ICE in object_safety.rs due to method receiver not having a layout

[mikeyhew]

The following code causes an ICE, Error: the type `T` has an unknown layout:

#![feature(arbitrary_self_types, dispatch_from_dyn)]

use std::ops::{Deref, DispatchFromDyn};

trait Trait<T: Deref<Target=Self> + DispatchFromDyn<T>> {
    fn foo(self: T) -> dyn Trait<T>;
}

Similar to #56806, receiver_is_dispatchable succeeds, and then there is an ICE during the layout sanity checks. In this case, it is because the method receiver is a type parameter and has no layout.

receiver_is_dispatchable checks that the following predicate holds:

forall (U: Trait + ?Sized) {
    if (Self: Unsize<U>) {
        Receiver: DispatchFromDyn<Receiver[Self => U]>
    }
}

In this case, it reduces to T: DispatchFromDyn<T>, which is provided by a where clause. In #56806, it reduced to Box<dyn Trait>: DispatchFromDyn<Box<dyn Trait>>. The check passes in both cases, and then there is an ICE during the layout sanity checks.

One way to fix both of these cases would be to add an extra requirement to receiver_is_dispatchable: that Receiver and Receiver[Self => U] are not the same type. I'm not sure if there are any edge cases that that doesn't cover.

cc @varkor #57229

[mikeyhew]

There is also the straightforward solution of not doing the layout checks if the layout is un-computable. But it seems like receiver_is_dispatchable should return false for this case, where changing Self doesn't change the receiver type. If adding a check for this case makes it impossible to trigger the ICE in user code, then maybe we should bug! unconditionally

[JohnTitor]

Triage: This is no longer ICE with the latest nightly, seems #78422 fixed it.

[JohnTitor]

It's ICE'd again (https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=dcac74ce4f9ee21c114a7659c5e86783), too late to add a regression test :(, removing the label.
The errors are:

error: internal compiler error: no errors encountered even though `delay_span_bug` issued

error: internal compiler error: error: the type `T` has an unknown layout
                                 while computing layout for type T
 [--> src/main.rs:6:5
](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)  |
6 |     fn foo(self: T) -> dyn Trait<T>;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: delayed at compiler/rustc_trait_selection/src/traits/object_safety.rs:470:34

error: internal compiler error: receiver when `Self = ()` should have a Scalar ABI; found None
 [--> src/main.rs:6:5
](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)  |
6 |     fn foo(self: T) -> dyn Trait<T>;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: delayed at compiler/rustc_trait_selection/src/traits/object_safety.rs:486:30

error: internal compiler error: receiver when `Self = (dyn Trait<T> + 'static)` should have a ScalarPair ABI; found None
 [--> src/main.rs:6:5
](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)  |
6 |     fn foo(self: T) -> dyn Trait<T>;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: delayed at compiler/rustc_trait_selection/src/traits/object_safety.rs:506:30

[adetaylor]

I'd like to fix this as we're on the way to stabilizing arbitrary self types in #135881.

As a first proposal, a simple "fix" would be to switch from an ICE to an error message: PR #136195. I feel that's a legitimate and defensible thing to tell the user about; but if not, at least I'm hoping that this PR provokes some discussion about the "correct" fix.