ICE on casting to trait object pointer

[michaelwu]

Playpen: http://is.gd/BpXFCU

trait Foo {
    fn get(&self) -> u32;
}

struct Bar {
    something: u32
}

impl Foo for Bar {
    fn get(&self) -> u32 {
        self.something
    }
}

fn main() {
    let x = Bar { something: 4 };
    let y = &x;
    let z: *const Foo = y as *const _ as *const _;
    let value = unsafe { (*z).get() };
    println!("Hello, world! {}", value);
}

Outputs:

error: internal compiler error: translating unsupported cast: *const Bar (cast_pointer) -> *const Foo (cast_other)
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
note: run with `RUST_BACKTRACE=1` for a backtrace
thread 'rustc' panicked at 'Box<Any>', /home/rustbuild/src/rust-buildbot/slave/beta-dist-rustc-linux/build/src/libsyntax/diagnostic.rs:209

[luqmana]

Probably addressed in #24333 or should be if it isn't.

But as a quick fix, you don't need that last as *const _ cast. The first one will take you to *const Bar and then since you have z: *const Foo it'll get coerced to a fat pointer there.

[Stebalien]

This has already been fixed on nightly so it can't be #24333. #25038?

[michaelwu]

Seems to be fixed on nightly according to playpen.