"implement this trait" suggestions should only list traits for which that is actually legal

[huonw]

E.g.

fn main() {
    1us.is_empty();
}

<anon>:2:9: 2:19 error: type `usize` does not implement any method in scope named `is_empty`
<anon>:2     1us.is_empty();
                 ^~~~~~~~~~
<anon>:2:19: 2:19 help: methods from traits can only be called if the trait is implemented and in scope; the following traits define a method `is_empty`, perhaps you need to implement one of them:
<anon>:2:19: 2:19 help: candidate #1: `core::slice::SliceExt`
<anon>:2:19: 2:19 help: candidate #2: `core::slice::SliceExt`
<anon>:2:19: 2:19 help: candidate #3: `core::str::StrExt`
<anon>:2:19: 2:19 help: candidate #4: `core::str::StrExt`
<anon>:2:19: 2:19 help: candidate #5: `collections::slice::SliceExt`
<anon>:2:19: 2:19 help: candidate #6: `collections::slice::SliceExt`
<anon>:2:19: 2:19 help: candidate #7: `collections::slice::SliceExt`
<anon>:2:19: 2:19 help: candidate #8: `collections::str::StrExt`
<anon>:2:19: 2:19 help: candidate #9: `collections::str::StrExt`
<anon>:2:19: 2:19 help: candidate #10: `collections::str::StrExt`

(The repetition is #21405.)

However, all of these are useless: because one cannot implement them for usize anyway, it would be better to just filter them out, e.g. reject external traits when the self type itself is external.

cc @nikomatsakis

[nikomatsakis]

Good point, although it's not always as simple as that -- with multidispatch it is plausible you COULD implement the trait for usize if another type parameter were local, though not with the current orphan rules. Nonetheless, it might be good to just screen them out anyway, as it is unlikely to be what you plan to do.