New "lifetime bound not satisfied" error on associated type

[laurmaedje]

Code

I tried this code:

pub struct Foo<'a> {
    // We need to override the constraint's lifetime here so that `Tracked` is
    // covariant over the constraint.
    _tracked: Tracked<'a, Self, <Foo<'static> as Validate>::Constraint>,
}

impl<'a> Validate for Foo<'a> {
    type Constraint = Constraint;
}

pub trait Validate {
    type Constraint;
}

pub struct Constraint;

pub struct Tracked<'a, T, C = <T as Validate>::Constraint>
where
    T: Validate + ?Sized,
{
    pub value: &'a T,
    pub constraint: Option<&'a C>,
}

I expected to see this happen: Compiles.

Instead, this happened: Fails with:

error[E0478]: lifetime bound not satisfied
 --> src/lib.rs:9:5
  |
9 |     type Constraint = Constraint;
  |     ^^^^^^^^^^^^^^^
  |
note: lifetime parameter instantiated with the lifetime `'a` as defined here
 --> src/lib.rs:8:6
  |
8 | impl<'a> Validate for Foo<'a> {
  |      ^^
  = note: but lifetime parameter must outlive the static lifetime

For more information about this error, try `rustc --explain E0478`.

The whole setup with the default type parameter that is overridden is required to get Tracked to be covariant over the constraint. The code above is a minimal extract from a usage of comemo in typst. Linked issue: typst/typst#2600

See also this discussion on Zulip for how this setup came to be in the first place: https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/.E2.9C.94.20Variance.20with.20associated.20types

Version it worked on

It most recently worked on:

rustc 1.73.0 (cc66ad468 2023-10-03)
binary: rustc
commit-hash: cc66ad468955717ab92600c770da8c1601a4ff33
commit-date: 2023-10-03
host: aarch64-apple-darwin
release: 1.73.0
LLVM version: 17.0.2

Version with regression

rustc 1.75.0-nightly (189d6c71f 2023-11-06)
binary: rustc
commit-hash: 189d6c71f3bb6c52113b5639a80839791974fd22
commit-date: 2023-11-06
host: aarch64-apple-darwin
release: 1.75.0-nightly
LLVM version: 17.0.4

[lqd]

I believe this is some other fallout from #117131, like #117598, and that it will be fixed by #117542 which should land real soon.

[Finchiedev]

Running cargo bisect-rustc seems to confirm #117131 is the culprit:

searched nightlies: from nightly-2023-11-03 to nightly-2023-11-07
regressed nightly: nightly-2023-11-04
searched commit range: a2f5f96...1bb6553
regressed commit: 2520ca8

bisected with cargo-bisect-rustc v0.6.7

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc -- check 

[Jules-Bertholet]

@rustbot label -regression-untriaged regression-from-stable-to-nightly

@compiler-errors

[Finchiedev]

With #117542 merged, I believe this issue has been resolved? I am no longer able to reproduce the error on the latest nightly (rustc 1.75.0-nightly (edf0b1db0 2023-11-10))

[apiraino]

@laurmaedje can you confirm the above comment? That is, is this issue fixed for you, too? Thanks

[laurmaedje]

Yes, it's resolved for me. Thanks!