compiler crash using specialization

[avl]

I tried this code:

#![feature(specialization)]

pub enum Either<L,R> {
    Left(L),
    Right(R),
}

default impl<L,R> From<L> for Either<L,R> {
    fn from(l: L) -> Self {
        Either::Left(l)
    }
}
impl<L,R> From<R> for Either<L,R> {
    fn from(r: R) -> Self {
        Either::Right(r)
    }
}

To be completely honest, I wasn't sure this would really work. But I suppose it could be argued that the compiler shouldn't crash.

Instead, this happened:

Compiling playground v0.0.1 (/playground)
warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
 --> src/lib.rs:1:12
  |
1 | #![feature(specialization)]
  |            ^^^^^^^^^^^^^^
  |
  = note: `#[warn(incomplete_features)]` on by default
  = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information

error[E0275]: overflow evaluating the requirement `Either<!, _>: From<!>`
  |
  = help: consider adding a `#![recursion_limit="256"]` attribute to your crate (`playground`)
  = note: required because of the requirements on the impl of `From<!>` for `Either<!, _>`
  = note: required because of the requirements on the impl of `From<!>` for `Either<!, _>`
  = note: required because of the requirements on the impl of `From<!>` for `Either<!, _>`
  = note: required because of the requirements on the impl of `From<!>` for `Either<!, _>`
  = note: required because of the requirements on the impl of `From<!>` for `Either<!, _>`
  = note: required because of the requirements on the impl of `From<!>` for `Either<!, _>`
  = note: required because of the requirements on the impl of `From<!>` for `Either<!, _>`
  = note: required because of the requirements on the impl of `From<!>` for `Either<!, _>`
  = note: required because of the requirements on the impl of `From<!>` for `Either<!, _>`
  = note: required because of the requirements on the impl of `From<!>` for `Either<!, _>`
  = note: required because of the requirements on the impl of `From<!>` for `Either<!, _>`
  = note: required because of the requirements on the impl of `From<!>` for `Either<!, _>`
  = note: required because of the requirements on the impl of `From<!>` for `Either<!, _>`
  = note: required because of the requirements on the impl of `From<!>` for `Either<!, _>`
  = note: required because of the requirements on the impl of `From<!>` for `Either<!, _>`
  = note: required because of the requirements on the impl of `From<!>` for `Either<!, _>`
  = note: required because of the requirements on the impl of `From<!>` for `Either<!, _>`

etc etc etc

This happened just now on 2020-09-21 at the Rust playground: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=005db744fc083053388b84c8b68d1751 .

I know specialization isn't stable, so there are no guarantees, and maybe this is a known issue. But if someone is currently working on stabilizing specialization, maybe they'd be interested in cleaning up the above error message.

Anyway, a great thank you to all the fantastic people who work on making Rust more awesome!

[steffahn]

@camelid that’s not a crash though, right? @rustbot label -I-crash

[avl]

@camelid that’s not a crash though, right? @rustbot label -I-crash

Ah, you're right. I saw the "error[E0275]: overflow evaluating the requirement Either<!, _>: From<!>", and thought for some reason it was an overflow in the compiler itself. But it's more an overflow resulting from the parsing of the input program, not quite the same.

[camelid]

@camelid that’s not a crash though, right? @rustbot label -I-crash

Correct, thanks for correcting that! It's possible this isn't even a bug in the compiler, but I don't know enough about specialization to tell if this code should work.