False positive `unused_lifetime`

[matthiaskrgr]

I tried this code:

#![warn(unused_lifetimes)]
struct S;

pub fn repro()
    where
        for<'a> S: Clone,
    {
    }
    
pub fn main() {}

=>

warning: lifetime parameter `'a` never used
 --> src/main.rs:6:13
  |
6 |         for<'a> S: Clone,
  |         ----^^-- help: elide the unused lifetime
  |
note: the lint level is defined here
 --> src/main.rs:1:9
  |
1 | #![warn(unused_lifetimes)]

suggested code (cargo fix)

#![warn(unused_lifetimes)]
struct S;

pub fn repro()
    where
        S: Clone,
    {
    }
    
pub fn main() {}

does not compile:

error[E0277]: the trait bound `S: Clone` is not satisfied
 --> src/main.rs:6:9
  |
6 |         S: Clone,
  |         ^^^^^^^^ the trait `Clone` is not implemented for `S`
  |
  = help: see issue #48214
help: consider annotating `S` with `#[derive(Clone)]`
  |
2 + #[derive(Clone)]
3 | struct S;
  |
help: add `#![feature(trivial_bounds)]` to the crate attributes to enable
  |
2 + #![feature(trivial_bounds)]
  |

For more information about this error, try `rustc --explain E0277`.
error: could not compile `f` (bin "f") due to 1 previous error

rustc 1.89.0-nightly (1bbd62e54 2025-05-29)
binary: rustc
commit-hash: 1bbd62e547ba5cc08ccb44c27def3d33195d2dd5
commit-date: 2025-05-29
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.5

[Noratrieb]

This is definitely an abuse of higher ranked trait bounds, so I don't think the lint should handle this