error[E0599]: the function or associated item `partial_cmp` exists for struct `A`, but its trait bounds were not satisfied
  --> $DIR/derive-sugg-arg-arity.rs:5:23
   |
LL | pub struct A;
   | ------------
   | |
   | function or associated item `partial_cmp` not found for this struct
   | doesn't satisfy `A: Iterator`
   | doesn't satisfy `A: PartialOrd<_>`
...
LL |         _ => match A::partial_cmp() {},
   |                       ^^^^^^^^^^^ function or associated item cannot be called on `A` due to unsatisfied trait bounds
   |
   = note: the following trait bounds were not satisfied:
           `A: PartialOrd<_>`
           which is required by `&A: PartialOrd<&_>`
           `A: PartialOrd<_>`
           which is required by `&mut A: PartialOrd<&mut _>`
           `A: Iterator`
           which is required by `&mut A: Iterator`
note: the trait `Iterator` must be implemented
  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
help: consider annotating `A` with `#[derive(PartialEq, PartialOrd)]`
   |
LL + #[derive(PartialEq, PartialOrd)]
LL | pub struct A;
   |

error: aborting due to previous error

For more information about this error, try `rustc --explain E0599`.