Warning about "type alias never used", but alias is used for impl block

[RalfJung]

The following code

enum MyOption<T> { None, Some(T) }
use MyOption::*;

type MyType = MyOption<i32>;

impl MyType {
    fn foo(self) -> Self {
        None
    }
}

fn main() {
    Some(3).foo();
}

issues a warning in current stable and nightly:

rustc 1.18.0 (03fc9d622 2017-06-06)
warning: type alias is never used: `MyType`
 --> <anon>:4:1
  |
4 | type MyType = MyOption<i32>;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(dead_code)] on by default

However, the type alias is actually used -- if I remove it, the code stops compiling because the impl block references the alias. Hence, there should be no warning.

[durka]

Nothing is public so I believe this is intentional. I complained about a similar case in the past, with enum variants used in private functions, and was told the dead_code lint is not supposed to be a general-purpose "you may delete this code" (what is it for then? I don't know).

[durka]

However, a duplicate is #39531.

[RalfJung]

I don't think that's a duplicate... in #39531, the functions indeed don't get used, to if you remove all of what "dead code" complains about, the resulting program still works.

Here, I would need to refactor to make the lint go away. That seems silly.

It not being public can't be the point either. Just adding a let _x : MyType; somewhere makes the warning go away, and still nothing is public.

[cramertj]

As I initially reported in #43328, struct definitions also aren't considered when determining if a type alias is used.

[Mark-Simulacrum]

Closing in favor of #18290.