How to handle reservation impls in rustdoc / error messages

[nikomatsakis]

I've opened this issue for discussion on how to handle reservation impls (see #64631) in rustdoc and how to handle their error messages.

[nikomatsakis]

My take:

We should not show reservation impls in rustdoc at all. They don't exist and they may never exist. I'm not convinced users want to see them. Moreover, they are only relevant if you're trying to rely on negative reasoning about From<!> for T, which seems unusual in practice.

[nikomatsakis]

The current error message is this:

error[E0119]: conflicting implementations of trait `OtherTrait` for type `()`:
  --> $DIR/reservation-impl-coherence-conflict.rs:13:1
   |
LL | impl OtherTrait for () {}
   | ---------------------- first implementation here
LL | impl<T: MyTrait> OtherTrait for T {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `()`
   |
   = note: this impl is reserved

error: aborting due to previous error

I think this is pretty clearly not good enough. However, I also don't think we're going to do a lot better in the error message. I suggest opening an issue for each use of #[rustc_reservation_impl] and explaining the situation there. Then we can say something like:

   = note: permitting this impl would forbid us from adding `impl<T> From<!> for T` later; see rust-lang/rust#12345 for details

[nikomatsakis]

Nominating for @rust-lang/lang discussion -- I'd love to get feedback on how to handle these errors, so that we can land this feature and unblock the ! stabilization.

[joshtriplett]

@nikomatsakis That error text sounds good to me.

[Centril]

I think diagnostics and rustdoc is squarely a T-compiler and T-rustdoc matter. I have no opinion other than to say that @nikomatsakis's suggestion seems reasonable but this is not my decision to make.

[joshtriplett]

I think there ought to be documentation of "you may not make any impl like this" somewhere. How about this: we lint on a rustc_reservation_impl that doesn't have a doc comment, we explain in the doc comment, and rustdoc just emits the doc comment next to the list of documented impls?