Lint for unused lifetime parameters

[jruderman]

This code in borrow.rs could trip a warning about having unnecessary lifetime parameters:

pub fn ref_eq<'a, 'b, T>(thing: &'a T, other: &'b T) -> bool {
    to_uint(thing) == to_uint(other)
}

[nikomatsakis]

cc me

[reem]

Triage: This is even more important given lifetime elision - we should warn when lifetimes are included that could be elided, not just when they aren't used.

[huonw]

I disagree about elision. It's very reasonable to include explicit lifetimes for clarity or to be 100% sure that unsafe code is doing what you want. (I could imagine a separate elidable_lifetimes lint, on allow by default.)

[zwarich]

I agree with @huonw. In Servo, where we have a lot of unsafe code, explicit lifetime parameters make things more understandable. The &self rule makes a lot of programs typecheck, but it is sometimes tricky for people to unravel when they have to investigate an issue.

[reem]

That seems like a perfectly reasonable approach. I'll make an issue for a new lint for elidable lifetimes.

[sanxiyn]

This is implemented as needless_lifetimes in Clippy. @jruderman's example warns. Closing.