Wrong unused warning for type alias

[therustmonk]

This code:

struct Runner;

type RuntimeImpl = Runner;

trait Runtime {
    fn run(&mut self);
}

impl Runtime for &mut RuntimeImpl {
    fn run(&mut self) { }    
}

fn main() {
    let mut runner = Runner;
    (&mut runner).run();
}

takes the warning:

   Compiling playground v0.0.1 (/playground)
warning: type alias is never used: `RuntimeImpl`
 --> src/main.rs:3:1
  |
3 | type RuntimeImpl = Runner;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(dead_code)] on by default

But removing this alias introduces a compilation error 🤔

The link to the nightly playground example: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=22a044608f4d67680bcf1cc7fd23a381

[Lonami]

There is no compilation error if you remove the alias (and of course rename impl ... for &mut Runner), at least for me.

[Patryk27]

@Lonami: If you have to remove the alias and rename the impl, it's wrong to say that the alias was not used in the first place - isn't it? It seems that Rust ignores fact that aliases can have impls and shows the type alias is never used warning anyway.

[Lonami]

@Patryk27 I'm not talking about the warning, which is what the issue is for (and it is a valid issue), I'm talking about:

But removing this alias introduces a compilation error 🤔

[Patryk27]

If you remove the alias, there is a compilation error: cannot find type 'RuntimeImpl' in this scope. What @deniskolodin meant, as I think, is that this error message contradicts the type alias is never used warning.

[therustmonk]

Yeah, I meant the alias is actually used and cannot be removed.

[taiki-e]

Related (or possibly the same) problem: #47131

[varkor]

So, #59486 will fix part of this issue, which means that if Runner and Runtime are made pub, there will no longer be a dead code warning. However, there's a second issue, which is that the usages in code are not being picked up. I think this is because they're being attributed to the original type rather than the alias.

[varkor]

Minimised:

struct Runner;

type RuntimeImpl = Runner;

trait Runtime {
    fn run(&self) {}
}

impl Runtime for RuntimeImpl {}

fn main() {
    Runner.run();
}

[varkor]

A similar issue was encountered in #47131 (comment):

trait Trait<S> {}

type Alias = ();

trait Trait2 {
    type Type: Trait<Alias>;
}

If these are not fixed at the same time, we should open a new issue for the unaddressed test case.

[Spoonbender]

triage: partially resolved

I'm not seeing errors for the original post nor for this example, but 2nd example still generates an incorrect warning.