useless-transmute doesn't take lifetimes into account

[ndmitchell]

Given:

use std::cell::Cell;
pub struct V<'v>(Cell<&'v ()>);

pub unsafe fn downcast_v<'v>(x: V<'static>) -> V<'v> {
    std::mem::transmute(x)
}

If you enable -D clippy::useless-transmute then Clippy warns:

error: transmute from a type (`foo:V`) to itself

But it's actually V<'static> to V<'v>, so a transmute is required.

$ cargo clippy -V
clippy 0.0.212 (891e1a85 2020-04-18)

[flip1995]

Duplicate of #5343