Auto merge of #98655 - nnethercote:dont-derive-PartialEq-ne, r=dtolnay

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.

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -1625,19 +1625,3 @@ where
         StaticEnum(..) | StaticStruct(..) => cx.span_bug(trait_span, "static function in `derive`"),
     }
 }
-
-/// Returns `true` if the type has no value fields
-/// (for an enum, no variant has any fields)
-pub fn is_type_without_fields(item: &Annotatable) -> bool {
-    if let Annotatable::Item(ref item) = *item {
-        match item.kind {
-            ast::ItemKind::Enum(ref enum_def, _) => {
-                enum_def.variants.iter().all(|v| v.data.fields().is_empty())
-            }
-            ast::ItemKind::Struct(ref variant_data, _) => variant_data.fields().is_empty(),
-            _ => false,
-        }
-    } else {
-        false
-    }
-}