incorrect error message for duplicate Send impl

[ultrasaurus]

For a specific case, the compiler reports an incorrect error message when there is a duplicate error

I tried this code:

pub struct Foo(*const ());

unsafe impl Send for Foo {}
unsafe impl Send for Foo {}

impl Foo {
    fn bar(&self) {}
}

fn assert_send<T: Send>(_: T) {}

fn main() {
    let foo = Foo(&());
    assert_send(async move {
         foo.bar();
    })
}

it reports this error:

  |
10 | fn assert_send<T: Send>(_: T) {}
   |    -----------    ---- required by this bound in `assert_send`
...
13 |     let foo = Foo(&());
   |         --- consider giving `foo` a type
14 |     assert_send(async move {
   |     ^^^^^^^^^^^ cannot infer type for struct `Foo`
   |
   = note: cannot resolve `Foo: std::marker::Send`
   = note: required because it appears within the type `[static generator@src/main.rs:14:28: 16:6 foo:Foo {}]`
   = note: required because it appears within the type `std::future::GenFuture<[static generator@src/main.rs:14:28: 16:6 foo:Foo {}]>`
   = note: required because it appears within the type `impl std::future::Future

If I comment out the line that calls foo.bar() I get the expected error:

 |
3 | unsafe impl Send for Foo {}
  | ------------------------ first implementation here
4 | unsafe impl Send for Foo {}
  | ^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Foo`

Here's a playground

Meta

Reproduced in 1.14.1, It also happens in nightly

originally found in
rustc --version --verbose:

rustc --version --verbose
rustc 1.41.0 (5e1a79984 2020-01-27)
binary: rustc
commit-hash: 5e1a799842ba6ed4a57e91f7ab9435947482f7d8
commit-date: 2020-01-27
host: x86_64-apple-darwin
release: 1.41.0
LLVM version: 9.0

[ultrasaurus]

thx @sgrif for helping to create an isolated example!

[sgrif]

/cc @estebank

[tmandry]

Seems related to #67651, and may be fixed by #68884.

[csmoe]

Fixed in latest nightly

error[E0119]: conflicting implementations of trait `std::marker::Send` for type `Foo`:
 --> src/main.rs:4:1
  |
3 | unsafe impl Send for Foo {}
  | ------------------------ first implementation here
4 | unsafe impl Send for Foo {}
  | ^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Foo`