Negative trait impl gives wrong error message

[DutchGhost]

The following code:

#![feature(negative_impls)]

struct MyType;

impl !Clone for &mut MyType {}

fn main() {}

produces:

  Compiling playground v0.0.1 (/playground)
error[E0119]: conflicting implementations of trait `std::clone::Clone` for type `&mut MyType`:
 --> src/main.rs:5:1
  |
5 | impl !Clone for &mut MyType {}
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: conflicting implementation in crate `core`:
          - impl<T> std::clone::Clone for &mut T
            where T: ?Sized;

error: aborting due to previous error

For more information about this error, try `rustc --explain E0119`.
error: could not compile `playground`.

To learn more, run the command again with --verbose.

which reads as: any &mut T where T: ?Sized implements Clone, which isn't realy true. It realy implements !Clone.

Perhaps the error message should add a ! in front of the impl to make it more obvious, like this:

= note: conflicting implementation in crate `core`:
          - impl<T> !std::clone::Clone for &mut T
            where T: ?Sized;

[heiseish]

Hi @fmease , I'd like to take up this issue if it has not been assigned yet. However this would be my first contribution so I just want to confirm it's a good first task. Thanks!

[fmease]

@heiseish Yeah, for sure! It should be as simple as updating to_pretty_impl_header (see) to account for the polarity. You can query/obtain the impl polarity with tcx.impl_polarity(impl_def_id). Feel free to reach out here or on Zulip if you have any questions.