False positive for useless_transmute

[digama0]

Source: (SO question, playground link)

#![allow(clippy::needless_lifetimes)]
#![warn(clippy::useless_transmute)]

pub fn insert<'a, K: Eq, V>(this: &'a mut Vec<(K, V)>, key: K, val: V) -> &'a mut V {
    for (key1, val1) in & /* 'b */ mut *this {
        if key == *key1 {
            return unsafe {
                std::mem::transmute::<&/* 'b */ mut V, &/* 'a */ mut V>(val1)
            }
        }
    }
    let this = & /* 'c */ mut *this;
    this.push((key, val));
    &mut this.last_mut().unwrap().1
}

Result:

warning: transmute from a type (`&mut V`) to itself
 --> src/lib.rs:8:17
  |
8 |                 std::mem::transmute::<&/* 'b */ mut V, &/* 'a */ mut V>(val1)
  |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: the lint level is defined here
 --> src/lib.rs:2:9
  |
2 | #![warn(clippy::useless_transmute)]
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_transmute

The transmute is not useless because without it the code does not compile. The comments make it a little clearer what's going on: the transmute is being used to change lifetimes, but since this is not represented in the syntax it looks like it's casting a type to itself. My suggested fix would be to skip the lint if the type being cast to itself contains embedded lifetimes (either a shared or unique reference, or a lifetime parameter in the type).

Meta

• cargo clippy -V: clippy 0.0.212 (1773f60 2020-11-08)
• rustc -Vv:

  rustc 1.49.0-nightly (1773f60ea 2020-11-08)
  binary: rustc
  commit-hash: 1773f60ea5d42e86b8fdf78d2fc5221ead222bc1
  commit-date: 2020-11-08
  host: x86_64-unknown-linux-gnu
  release: 1.49.0-nightly
  

[rail-rain]

I think this is essentially a duplicate of #5343 (this also means the cause is actualy not the implicit syntax, but because Clippy checks types without running the borrow checker). The suggested fix is interesting though. Unlike #5343, which suggests using the real borrow checker; ignoring every types with some lifetimes is far easier to implement although it would introduce a false negative where U outlives T.