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 { }