Duplicate 'upstream crates may add a new impl of trait' notes on inter-crate ambiguous impl of traits

[rhysd]

Given the following code:

struct S;

impl From<()> for S {
    fn from(x: ()) -> Self {
        S
    }
}

impl<I> From<I> for S
where
    I: Iterator<Item = ()>,
{
    fn from(x: I) -> Self {
        S
    }
}

fn main() {}

The current output is:

error[E0119]: conflicting implementations of trait `std::convert::From<()>` for type `S`
  --> foo.rs:9:1
   |
3  |   impl From<()> for S {
   |   ------------------- first implementation here
...
9  | / impl<I> From<I> for S
10 | | where
11 | |     I: Iterator<Item = ()>,
12 | | {
...  |
15 | |     }
16 | | }
   | |_^ conflicting implementation for `S`
   |
   = note: upstream crates may add a new impl of trait `std::iter::Iterator` for type `()` in future versions
   = note: upstream crates may add a new impl of trait `std::iter::Iterator` for type `()` in future versions

error: aborting due to previous error

For more information about this error, try `rustc --explain E0119`.

Ideally the output should look like:

error[E0119]: conflicting implementations of trait `std::convert::From<()>` for type `S`
  --> foo.rs:9:1
   |
3  |   impl From<()> for S {
   |   ------------------- first implementation here
...
9  | / impl<I> From<I> for S
10 | | where
11 | |     I: Iterator<Item = ()>,
12 | | {
...  |
15 | |     }
16 | | }
   | |_^ conflicting implementation for `S`
   |
   = note: upstream crates may add a new impl of trait `std::iter::Iterator` for type `()` in future versions

error: aborting due to previous error

For more information about this error, try `rustc --explain E0119`.

[rhysd]

I'll make a PR for fixing this.

[compiler-errors]

Haha, I was already working on one before I saw that message below, but I'll throw away my changes.

@rhysd, I recommend using a FxIndexSet to track the inter-crate ambiguity causes. That's what I was gonna try.

[rhysd]

@compiler-errors Ah, I'm sorry for interrupting your work... I was thinking to remove duplicates from notes after making messages as String. But your approach would be better. I'll try it.

[compiler-errors]

No you did not interrupt, I just didn't see your comment saying that you were interested in doing the change yourself. Thanks for contributing.