Unused trait parameter error recommends unstable feature

[jethrogb]

Compiling this with 1.0.0:

trait Foo {}
struct Bar<T: Foo>;

gives:

<anon>:2:12: 2:13 error: parameter `T` is never used
<anon>:2 struct Bar<T: Foo>;
                    ^
<anon>:2:12: 2:13 help: consider removing `T` or using a marker such as `core::marker::PhantomData`

This should recommend std::marker::PhantomData instead, because using core both sounds scary and is unstable.

[jethrogb]

This has been fixed. core is stable now. nightly actually says std now.