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Remove requirement that forces symmetric and transitive PartialEq impls to exist

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David Tolnay 2021-01-19 14:26:02 -08:00
parent cf04ae54e6
commit 8758083aad
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1 changed files with 10 additions and 6 deletions
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library/core/src/cmp.rs
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@ -31,14 +31,18 @@ use self::Ordering::*;
/// equivalence relation. For example, in floating point numbers `NaN != NaN`,
/// so floating point types implement `PartialEq` but not [`trait@Eq`].
///
/// Formally, the equality must be (for all `a`, `b` and `c`):
/// Formally, the equality must be (for all `a`, `b`, `c` of type `A`, `B`,
/// `C`):
///
/// - symmetric: `a == b` implies `b == a`; and
/// - transitive: `a == b` and `b == c` implies `a == c`.
/// - **Symmetric**: if `A: PartialEq<B>` and `B: PartialEq<A>`, then **`a == b`
/// implies `b == a`**; and
///
/// Note that these requirements mean that the trait itself must be implemented
/// symmetrically and transitively: if `T: PartialEq<U>` and `U: PartialEq<V>`
/// then `U: PartialEq<T>` and `T: PartialEq<V>`.
/// - **Transitive**: if `A: PartialEq<B>` and `B: PartialEq<C>` and `A:
/// PartialEq<C>`, then **`a == b` and `b == c` implies `a == c`**.
///
/// Note that the `B: PartialEq<A>` (symmetric) and `A: PartialEq<C>`
/// (transitive) impls are not forced to exist, but these requirements apply
/// whenever they do exist.
///
/// ## Derivable
///