False positives when using `transmute()` with raw trait object pointers to change trait object lifetimes

[okready]

With clippy 0.0.212, I'm getting what appears to be false positives when using transmute() to cast either to or from a raw trait object pointer where the input and output types have invariant lifetimes, or if the output type has a longer lifetime such as 'static (casting using as isn't allowed in such cases). No warnings are triggered if the source and target types are both references.

use std::mem::transmute;

pub trait DummyTrait {}

pub unsafe fn ref_to_ref<'a, 'b>(obj: &'a (DummyTrait + 'a)) -> &'b (DummyTrait + 'b) {
    // No warnings.
    transmute(obj)
}

pub unsafe fn ptr_to_ptr<'a, 'b>(obj: *const (DummyTrait + 'a)) -> *const (DummyTrait + 'b) {
    // Triggers `transmute_ptr_to_ptr` warning.
    transmute(obj)
}

pub unsafe fn ptr_to_ref<'a, 'b>(obj: *const (DummyTrait + 'a)) -> &'b (DummyTrait + 'b) {
    // Triggers `transmute_ptr_to_ref` warning.
    transmute(obj)
}

pub unsafe fn ref_to_ptr<'a, 'b>(obj: &'a (DummyTrait + 'a)) -> *const (DummyTrait + 'b) {
    // Triggers `useless_transmute` warning.
    transmute(obj)
}

[rail-rain]

The warnings has changed due to #5343. Here is the lastest one for your convinience.

#![allow(clippy::missing_safety_doc)]
// `useless_transmute` is now an allowed-by-default lint.
#![warn(clippy::useless_transmute)]

use std::mem::transmute;

pub trait DummyTrait {}

pub unsafe fn ref_to_ref<'a, 'b>(obj: &'a (dyn DummyTrait + 'a)) -> &'b (dyn DummyTrait + 'b) {
    // Triggers `useless_transmute` warning.
    // Note: this is the false positive introduced #5343
    transmute(obj)
}

pub unsafe fn ptr_to_ptr<'a, 'b>(obj: *const (dyn DummyTrait + 'a)) -> *const (dyn DummyTrait + 'b) {
    // Triggers `useless_transmute` warning.
    // Note: `useless_transmute` took over because `useless_transmute` has a higher priority than `transmute_ptr_to_ptr`
    // besides the false positive above.
    transmute(obj)
}

pub unsafe fn ptr_to_ref<'a, 'b>(obj: *const (dyn DummyTrait + 'a)) -> &'b (dyn DummyTrait + 'b) {
    // Triggers `transmute_ptr_to_ref` warning.
    transmute(obj)
}

pub unsafe fn ref_to_ptr<'a, 'b>(obj: &'a (dyn DummyTrait + 'a)) -> *const (dyn DummyTrait + 'b) {
    // Triggers `useless_transmute` warning.
    transmute(obj)
}

This issue means (every?) lints for transmute don't account lifetimes, including transmutes_expressible_as_ptr_cast (if you commented out other transmute related lints, it would fire on the example).

[rail-rain]

@rustbot label +E-medium +L-bug +L-false-positive