Lifetime parameters in type aliases do not need to be used in the definition

[Darksonn]

The following code compiles, even though it would emit an error about the lifetime being unused in any other context:

type Huh<'a> = String;

Interestingly enough,

type Huh<'a> = String;

struct MyStruct<'a> {
    foo: Huh<'a>,
}

fails to compile with

error[E0392]: parameter `'a` is never used
 --> src/lib.rs:3:17
  |
3 | struct MyStruct<'a> {
  |                 ^^ unused parameter
  |
  = help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`

I'm not sure what exactly is going on here, but if nothing else, this error message seems pretty confusing.

context: link

[ada4a]

Quoting #119236 (comment):

This is only tangentially related but the specific code you posted will stop compiling anyway once lazy type aliases (#112792) become the default in Rust 202X:

error[E0392]: parameter `'a` is never used
 --> src/main.rs:1:10
  |
1 | type Huh<'a> = String;
  |          ^^ unused parameter
  |
  = help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`

[Darksonn]

I'm going to close this, as I don't think it's particularly actionable. I added the above example to a test in #163176 so it won't be missed as a regression if this behavior is changed.