Unused trait parameter error recommends deprecated PhantomFn

[lilyball]

The error message for a trait with an unused parameter recommends the use of PhantomFn even though it's deprecated. It should presumably recommend PhantomData instead, and the documentation on PhantomData should be updated to include an example of how to use it with a trait.

Example (play):

trait Foo<T> {
    fn foo(&self);
}

yields

<anon>:1:11: 1:12 error: parameter `T` is never used
<anon>:1 trait Foo<T> {
                   ^
<anon>:1:12: 1:12 help: consider removing `T` or using a marker such as `core::marker::PhantomFn`
error: aborting due to previous error

You also get the same error today (suggesting PhantomFn) when using a "marker-like" trait with no functions, which is filed as #22598 (it needs a better error message).

[lilyball]

I admit to being slightly confused as to why PhantomFn and MarkerTrait are deprecated. This happened in #23938 (Remove variances from traits and deprecate PhantomFn/MarkerTrait). But MarkerTrait is still used as the super trait of the other markers (Send, Sized, and Reflect).

As near as I can tell, without using either of these two deprecated traits, the only way to declare a marker-like trait (e.g. one that doesn't use Self) is to use an associated type with PhantomData, e.g.

trait Foo {
    type _phantom = PhantomData<Self>;
}

And it's not even clear if this is "correct". #23938 says that traits no longer have variance, so I guess it's ok? But I don't actually know.

/cc @nikomatsakis

[apasel422]

What version of rustc are you using? The following compiles successfully with rustc 1.0.0-nightly (083b8a404 2015-04-05) (built 2015-04-05)

trait Foo<T> {
    fn foo(&self);
}

fn main() {}

[lilyball]

Huh, you're right. I apparently had the beta on my system (it seems rustup.sh started installing that by default), and I'm not sure what version play.rust-lang.org is using right now (Edit: I'm told it's also on the beta).

Anyway, it seems PhantomFn stopped being recommended as of #23938.