Allow pass by reference if we return a reference

[etaoins]

Currently this code will trigger trivally_copy_pass_by_ref:

struct OuterStruct {
    field: [u8; 8],
}

fn return_inner(outer: &OuterStruct) -> &[u8] {
    &outer.field
}

If we change the outer to be pass-by-value it will not live long enough for us to return the reference. The above example is trivial but I've hit this in real code that either returns a reference to either the argument or in to self.

This suppresses the trivally_copy_pass_by_ref lint if we return a reference and it has the same lifetime as the argument. This will likely miss complex cases with multiple lifetimes bounded by each other but it should cover the majority of cases with little effort.

Fixes #2946

[flip1995]

Yeah this won't catch

fn good_return_explicit_lt_ref_bound<'a, 'b: 'a>(foo: &'b Foo) -> &'a u32 {
    &foo.0
}

I think we need to check somehow if the lifetime of the argument outlives the lifetime of the return value. But I don't know how to do this off the top of my head.

It would be great if you could add this to the Known Problems section of the lint. Everything else LGTM.

[etaoins]

@flip1995 Updated with a note in the Known Problems section

[flip1995]

Thanks!