diff options
| -rw-r--r-- | library/core/src/marker.rs | 31 |
1 files changed, 13 insertions, 18 deletions
diff --git a/library/core/src/marker.rs b/library/core/src/marker.rs index b1e67e4e900..4af9b666e54 100644 --- a/library/core/src/marker.rs +++ b/library/core/src/marker.rs @@ -190,24 +190,19 @@ pub trait Unsize<T: ?Sized> { /// Required trait for constants used in pattern matches. /// -/// Any type that derives `PartialEq` automatically implements this trait, -/// *regardless* of whether its type-parameters implement `PartialEq`. -/// -/// If a `const` item contains some type that does not implement this trait, -/// then that type either (1.) does not implement `PartialEq` (which means the -/// constant will not provide that comparison method, which code generation -/// assumes is available), or (2.) it implements *its own* version of -/// `PartialEq` (which we assume does not conform to a structural-equality -/// comparison). -/// -/// In either of the two scenarios above, we reject usage of such a constant in -/// a pattern match. -/// -/// See also the [structural match RFC][RFC1445], and [issue 63438] which -/// motivated migrating from an attribute-based design to this trait. -/// -/// [RFC1445]: https://github.com/rust-lang/rfcs/blob/master/text/1445-restrict-constants-in-patterns.md -/// [issue 63438]: https://github.com/rust-lang/rust/issues/63438 +/// Constants are only allowed as patterns if (a) their type implements +/// `PartialEq`, and (b) interpreting the value of the constant as a pattern +/// is equialent to calling `PartialEq`. This ensures that constants used as +/// patterns cannot expose implementation details in an unexpected way or +/// cause semver hazards. +/// +/// This trait ensures point (b). +/// Any type that derives `PartialEq` automatically implements this trait. +/// +/// Implementing this trait (which is unstable) is a way for type authors to explicitly allow +/// comparing const values of this type; that operation will recursively compare all fields +/// (including private fields), even if that behavior differs from `PartialEq`. This can make it +/// semver-breaking to add further private fields to a type. #[unstable(feature = "structural_match", issue = "31434")] #[diagnostic::on_unimplemented(message = "the type `{Self}` does not `#[derive(PartialEq)]`")] #[lang = "structural_peq"] |