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Description
Starting in Rust nightly-2025-06-24, I'm seeing a new "type annotations needed" error, in tests/path/arg.rs in rustix. The code below is a reduced standalone testcase.
I tried this code:
use std::ffi::CStr;
use std::borrow::{Borrow, Cow};
fn red() -> &'static CStr {
todo!()
}
fn green() -> Cow<'static, CStr> {
todo!()
}
fn main() {
assert!(red() == Borrow::borrow(&green()))
}I expected to see this happen: The code compiles.
Instead, this happened: Starting with Rust nightly-2025-06-24, and still with nightly-2025-06-27, I get this error:
error[E0790]: cannot call associated function on trait without specifying the corresponding `impl` type
--> t.rs:12:22
|
12 | assert!(red() == Borrow::borrow(&green()))
| ^^^^^^^^^^^^^^^^^^^^^^^^ cannot call associated function of trait
|
help: use a fully-qualified path to one of the available implementations
|
12 | assert!(red() == <OsString as Borrow>::borrow(&green()))
| ++++++++++++ +
12 | assert!(red() == <PathBuf as Borrow>::borrow(&green()))
| +++++++++++ +
12 | assert!(red() == <&T as Borrow>::borrow(&green()))
| ++++++ +
12 | assert!(red() == <&mut T as Borrow>::borrow(&green()))
| ++++++++++ +
and 13 other candidates
error[E0283]: type annotations needed
--> t.rs:12:22
|
12 | assert!(red() == Borrow::borrow(&green()))
| -- ^^^^^^^^^^^^^^ cannot infer type of the type parameter `Borrowed` declared on the trait `Borrow`
| |
| type must be known at this point
|
= note: multiple `impl`s satisfying `CStr: PartialEq<_>` found in the following crates: `alloc`, `core`:
- impl PartialEq for CStr;
- impl PartialEq<&CStr> for CStr;
- impl PartialEq<CString> for CStr;
- impl PartialEq<Cow<'_, CStr>> for CStr;
= note: required for `&CStr` to implement `PartialEq<&_>`
help: consider specifying the generic argument
|
12 | assert!(red() == Borrow::<Borrowed>::borrow(&green()))
| ++++++++++++
Some errors have detailed explanations: E0283, E0790.
For more information about an error, try `rustc --explain E0283`.
error: could not compile `rustix` (example "t") due to 2 previous errors
Meta
The first nightly version with the error is:
rustc 1.90.0-nightly (28f1c8079 2025-06-24)
binary: rustc
commit-hash: 28f1c807911c63f08d98e7b468cfcf15a441e34b
commit-date: 2025-06-24
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.7
The last nightly version without the error is:
rustc 1.90.0-nightly (706f244db 2025-06-23)
binary: rustc
commit-hash: 706f244db581212cabf2e619e0113d70999b2bbe
commit-date: 2025-06-23
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.7