Use of `ty` macro non-terminal in trait bounds

[stuglaser]

macro_rules! impl_somethingable {
    ($Base: ty, $Block:ty) => {
        impl<'a, T> Somethingable for $Block where T: $Base {
        }
    }
}

impl_somethingable!(ATrait, A);

<anon>:15:52: 15:60 error: each predicate in a `where` clause must have at least one bound in it
<anon>:15         impl<'a, T> Somethingable for $Block where T: $Base {
                                                             ^~~~~~~~
<anon>:15:56: 15:60 error: expected one of `,` or `{`, found `ATrait`
<anon>:15         impl<'a, T> Somethingable for $Block where T: $Base {
                                                                 ^~~~

The solution is to use $ident instead of $ty, but this is non-obvious to the user, especially since it's perfectly ok to have ATrait in that location.

Playpen: http://is.gd/AMUJDw

cc @huonw

[huonw]

Triage: still an issue.

[estebank]

Current output, still doesn't point at the possibility of changing $Base:

error: expected one of `(`, `,`, `?`, `for`, `{`, lifetime, or path, found `ATrait`
 --> src/main.rs:4:55
  |
4 |         impl<'a, T> Somethingable for $Block where T: $Base {
  |                                                       ^^^^^ expected one of 7 possible tokens here
...
9 | impl_somethingable!(ATrait, A);
  | ------------------------------- in this macro invocation

[dtolnay]

I removed A-diagnostics because I would like the original code using $:ty to work as written. Refusing to treat $:ty containing path as a trait bound is an unnecessary limitation.

[diwic]

Also ident is not always a possible solution, like if I modify the example slightly:

trait ATrait {
    type AType;
}

macro_rules! impl_somethingable {
    ($Base: ident, $Block:ty) => {
        impl<'a, T> Somethingable for $Block where T: $Base {
        }
    }
}

impl_somethingable!(ATrait<AType=u32>, A);