Rust: Warn on unused lifetime parameters

[clarfonthey]

Somehow this made it into master without being warned:

#[stable(feature = "os_string_from_box", since = "1.17.0")]
impl<'a> From<Box<OsStr>> for OsString {
    fn from(boxed: Box<OsStr>) -> OsString {
        boxed.into_os_string()
    }
}

This should definitely trigger a warning.

[Thiez]

Yikes, no warnings at all.

fn main() {
    trait T {}
    impl<'a> T for u8 {}
}

[estebank]

Triage: no change.

[varkor]

Note that this is currently intentional behaviour:
https://github.com/rust-lang/rust/blob/e57577314120bb5504d5ce4b180fb6befc86f672/src/librustc_typeck/impl_wf_check.rs#L115
https://github.com/rust-lang/rust/blob/e57577314120bb5504d5ce4b180fb6befc86f672/src/librustc_typeck/impl_wf_check.rs#L139-L156

[clarfonthey]

@varkor they should be accepted, just linted.

[scottmcm]

I think this still needs a warn-by-default lint, because I just spotted this in liballoc:

https://github.com/rust-lang/rust/blob/9a3392e174a432a5f06c1157befc1ce0fae74bec/src/liballoc/collections/vec_deque.rs#L2227-L2236

Edit: found another one:

https://github.com/rust-lang/rust/blob/9a3392e174a432a5f06c1157befc1ce0fae74bec/src/libstd/panic.rs#L322-L330

[JustAPerson]

pub type UnusedHrtb = for<'a> fn();

Found something like this in my codebase today. No warning from rustc or clippy.

[clarfonthey]

Any plans to add a warn-by-default lint?

[Mark-Simulacrum]

We have an unused_lifetimes lint now.

[estebank]

@Mark-Simulacrum it is not warn by default and I think it probably should be (gated may be on ecosystem prevalence).

[Mark-Simulacrum]

Hm, yeah, that makes sense. Reopening. I personally think we can probably "just" land this as a PR with FCP T-lang.

[varkor]

https://github.com/rust-lang/rust/issues/64493 should be considered blocking for making this warn-by-default.