Unused warning for type alias doesn't take into account inherent impls.

[cristicbz]

The following code (playground):

pub struct GenericFoo<T>(T);

type Foo = GenericFoo<u32>;

impl Foo {
    fn bar(self) -> u8 {
        0
    }
}

fn main() {    
    println!("{}", GenericFoo(0).bar());
}

results in the warning:

warning: type alias is never used: `Foo`
 --> src/main.rs:3:1
  |
3 | type Foo = GenericFoo<u32>;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(dead_code)] on by default

This is incorrect, since the Foo impl uses it (and it can be unwieldy to write it out otherwise).

[jsgf]

This looks like a similar case: type aliases used for trait bounds on inherent impl are not considered used: playground

#![deny(warnings)]

type FooBar<F> = <F as Foo>::Bar;

trait Foo {
    type Bar;
}

struct Blop<F>(F);

impl<F: Foo> Blop<F>
where
    FooBar<F>: Clone,
{
}

impl Foo for () {
    type Bar = ();
}

fn main() {
    let _ = Blop(());
}

[shepmaster]

Additional example:

type Inner = Box<i32>;
struct Wrapper(Inner);

fn main() {
    Wrapper(Box::new(42));
}

warning: type alias is never used: `Inner`
 --> src/main.rs:4:1
  |
4 | type Inner = Box<i32>;
  | ^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(dead_code)] on by default

Rust 1.32.0

[Elrendio]

Hello,

I'm still having this issue on rustc 1.39.0 (4560ea7 2019-11-04), how can I know when this will be released ?

Thanks a lot !

[shepmaster]

, how can I know when this will be released

Click on the PR that closed the issue:

Click on the commits:

Pick any of the commits, then look for the tagged version:

[shepmaster]

I'm still having this issue

Please provide a minimized example that demonstrates the problem.

[Elrendio]

I'm still having this issue

Please provide a minimized example that demonstrates the problem.

Here's a link to the playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=064177eade4f98748bacce0167eb354b

StructAlias is warned as never used yet used in the requirements of the AssociatedType of Trait2

[shepmaster]

Further reduced:

trait Trait<S> {}

type Alias = ();

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

warning: type alias is never used: `Alias`
 --> src/lib.rs:3:1
  |
3 | type Alias = ();
  | ^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

/cc @varkor — should we reopen or create a new issue?

[varkor]

This isn't the same as the original issue, so let's open a new one (there are already related issues, so it might be worth checking if this is already open elsewhere).

[Elrendio]

This isn't the same as the original issue, so let's open a new one (there are already related issues, so it might be worth checking if this is already open elsewhere).

Do you think it's the same as #59333 ?

[varkor]

@Elrendio: that's the one I was thinking of, thanks. I've added your example there — I think they probably are the same bug (but if not, we can split out another issue then).