regression: the method `as_ref` exists for reference

[BoxyUwU]

• https://crater-reports.s3.amazonaws.com/beta-1.91-2/beta-2025-09-21/gh/ndchorley.pipeline-runner/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.91-2/beta-2025-09-21/gh/RouHim.beammp-server-beiwagen/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.91-2/beta-2025-09-21/gh/RouHim.envini/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.91-2/beta-2025-09-21/gh/Stephen-X.rust-csharp-async-ffi/log.txt

[INFO] [stdout] error[E0599]: the method `as_ref` exists for reference `&Cow<'_, T>`, but its trait bounds were not satisfied
[INFO] [stdout]   --> /opt/rustwide/cargo-home/registry/src/index.crates.io-1949cf8c6b5b557f/assertor-0.0.3/src/assertions/cow.rs:77:35
[INFO] [stdout]    |
[INFO] [stdout] 77 |         let value = self.actual().as_ref().to_owned();
[INFO] [stdout]    |                                   ^^^^^^ method cannot be called on `&Cow<'_, T>` due to unsatisfied trait bounds
[INFO] [stdout]    |
[INFO] [stdout]    = note: the following trait bounds were not satisfied:

[theemathas]

All four regressions are via the dependency crate assertor. The code in that crate is minimized to the following:

use std::borrow::Cow;

pub fn foo<'a, T: ToOwned<Owned = U>, U>(x: Cow<'a, T>) {
    let _ = x.as_ref();
}

Error on beta:

error[E0599]: the method `as_ref` exists for enum `Cow<'a, T>`, but its trait bounds were not satisfied
   --> src/lib.rs:4:15
    |
  4 |     let _ = x.as_ref();
    |               ^^^^^^ method cannot be called on `Cow<'a, T>` due to unsatisfied trait bounds
    |
   ::: /playground/.rustup/toolchains/beta-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/borrow.rs:181:1
    |
181 | pub enum Cow<'a, B: ?Sized + 'a>
    | -------------------------------- doesn't satisfy `Cow<'_, T>: AsRef<T>`
    |
    = note: the following trait bounds were not satisfied:
            `U: Borrow<T>`
            which is required by `Cow<'_, T>: AsRef<T>`
help: consider restricting the type parameter to satisfy the trait bound
    |
  3 | pub fn foo<'a, T: ToOwned<Owned = U>, U>(x: Cow<'a, T>) where U: Borrow<T> {
    |                                                         ++++++++++++++++++

For more information about this error, try `rustc --explain E0599`.

[theemathas]

Probably regressed in #145279
cc @clarfonthey

[joshtriplett]

Looking at that minimal reduction:

All four regressions are via the dependency crate assertor. The code in that crate is minimized to the following:

use std::borrow::Cow;

pub fn foo<'a, T: ToOwned<Owned = U>, U>(x: Cow<'a, T>) {
    let _ = x.as_ref();
}

This seems very strange to me. The bound T: ToOwned<Owned = U>, given that ToOwned has type Owned: Borrow<Self>;, should imply U: Borrow<T>, but that error suggests that the trait solver doesn't do that.

Could we get some trait solver expertise in here to explain 1) why this breaks and 2) what changed to trigger this in this release?

[joshtriplett]

ping @rust-lang/types