Use of ty macro non-terminal in trait bounds

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

1. Triage: still an issue.

   Huon Wilson at 2015-11-18 06:49:29

2. 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
   

   Esteban Kuber at 2017-08-10 17:05:32

3. I removed <kbd>A-diagnostics</kbd> 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.

   David Tolnay at 2018-06-09 19:36:31

4. 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);
   

   diwic at 2018-06-17 08:30:49