Diagnostic recommends `+ '_` instead of `+ use<'_>` or nothing to express capturing

[purplesyringa]

Code

use std::fmt::Debug;

fn f(x: &i32) -> (Box<dyn Debug>, impl Debug) {
    (Box::new(x), x)
}

Current output

error: lifetime may not live long enough
 --> src/lib.rs:4:5
  |
3 | fn f(x: &i32) -> (Box<dyn Debug>, impl Debug) {
  |         - let's call the lifetime of this reference `'1`
4 |     (Box::new(x), x)
  |     ^^^^^^^^^^^^^^^^ returning this value requires that `'1` must outlive `'static`
  |
help: to declare that the trait object captures data from argument `x`, you can add an explicit `'_` lifetime bound
  |
3 | fn f(x: &i32) -> (Box<dyn Debug + '_>, impl Debug) {
  |                                 ++++
help: to declare that `impl Debug` captures data from argument `x`, you can add an explicit `'_` lifetime bound
  |
3 | fn f(x: &i32) -> (Box<dyn Debug>, impl Debug + '_) {
  |                                              ++++

Desired output

Rationale and extra context

Legacy from #72543 when we didn't have precise capturing and a good understanding of outlives vs captures relationships.

We should probably replace the advice for the RPIT with impl Debug + use<'_>. On edition 2024, the RPIT hint should be removed altogether -- there doesn't seem to be any check against recommending implicit captures currently.

I have no idea about the right solution for dyn. We could maybe leave it as-is, but

fn f<'a, 'b>(x: &'a i32, y: &'b i32) -> Box<dyn Debug> {
    if true {
        Box::new(x)
    } else {
        Box::new(y)
    }
}

currently recommends to add both + 'a and + 'b, which obviously doesn't work. Does anyone know if there's ongoing work on adding use to dyn?

Other cases

Rust Version

rustc 1.90.0-nightly (a00149764 2025-07-14)
binary: rustc
commit-hash: a001497644bc229f1abcc5b2528733386591647f
commit-date: 2025-07-14
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.8

Anything else?

No response