clippy::manual_map gives incorrect advice when dyn coercion is involved

[mcy]

I tried this code:

trait T {}
struct K(Option<Box<dyn T>>);

fn compiles(k: &mut K) -> Option<&mut dyn T> {
    match &mut k.0 {
        Some(x) => Some(&mut **x),
        None => None,
    }   
}

Clippy suggests rewriting this to

fn advice(k: &mut K) -> Option<&mut dyn T> {
    k.0.as_mut().map(|x| &mut **x)
}

which fails to compile with the classic lifetime inference error:

error[E0308]: mismatched types
  --> src/lib.rs:13:5
   |
13 |     k.0.as_mut().map(|x| &mut **x)
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
   |
   = note: expected enum `Option<&mut dyn T>`
              found enum `Option<&mut (dyn T + 'static)>`
note: the anonymous lifetime defined on the function body at 12:14...
  --> src/lib.rs:12:14
   |
12 | fn advice(k: &mut K) -> Option<&mut dyn T> {
   |              ^^^^^^
   = note: ...does not necessarily outlive the static lifetime

In the latter case, Rust's type inference does not correctly infer that a coersion from dyn T + 'static to dyn T + '_ needs to happen.

Clippy should instead suggest

fn correct(k: &mut K) -> Option<&mut dyn T> {
    k.0.as_mut().map(|x| &mut **x as &mut dyn T)
}

(TBH, this might just be a rustc inference bug?)

Meta

• cargo clippy -V: clippy 0.1.52 (9bc8c42 2021-05-09)
• rustc -Vv:

  rustc 1.52.1 (9bc8c42bb 2021-05-09)
  binary: rustc
  commit-hash: 9bc8c42bb2f19e745a63f3445f1ac248fb015e53
  commit-date: 2021-05-09
  host: x86_64-unknown-linux-gnu
  release: 1.52.1
  LLVM version: 12.0.0
  

[ghost]

We've fixed similar bugs by making the suggestion include as _ when necessary. match_as_ref does it for example. See #4437.

[Jarcho]

This no longer occurs. Would have to run backwards through releases to find which version fixed it.