FP: `unused_self` triggers if only the lifetime of `self` is used

[FaultyRAM]

Tested on clippy shipping with Rust 1.40.0:

C:\>cargo clippy -V
clippy 0.0.212 (c8e3cfbd 2019-10-28)

Here's a contrived example that triggers the unused_self lint and fails to build:

#![deny(clippy::unused_self)]

use std::marker::PhantomData;

pub struct A;

pub struct B<'a>(PhantomData<&'a A>);

impl A {
    pub fn b(&self) -> B<'_> {
        B(PhantomData)
    }
}

This is actually a valid pattern. In FFI code for instance, A might be responsible for initialising and freeing some part of a library, and the function b is used to create objects that belong to the library, and therefore can't outlive it. So A can just be a ZST while B uses PhantomData<&'a A> to convey ownership semantics.

If we rewrite the example so it directly uses self it works:

#![deny(clippy::unused_self)]

pub struct A;

pub struct B<'a>(&'a A);

impl A {
    pub fn b(&self) -> B<'_> {
        B(self)
    }
}

But we don't want to hold pointers to A, we just want to express that instances of B belong to an instance of A.

tl;dr unused_self doesn't catch edge cases where self is used without actually being used.