False positive useless_transmute?

[afck]

If you (ab)use std::mem::transmute like this:

fn convert<'a>(foobar: &mut Foo<'a>) -> *mut Foo<'static> {
    unsafe { transmute(foobar) }
}

Clippy 0.0.212 complains:

warning: transmute from a reference to a pointer
  --> src/main.rs:10:14
   |
10 |     unsafe { transmute(foobar) }
   |              ^^^^^^^^^^^^^^^^^ help: try: `foobar as *mut Foo<'a> as *mut Foo<'static>`
   |
   = note: #[warn(clippy::useless_transmute)] on by default
   = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#useless_transmute

But Clippy's suggestion,

fn convert<'a>(foobar: &mut Foo<'a>) -> *mut Foo<'static> {
    unsafe { foobar as *mut Foo<'a> as *mut Foo<'static> }
}

does not compile:

error[E0308]: mismatched types
 --> src/main.rs:8:14
  |
8 |     unsafe { foobar as *mut Foo<'a> as *mut Foo<'static> }
  |              ^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
  |
  = note: expected type `*mut Foo<'static>`
             found type `*mut Foo<'a>`
note: the lifetime 'a as defined on the function body at 7:12...
 --> src/main.rs:7:12
  |
7 | fn convert<'a>(foobar: &mut Foo<'a>) -> *mut Foo<'static> {
  |            ^^
  = note: ...does not necessarily outlive the static lifetime

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.

https://play.rust-lang.org/?version=stable&mode=debug&edition=2015&gist=55287e186a312efc65617fbd87ff288a

[HMPerson1]

This could be considered a rustc bug since casting between raw pointers should always succeed.

What happened here was that, in the outer cast, rustc first tried to do the cast as a coercion, which succeeded, so it doesn't bother checking for the other kinds of casts, but the coercion generated the obligation that 'a: 'static, which later caused the compile to fail.

Note that foobar as *mut Foo<'a> as *mut u8 as *mut Foo<'static> works.