The mismatched types were not detected, and a segmentation fault occurred at runtime.

[rucsesec]

I tried this code:

use std::fmt::Display;

trait Trait{
    type Associated;
}

impl<T: ?Sized> Trait for T {
    type Associated = Box<dyn Display>;
}

fn foo<T: ?Sized>(_: <T as Trait>::Associated) {}

fn main() {
    let x = "Hello World".to_string();
    foo::<dyn Trait<Associated = &str>>( &x );
}

I expected to see this happen: mismatched types：expected “Box<dyn Display>”， found “&str”.
Instead, this happened: The code compiles successfully but encounters a segmentation fault at runtime.

Program returned: 139
Program stderr
Program terminated with signal: SIGSEGV

Demo: godbolt permalink
Demo: on current playground

Meta

rustc --version --verbose:

rustc 1.88.0-nightly (0fe8f3454 2025-04-10)

[jieyouxu]

This also segfaults on stable

[lcnr]

Thank you for finding this unsoundness and reporting it. This is already a known issue which is difficult to fix. We do intend to fix this in the medium term however. Closing in favor of #57893