Wrong message from object safety checker

[qnighy]

UPDATE: There are mentoring instructions to be found in a comment below.

---

Object safety checker produces messages with "supertrait something" even when the cause is in where-clause.

trait Bar<X:?Sized> {}
trait Foo where Box<Self>: Bar<Self> {}
fn f(x: Box<Foo>) {}
fn main () {
}

rustc 1.17.0-nightly (b1e31766d 2017-03-03)
error[E0038]: the trait `Foo` cannot be made into an object
 --> <anon>:3:1
  |
3 | fn f(x: Box<Foo>) {}
  | ^^^^^^^^^^^^^^^^^^^^ the trait `Foo` cannot be made into an object
  |
  = note: the trait cannot use `Self` as a type parameter in the supertrait listing

error: aborting due to previous error

Rust Playground: https://is.gd/mAvVNW

Indeed this seems object-unsafe, but the message should be something about "where-clause", not "supertrait listing".

[nikomatsakis]

I tend to agree. This message is generated in src/librustc/traits/object_safety.rs, which houses the object safety analysis. The simplest fix would just be to adjust the wording to say "in the supertraits or where-clauses". It would be difficult -- but not impossible -- for us to distinguish the two, since at this phase in the compiler they are effectively unified. Doesn't seem worth the effort.

[mandeep]

Currently, working on this. Hope to submit a pull request tonight.

[qnighy]

May I (or should I) close this issue?

[nikomatsakis]

Yep! Thanks. =)