Compiler allows type aliases with unused lifetime parameters

[abonander]

First noticed with this funky type alias, which probably needed a lifetime at one point but doesn't anymore.

Simplest case is the most trivial: create a type alias with a lifetime parameter, but don't use it on the RHS:

// No Error
pub type MyType<'a> = String;

// Errors
// pub type MyType<T> = String;

// Errors
// pub struct MyType<'a> {}

Playground

The linked example compiles on all three channels.

As far as I understand it, unused type parameters are always supposed to be hard errors, as established in RFC 738, so this must be a corner case that was never implemented, or otherwise a regression. I haven't tested previous releases, I might do that tomorrow if someone hasn't gotten to this already,

[eddyb]

You can't misuse it because the checks are done on the semantic type, so it sees right through type aliases, e.g. impl<'a> Foo for MyType<'a> will fail because Foo for String doesn't mention 'a.

[abonander]

@eddyb Are you sure about that?

[eddyb]

@cybergeek94 You're not using it anywhere so it's actually ignored, apparently. Here's the error.

[Mark-Simulacrum]

I'm going to mark this is a diagnostics issue since it's likely an error centering on the type alias's definition site would be a lot more helpful here.

[Mark-Simulacrum]

Closing in favor of #35160.

[abonander]

@Mark-Simulacrum you just closed this issue in favor of itself.

[Mark-Simulacrum]

Someone fix clipboards please; #41960.