Replace named generic with impl assist when the type variable occurs more than once

The "Replace named generic with impl trait" assist can change the semantics of the code if the type variable being replaced occurs more than once. For example, the following code fails to compile after applying the assist (in other cases it can be a silent change):

fn common_count<T: Eq>(slice: &[(T, T)]) -> usize {
    slice.iter().filter(|(a, b)| a == b).count()
}

Either the assist should not apply in such cases or at least this behavior should be documented.

rust-analyzer version: 0.3.1689-standalone

rustc version: rustc 1.73.0 (cc66ad468 2023-10-03)

relevant settings: None

Start by locating the implementation and tests for the "Replace named generic with impl trait" assist. Reproduce the transformation with the provided common_count example, then determine whether repeated uses of the type variable should prevent the assist or be documented; done means the semantic regression is covered by the assist's tests.