ICE when abusing CoerceUnsized through specialization

[arielb1]

I was attempting to figure out the feature landscape for #27941, and I found this ICE when playing with it and specialization:

Code

#![feature(specialization)]
#![feature(unsize, coerce_unsized)]

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

pub struct SmartassPtr<A: Smartass+?Sized>(A::Data);

pub trait Smartass {
    type Data;
    type Data2: CoerceUnsized<*const [u8]>;
}

pub trait MaybeObjectSafe {
// fn not_object_safe(self: SmartassPtr<Self>);
}

impl MaybeObjectSafe for () {}

impl<T> Smartass for T {
    type Data = <Self as Smartass>::Data2;
    default type Data2 = ();
}

impl Smartass for () {
    type Data2 = *const [u8; 1];
}

impl Smartass for MaybeObjectSafe {
    type Data = *const [u8];
    type Data2 = *const [u8; 0];
}

impl<U: Smartass+?Sized, T: Smartass+?Sized> CoerceUnsized<SmartassPtr<T>> for SmartassPtr<U>
    where <U as Smartass>::Data: std::ops::CoerceUnsized<<T as Smartass>::Data>
{}

pub fn conv(s: SmartassPtr<()>) -> SmartassPtr<MaybeObjectSafe> {
    s
}

fn main() {
}

ICE:

error: internal compiler error: /checkout/src/librustc_trans/collector.rs:820: find_vtable_types_for_unsizing: invalid coercion <() as Smartass>::Data -> <MaybeObjectSafe as Smartass>::Data

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: rustc 1.22.0-nightly (6c476ce46 2017-09-25) running on x86_64-unknown-linux-gnu

thread 'rustc' panicked at 'Box<Any>', /checkout/src/librustc_errors/lib.rs:492:8
note: Run with `RUST_BACKTRACE=1` for a backtrace

[Seeker14491]

The given code builds fine on current nightly.

[JohnTitor]

This is still an ICE on current nightly when not cargo run. playground

[Seeker14491]

This code compiles without a problem whether I do cargo build or cargo run using current nightly: rustc 1.40.0-nightly (1721c9685 2019-10-12). Tested on Windows 10 x64 and Ubuntu x64.

[JohnTitor]

@Seeker14491 Really? But on the playground, the ICE still appears. Can you check the above link?

[Seeker14491]

Yes, I get an ICE in the playground when I just build.

[mati865]

It doesn't ICE on the playground any more.
It does ICE when using build instead of run.

[JohnTitor]

@mati865 You need cargo build or cargo test, not cargo run. It seems like a still ICE to me.

[JohnTitor]

Triage: the latest nightly rejects the code without an ICE. Marking as E-needs-test.