src/test/mir-opt/issue-41697.rs

// Regression test for #41697. Using dump-mir was triggering
// artificial cycles: during type-checking, we had to get the MIR for
// the constant expressions in `[u8; 2]`, which in turn would trigger
// an attempt to get the def-path, which in turn would request the
// types of the impl, which would trigger a cycle. We suppressed this
// cycle now by forcing mir-dump to avoid asking for types of an impl.

#![feature(rustc_attrs)]

use std::sync::Arc;

trait Foo {
    fn get(&self) -> [u8; 2];
}

impl Foo for [u8; 2] {
    fn get(&self) -> [u8; 2] {
        *self
    }
}

struct Bar<T: ?Sized>(T);

fn unsize_fat_ptr<'a>(x: &'a Bar<Foo + Send + 'a>) -> &'a Bar<Foo + 'a> {
    x
}

fn unsize_nested_fat_ptr(x: Arc<Foo + Send>) -> Arc<Foo> {
    x
}

fn main() {
    let x: Box<Bar<Foo + Send>> = Box::new(Bar([1,2]));
    assert_eq!(unsize_fat_ptr(&*x).0.get(), [1, 2]);

    let x: Arc<Foo + Send> = Arc::new([3, 4]);
    assert_eq!(unsize_nested_fat_ptr(x).get(), [3, 4]);
}