spurious "error: parameter Self is never used" for trivial trait T {}`

[cjsut]

Given the input

trait T {}

rustc will complain:

<anon>:2:1: 2:11 error: parameter `Self` is never used
<anon>:2 trait T {}
         ^~~~~~~~~~
<anon>:2:1: 2:11 help: consider removing `Self` or using a marker such as `core::marker::PhantomFn`
<anon>:2 trait T {}
         ^~~~~~~~~~

This may just be a diagnostics issue, but I'll let someone more familiar with Rust make that judgement.

[alexcrichton]

This is actually intended due to a recently merged RFC.

You can find some more info about this in the RFC as well.

[cjsut]

This is still a diagnostics error. There is no parameter Self, so the error should explain why it thinks there is one.
As the RFC says,

The goal is to help users as concretely as possible.

The current error message is confusing and misleading. This does not help users.

[huonw]

Reopening to cover the error message.

[bluss]

I've had a useful trait stop working with this error:

pub trait Graphlike {
    type NodeId: Clone;
}

error: parameter `Self` is never used

Another example:

/// A graph's edge type determines whether is has directed edges or not.
pub trait EdgeType {
    fn is_directed() -> bool;
}

impl EdgeType for Directed {
    #[inline]
    fn is_directed() -> bool { true }
}

impl EdgeType for Undirected {
    #[inline]
    fn is_directed() -> bool { false }
}

[cjsut]

@bluss, read the RFC linked by alexcrichton: you need to use PhantomFn.

[shepmaster]

or better (from the docs):

use std::marker::MarkerTrait;
trait Graphlike : MarkerTrait { }

[apasel422]

This can be closed, as the original example now compiles without error.

[cjsut]

@apasel422 The playpen linked in the original post does not compile.

I still think that the suggestion to "consider removing Self" is completely bogus and confusing.

[bluss]

It should compile in the nightly, even if playpen doesn't have that version. PhantomFn was deprecated after beta.

[cjsut]

Huh, did I accidentally close this one?

[apasel422]

The issue no longer exists in the nightly builds.

On Thursday, April 9, 2015, Collin J. Sutton notifications@github.com
wrote:

Huh, did I accidentally close this one?

—
Reply to this email directly or view it on GitHub
#22598 (comment).

[cjsut]

Oh! When did Playpen change the default version to "beta"? But now there's a different error...
Re-closing, in any case. :)