regression: conflicting implementations of trait `Visitable` for type `Box<_>`

[Mark-Simulacrum]

Not entirely clear if this is caused by a new impl in std or trait solver changes.

• https://crater-reports.s3.amazonaws.com/beta-1.80-4/beta-2024-06-22/reg/another-visitor-0.1.0/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.80-4/beta-2024-06-22/reg/derive-visitor-0.3.0/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.80-4/beta-2024-06-22/reg/gfx-xtra-0.2.0/log.txt

[INFO] [stdout] error[E0119]: conflicting implementations of trait `Visitable` for type `Box<_>`
[INFO] [stdout]   --> src/std_impls.rs:49:1
[INFO] [stdout]    |
[INFO] [stdout] 25 | / impl<C: 'static> Visitable for C
[INFO] [stdout] 26 | | where
[INFO] [stdout] 27 | |     for<'a> &'a C: IntoIterator,
[INFO] [stdout] 28 | |     for<'a> <&'a C as IntoIterator>::Item: AsVisitable<'a>,
[INFO] [stdout]    | |___________________________________________________________- first implementation here
[INFO] [stdout] ...
[INFO] [stdout] 49 |   impl<T: Visitable> Visitable for Box<T> {
[INFO] [stdout]    |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Box<_>`
[INFO] [stdout]    |
[INFO] [stdout]    = note: downstream crates may implement trait `std_impls::AsVisitable<'a>` for type `<&'a std::boxed::Box<_> as std::iter::IntoIterator>::Item`

[compiler-errors]

Bisected derive-visitor example, since that crate has come up a few times. This bisected to #117164 (comment) cc @lcnr @fmease, though that particular crate didn't get mentioned in the stabilization report.

[apiraino]

Labelling also this as P-high for now (Zulip discussion).

@rustbot label -I-prioritize +P-high

[lcnr]

all of these look like #114061 to me, I consider this to be intended breakage.

[apiraino]

all of these look like #114061 to me, I consider this to be intended breakage.

ok then I think it makes sense to add relnotes on #117164

thanks @lcnr :)