Enforce that dyn* coercions are actually pointer-sized

[compiler-errors]

Implement a perma-unstable, rudimentary PointerSized trait to enforce dyn* casts are usize-sized for now, at least to prevent ICEs and weird codegen issues from cropping up after monomorphization since currently we enforce nothing.

This probably can/should be removed in favor of a more sophisticated trait for handling dyn* conversions when we decide on one, but I just want to get something up for discussion and experimentation for now.

r? @eholk cc @tmandry (though feel free to claim/reassign)

Fixes #102141
Fixes #102173

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[workingjubilee]

what busts where Pointee::Metadata = ()?

[compiler-errors]

@workingjubilee -- are you asking why this is enforcing something that's stricter than just T: Pointee<Metadata = ()>? Well, the currently implementation of dyn* treats dyn* Trait as being usize-sized, so we need to enforce that the thing that's being coerced into a dyn* is that size as well.

We could probably relax this to check size <= sizeof(usize), but I wanted to keep it simple. Also I need to do an alignment check I guess.

If that isn't what you're asking, then could you maybe re-explain?

[compiler-errors]

Unless you were asking what happens in the case like this:

#![feature(ptr_metadata)]
#![feature(dyn_star)]

use std::fmt::Debug;
use std::ptr::Thin;

fn polymorphic<T: Debug + ?Sized + Thin>(t: &T) {
    let _ = t as dyn* Debug;
}

In which case:

error[E0277]: `&T` needs to be a pointer-sized type
 --> /home/gh-compiler-errors/test.rs:9:13
  |
9 |     let _ = t as dyn* Debug;
  |             ^ `&T` needs to be a pointer-sized type
  |
  = help: the trait `PointerSized` is not implemented for `&T`
help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
  |
8 | fn polymorphic<T: Debug + ?Sized + Thin>(t: &T) where &T: PointerSized {
  |                                                 ++++++++++++++++++++++

This happens. Layout computation is not smart enough to take in account that T: Thin implies sizeof(&T) = sizeof(usize) 🤔

[workingjubilee]

Hmm, I'm not sure which I was asking, honestly, but I am satisfied with that answer, aside from the question of whether or not layout computation could simply be taught that instead I suppose. But that can be tabled for now.

[compiler-errors]

Yeah, I have a follow-up commit to teach layout computation exactly that, but I want to separate that from this PR since it maybe needs a bit more inspection.

[bors]

☔ The latest upstream changes (presumably #104428) made this pull request unmergeable. Please resolve the merge conflicts.

[eholk]

@bors r+

[bors]

📌 Commit 5edbf2c has been approved by eholk

It is now in the queue for this repository.

[compiler-errors]

@bors r-

[compiler-errors]

@rustbot ready

[eholk]

@bors r+

[bors]

📌 Commit 4e20fa5209c868c48a6f044e76447bef004a87a7 has been approved by eholk

It is now in the queue for this repository.

[bors]

☔ The latest upstream changes (presumably #104555) made this pull request unmergeable. Please resolve the merge conflicts.

[compiler-errors]

Rebased past oli's Obligation constructor PR.

@bors r=eholk

[bors]

📌 Commit 39e076a has been approved by eholk

It is now in the queue for this repository.