Rust is about to start warning about this by default, which can trigger a lot of warnings, but unfortunately the warnings can't be fixed automatically:

#![warn(mismatched_lifetime_syntaxes)]

fn lifetimes(x: &str) -> std::borrow::Cow<str> {
    x.into()
}

Running these commands has no effect:

cargo fix
cargo fix --broken-code
cargo clippy --fix
cargo clippy --fix --broken-code

The code remains unfixed, and the warning keeps triggering.

cargo 1.90.0-nightly (9b296973b 2025-07-25)
rustc 1.90.0-nightly (f8e355c 2025-07-27)