Unused lifetime type parameters

[regexident]

Leaving an unused type parameter …

struct Foo;

impl<T> Foo {
    fn foo() {}
}

… triggers the following error:

error: the type parameter `T` is not constrained
by the impl trait, self type, or predicates [E0207]

Leaving an unused lifetime parameter however …

struct Bar;

impl<'a> Bar {
    fn bar() {}
}

does not.

We should lint these, shouldn't we? Or should I file a rustc bug report?

[llogiq]

There's the unused_lifetimes lint. Doesn't that catch your case?

[regexident]

No, apparently it doesn't.

This …

impl<'a> Bar {
    fn foo<'b>() { }
}

… only triggers one clippy warning. For fn foo:

src/lib.rs:4:12: 4:14 warning: this lifetime isn't used in the function definition,
#[warn(unused_lifetimes)] on by default
src/lib.rs:4     fn foo<'b>() { }
                        ^~
src/lib.rs:4:12: 4:14 help: for further information visit
https://github.com/Manishearth/rust-clippy/wiki#unused_lifetimes

[llogiq]

Ok, so this should be an extension to the unused_lifetimes lint that should also check ImplItems (and probably TraitImplItems), which it currently apparently does not.

[birkenfeld]

I'll note that this behavior is currently pretty nice for writing macros that handle reference and non-reference types, because it saves you some contortions that come from the fact that there is no macro matcher for Generics.

Of course, for clippy, that doesn't really matter since we'd exclude macro expansions anyway.

[varkor]

The unused_lifetimes lint now captures the example in #877 (comment), so it might now be sufficient.

[flip1995]

Is this a new addition to the rustc lint, that hasn't made it into the playground yet? Since it doesn't get linted on the Playground.

[varkor]

@flip1995: you were missing the !s — here's a working link.

[flip1995]

Ohhhhh, sorry 🤦‍♂️ Then this can be closed