clippy::useless_conversion suggested fix breaks code with lifetimes

[m-mueller678]

Summary

clippy::useless_conversion is issued when Into is used to convert to the same type but with different lifetime. The suggested fix breaks the code.

rust playground

Lint Name

useless_conversion

Reproducer

I tried this code:

pub trait MyTrait {
    type Borrowed<'a>;
}

pub fn make_static<T: MyTrait>(x: T::Borrowed<'_>) -> T::Borrowed<'static>
where
    for<'a> T::Borrowed<'a>: Into<T::Borrowed<'static>>,
{
    x.into()
}

I saw this happen:

  |
9 |     x.into()
  |     ^^^^^^^^ help: consider removing `.into()`: `x`

This breaks:

pub fn make_static<T: MyTrait>(x: T::Borrowed<'_>) -> T::Borrowed<'static>
where
    for<'a> T::Borrowed<'a>: Into<T::Borrowed<'static>>,
{
    x
}

9 |     x
  |     ^ returning this value requires that `'1` must outlive `'static`

Version

rust stable 1.85.0
clippy 0.1.85 (2025-02-17 4d91de4e48)

Additional Labels

@rustbot label +I-suggestion-causes-error