Commits on Jul 31, 2022

1. Don't derive PartialEq::ne. …

   Currently we skip deriving `PartialEq::ne` for C-like (fieldless) enums
   and empty structs, thus reyling on the default `ne`. This behaviour is
   unnecessarily conservative, because the `PartialEq` docs say this:
   
   > Implementations must ensure that eq and ne are consistent with each other:
   >
   > `a != b` if and only if `!(a == b)` (ensured by the default
   > implementation).
   
   This means that the default implementation (`!(a == b)`) is always good
   enough. So this commit changes things such that `ne` is never derived.
   
   The motivation for this change is that not deriving `ne` reduces compile
   times and binary sizes.
   
   Observable behaviour may change if a user has defined a type `A` with an
   inconsistent `PartialEq` and then defines a type `B` that contains an
   `A` and also derives `PartialEq`. Such code is already buggy and
   preserving bug-for-bug compatibility isn't necessary.
   
   Two side-effects of the change:
   - There is only one error message produced for types where `PartialEq`
     cannot be derived, instead of two.
   - For coverage reports, some warnings about generated `ne` methods not
     being executed have disappeared.
   
   Both side-effects seem fine, and possibly preferable.

   

   nnethercote committed Jul 31, 2022
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