!(f as *const (), square_local as *const ());
    assert_eq!(g(4), 16);
    assert_eq!(identity_coercion(g)(5), 25);

    assert_eq!(simple_array_coercion(&a), &a);
    let w = coerce_ptr_wrapper(PtrWrapper(2, 3, (), &a));
    assert!(w == PtrWrapper(2, 3, (), &a) as PtrWrapper<[u8]>);

    let w = coerce_triv_ptr_wrapper(TrivPtrWrapper(&a));
    assert_eq!(&w.0, &a);

    let z = coerce_fat_ptr_wrapper(PtrWrapper(2, 3, (), &square_local));
    assert_eq!((z.3)(6), 36);

    let z: PtrWrapper<dyn Fn(u32) -> u32> =
        coerce_ptr_wrapper_poly(PtrWrapper(2, 3, (), &square_local));
    assert_eq!((z.3)(6), 36);
}