Lifetime transmutation of Generic Associated Types does not work

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Thank you for your response and the idea with the monomorphization check. I'm not sure if mem::transmute_copy(&*mem::ManuallyDrop::new(src)) would be zero-cost though?

For me, the best solution probably is to not use transmute and instead (implicitly) pass a lifetime to get_unsafe as in my last code block.

Interestingly, I wasn't able to pass the lifetime explicitly to get_unsafe:

warning: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> src/lib.rs:914:42
    |
861 |         &mut self,
    |         - the late bound lifetime parameter is introduced here
...
914 |         unsafe { Ok(backend.get_unsafe::<'b, _, K, V>(self.env, db, key)?) }
    |                                          ^^
    |
    = note: `#[warn(late_bound_lifetime_arguments)]` on by default
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #42868 <https://github.com/rust-lang/rust/issues/42868>

But if I omit the 'b from the turbofish it works fine.

Now I totally forgot again what late-boundedness was. It had to do something with passing lifetimes explicitly. What was this rhyme again? … Late bound, un-sound, spinning around… :face_with_spiral_eyes:

No, it was: