fix type annotations to not require Self: 'this

[aliemjay]

Hi @WanderLanz,

Just a heads-up, lender will stop compiling after a recent soundness fix to the rust compiler in rust-lang/rust#104098. This was found by this crater run. The error message can be found here and you can try to build lender with that change by installing the toolchain first:

$ rustup-toolchain-install-master 6c5ba887f1fae50b77a36ca00e3d7164c1434657
$ cargo +6c5ba887f1fae50b77a36ca00e3d7164c1434657 build

A minimal example of what has changed in that PR is that the rust compiler used to incorrectly accept the following code:

trait Lending<'lend, _Witness = &'lend Self> {
    type Lend: 'lend;
}

struct Adapter<G>(G); // e.g. IntersperseWith

impl<'this, G> Lending<'this> for Adapter<G> {
    type Lend = (); // Lend does not depend on G
}

fn next<'this, G>() {
    let _: <Adapter<G> as Lending<'this>>::Lend;
    //~^ ERROR `G` may not live long enough
    // The full type is `<Adapter<G> as Lending<'this, &'this Adapter<G>>>::Lend`
    // The default type `&'this Adapter<G>` should require `T: 'this`
}

This can be fixed by adding a where clause G: 'this as suggested by the error message. Alternatively we can change the type annotation <Adapter<G> as Lending<'this>>::Lend to not require additional lifetime constraints. This is what I've done in this PR because it has the advantage of not breaking the public interface (although the other approach is only a theoretical breaking change. I would be surprised if it breaks real dependent code).

[vigna]

I'm really confused. The commit seem to have nothing to do with the post. Am I missing something?

[aliemjay]

I'm really confused. The commit seem to have nothing to do with the post. Am I missing something?

@vigna Before this change, the full annotated type here was <Self as Lending<'a, &'a Self>>::Lend, which requires &'a Self to be a "well-formed" reference type, which requires Self: 'a => RepeatWith<'a, L, F>: 'a => (F: 'a and L: 'a).
L: 'a is satified by the where clause, but F: 'a is not hence the error.

After this change, the full type is <L as Lending<'a, &'a L>>::Lend. This only requires L: 'a which is already satisfied by the bounds.

[WanderLanz]

Looks good, thank you for the heads up on future WF changes.

I'm not expecting vigna to have upstream tools to check, but if you do, please feel free to merge. Otherwise, I will merge/fix once I'm able to.

[vigna]

I tried to run crater on AWS, but documentation is 4yo and the build process dies on a too old version of CMake. I think it is something fairly easy to fix for someone using the toolchain normally, but after a couple of hours of trial and error I stopped.

I mean, I can check that this compiles now if that's sufficient for merge.

[aliemjay]

I'm not sure why you want to run crater yourself. The change I made here is definitely not a breaking change to downstream users and, if you want to test how lender builds against the proposed rustc change, you only need to use rustup-toolchain-install-master locally as described above.

[vigna]

Ah ok. I tried to read online how to get that command and I arrived to crater. I'll check again.

[vigna]

Ok. Command installed. I verified that the new nightly will stop compiling Lender. The proposed PR fixes this problem. All test pass.

[WanderLanz]

Awesome, thanks for the work.