Trait core::cmp::PartialEq

1.0.0 · source ·
pub trait PartialEq<Rhs: ?Sized = Self> {
    // Required method
    fn eq(&self, other: &Rhs) -> bool;

    // Provided method
    fn ne(&self, other: &Rhs) -> bool { ... }
}
Expand description

Trait for comparisons using the equality operator.

Implementing this trait for types provides the == and != operators for those types.

x.eq(y) can also be written x == y, and x.ne(y) can be written x != y. We use the easier-to-read infix notation in the remainder of this documentation.

This trait allows for comparisons using the equality operator, for types that do not have a full equivalence relation. For example, in floating point numbers NaN != NaN, so floating point types implement PartialEq but not Eq. Formally speaking, when Rhs == Self, this trait corresponds to a partial equivalence relation.

Implementations must ensure that eq and ne are consistent with each other:

  • a != b if and only if !(a == b).

The default implementation of ne provides this consistency and is almost always sufficient. It should not be overridden without very good reason.

If PartialOrd or Ord are also implemented for Self and Rhs, their methods must also be consistent with PartialEq (see the documentation of those traits for the exact requirements). It’s easy to accidentally make them disagree by deriving some of the traits and manually implementing others.

The equality relation == must satisfy the following conditions (for all a, b, c of type A, B, C):

  • Symmetry: if A: PartialEq<B> and B: PartialEq<A>, then a == b implies b == a; and

  • Transitivity: if A: PartialEq<B> and B: PartialEq<C> and A: PartialEq<C>, then a == b and b == c implies a == c. This must also work for longer chains, such as when A: PartialEq<B>, B: PartialEq<C>, C: PartialEq<D>, and A: PartialEq<D> all exist.

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.

Violating these requirements is a logic error. The behavior resulting from a logic error is not specified, but users of the trait must ensure that such logic errors do not result in undefined behavior. This means that unsafe code must not rely on the correctness of these methods.

§Cross-crate considerations

Upholding the requirements stated above can become tricky when one crate implements PartialEq for a type of another crate (i.e., to allow comparing one of its own types with a type from the standard library). The recommendation is to never implement this trait for a foreign type. In other words, such a crate should do impl PartialEq<ForeignType> for LocalType, but it should not do impl PartialEq<LocalType> for ForeignType.

This avoids the problem of transitive chains that criss-cross crate boundaries: for all local types T, you may assume that no other crate will add impls that allow comparing T == U. In other words, if other crates add impls that allow building longer transitive chains U1 == ... == T == V1 == ..., then all the types that appear to the right of T must be types that the crate defining T already knows about. This rules out transitive chains where downstream crates can add new impls that “stitch together” comparisons of foreign types in ways that violate transitivity.

Not having such foreign impls also avoids forward compatibility issues where one crate adding more PartialEq implementations can cause build failures in downstream crates.

§Derivable

This trait can be used with #[derive]. When derived on structs, two instances are equal if all fields are equal, and not equal if any fields are not equal. When derived on enums, two instances are equal if they are the same variant and all fields are equal.

§How can I implement PartialEq?

An example implementation for a domain in which two books are considered the same book if their ISBN matches, even if the formats differ:

enum BookFormat {
    Paperback,
    Hardback,
    Ebook,
}

struct Book {
    isbn: i32,
    format: BookFormat,
}

impl PartialEq for Book {
    fn eq(&self, other: &Self) -> bool {
        self.isbn == other.isbn
    }
}

let b1 = Book { isbn: 3, format: BookFormat::Paperback };
let b2 = Book { isbn: 3, format: BookFormat::Ebook };
let b3 = Book { isbn: 10, format: BookFormat::Paperback };

assert!(b1 == b2);
assert!(b1 != b3);
Run

§How can I compare two different types?

The type you can compare with is controlled by PartialEq’s type parameter. For example, let’s tweak our previous code a bit:

// The derive implements <BookFormat> == <BookFormat> comparisons
#[derive(PartialEq)]
enum BookFormat {
    Paperback,
    Hardback,
    Ebook,
}

struct Book {
    isbn: i32,
    format: BookFormat,
}

// Implement <Book> == <BookFormat> comparisons
impl PartialEq<BookFormat> for Book {
    fn eq(&self, other: &BookFormat) -> bool {
        self.format == *other
    }
}

// Implement <BookFormat> == <Book> comparisons
impl PartialEq<Book> for BookFormat {
    fn eq(&self, other: &Book) -> bool {
        *self == other.format
    }
}

let b1 = Book { isbn: 3, format: BookFormat::Paperback };

assert!(b1 == BookFormat::Paperback);
assert!(BookFormat::Ebook != b1);
Run

By changing impl PartialEq for Book to impl PartialEq<BookFormat> for Book, we allow BookFormats to be compared with Books.

A comparison like the one above, which ignores some fields of the struct, can be dangerous. It can easily lead to an unintended violation of the requirements for a partial equivalence relation. For example, if we kept the above implementation of PartialEq<Book> for BookFormat and added an implementation of PartialEq<Book> for Book (either via a #[derive] or via the manual implementation from the first example) then the result would violate transitivity:

#[derive(PartialEq)]
enum BookFormat {
    Paperback,
    Hardback,
    Ebook,
}

#[derive(PartialEq)]
struct Book {
    isbn: i32,
    format: BookFormat,
}

impl PartialEq<BookFormat> for Book {
    fn eq(&self, other: &BookFormat) -> bool {
        self.format == *other
    }
}

impl PartialEq<Book> for BookFormat {
    fn eq(&self, other: &Book) -> bool {
        *self == other.format
    }
}

fn main() {
    let b1 = Book { isbn: 1, format: BookFormat::Paperback };
    let b2 = Book { isbn: 2, format: BookFormat::Paperback };

    assert!(b1 == BookFormat::Paperback);
    assert!(BookFormat::Paperback == b2);

    // The following should hold by transitivity but doesn't.
    assert!(b1 == b2); // <-- PANICS
}
Run

§Examples

let x: u32 = 0;
let y: u32 = 1;

assert_eq!(x == y, false);
assert_eq!(x.eq(&y), false);
Run

Required Methods§

1.0.0 · source

fn eq(&self, other: &Rhs) -> bool

This method tests for self and other values to be equal, and is used by ==.

Provided Methods§

1.0.0 · source

fn ne(&self, other: &Rhs) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

Implementors§

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impl PartialEq for AsciiChar

1.34.0 · source§

impl PartialEq for Infallible

1.28.0 · source§

impl PartialEq for core::fmt::Alignment

1.7.0 · source§

impl PartialEq for IpAddr

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impl PartialEq for Ipv6MulticastScope

1.0.0 · source§

impl PartialEq for SocketAddr

1.0.0 · source§

impl PartialEq for FpCategory

1.55.0 · source§

impl PartialEq for IntErrorKind

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impl PartialEq for SearchStep

1.0.0 · source§

impl PartialEq for core::sync::atomic::Ordering

1.0.0 · source§

impl PartialEq for core::cmp::Ordering

1.0.0 · source§

impl PartialEq for bool

1.0.0 · source§

impl PartialEq for char

1.0.0 · source§

impl PartialEq for f16

1.0.0 · source§

impl PartialEq for f32

1.0.0 · source§

impl PartialEq for f64

1.0.0 · source§

impl PartialEq for f128

1.0.0 · source§

impl PartialEq for i8

1.0.0 · source§

impl PartialEq for i16

1.0.0 · source§

impl PartialEq for i32

1.0.0 · source§

impl PartialEq for i64

1.0.0 · source§

impl PartialEq for i128

1.0.0 · source§

impl PartialEq for isize

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impl PartialEq for !

1.0.0 · source§

impl PartialEq for str

1.0.0 · source§

impl PartialEq for u8

1.0.0 · source§

impl PartialEq for u16

1.0.0 · source§

impl PartialEq for u32

1.0.0 · source§

impl PartialEq for u64

1.0.0 · source§

impl PartialEq for u128

1.0.0 · source§

impl PartialEq for ()

1.0.0 · source§

impl PartialEq for usize

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impl PartialEq for AllocError

1.28.0 · source§

impl PartialEq for Layout

1.50.0 · source§

impl PartialEq for LayoutError

1.0.0 · source§

impl PartialEq for TypeId

1.27.0 · source§

impl PartialEq for CpuidResult

Available on x86 or x86-64 only.
1.34.0 · source§

impl PartialEq for CharTryFromError

1.9.0 · source§

impl PartialEq for DecodeUtf16Error

1.20.0 · source§

impl PartialEq for ParseCharError

1.59.0 · source§

impl PartialEq for TryFromCharError

1.69.0 · source§

impl PartialEq for FromBytesUntilNulError

1.64.0 · source§

impl PartialEq for FromBytesWithNulError

1.0.0 · source§

impl PartialEq for CStr

1.0.0 · source§

impl PartialEq for Error

1.33.0 · source§

impl PartialEq for PhantomPinned

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impl PartialEq for Assume

1.0.0 · source§

impl PartialEq for AddrParseError

1.0.0 · source§

impl PartialEq for Ipv4Addr

1.0.0 · source§

impl PartialEq for Ipv6Addr

1.0.0 · source§

impl PartialEq for SocketAddrV4

1.0.0 · source§

impl PartialEq for SocketAddrV6

1.0.0 · source§

impl PartialEq for ParseFloatError

1.0.0 · source§

impl PartialEq for ParseIntError

1.34.0 · source§

impl PartialEq for TryFromIntError

1.0.0 · source§

impl PartialEq for RangeFull

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impl PartialEq for core::ptr::Alignment

1.0.0 · source§

impl PartialEq for ParseBoolError

1.0.0 · source§

impl PartialEq for Utf8Error

1.36.0 · source§

impl PartialEq for RawWaker

1.36.0 · source§

impl PartialEq for RawWakerVTable

1.3.0 · source§

impl PartialEq for Duration

1.66.0 · source§

impl PartialEq for TryFromFloatSecsError

1.16.0 · source§

impl PartialEq<IpAddr> for Ipv4Addr

1.16.0 · source§

impl PartialEq<IpAddr> for Ipv6Addr

1.16.0 · source§

impl PartialEq<Ipv4Addr> for IpAddr

1.16.0 · source§

impl PartialEq<Ipv6Addr> for IpAddr

1.10.0 · source§

impl<'a> PartialEq for Location<'a>

1.79.0 · source§

impl<'a> PartialEq for Utf8Chunk<'a>

1.0.0 · source§

impl<A, B: ?Sized> PartialEq<&B> for &A
where A: PartialEq<B> + ?Sized,

1.0.0 · source§

impl<A, B: ?Sized> PartialEq<&B> for &mut A
where A: PartialEq<B> + ?Sized,

1.0.0 · source§

impl<A, B: ?Sized> PartialEq<&mut B> for &A
where A: PartialEq<B> + ?Sized,

1.0.0 · source§

impl<A, B: ?Sized> PartialEq<&mut B> for &mut A
where A: PartialEq<B> + ?Sized,

1.55.0 · source§

impl<B: PartialEq, C: PartialEq> PartialEq for ControlFlow<B, C>

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impl<Dyn: ?Sized> PartialEq for DynMetadata<Dyn>

1.4.0 · source§

impl<F: FnPtr> PartialEq for F

1.29.0 · source§

impl<H> PartialEq for BuildHasherDefault<H>

1.0.0 · source§

impl<Idx: PartialEq> PartialEq for Range<Idx>

1.0.0 · source§

impl<Idx: PartialEq> PartialEq for RangeFrom<Idx>

1.26.0 · source§

impl<Idx: PartialEq> PartialEq for RangeInclusive<Idx>

1.0.0 · source§

impl<Idx: PartialEq> PartialEq for RangeTo<Idx>

1.26.0 · source§

impl<Idx: PartialEq> PartialEq for RangeToInclusive<Idx>

1.41.0 · source§

impl<Ptr: Deref, Q: Deref> PartialEq<Pin<Q>> for Pin<Ptr>
where Ptr::Target: PartialEq<Q::Target>,

1.0.0 · source§

impl<T> PartialEq for (T₁, T₂, …, Tₙ)
where T: ?Sized + PartialEq,

This trait is implemented for tuples up to twelve items long.

1.21.0 · source§

impl<T> PartialEq for Discriminant<T>

1.28.0 · source§

impl<T> PartialEq for NonZero<T>

1.0.0 · source§

impl<T, U> PartialEq<[U]> for [T]
where T: PartialEq<U>,

1.0.0 · source§

impl<T, U, const N: usize> PartialEq<&[U]> for [T; N]
where T: PartialEq<U>,

1.0.0 · source§

impl<T, U, const N: usize> PartialEq<&mut [U]> for [T; N]
where T: PartialEq<U>,

1.0.0 · source§

impl<T, U, const N: usize> PartialEq<[U; N]> for &[T]
where T: PartialEq<U>,

1.0.0 · source§

impl<T, U, const N: usize> PartialEq<[U; N]> for &mut [T]
where T: PartialEq<U>,

1.0.0 · source§

impl<T, U, const N: usize> PartialEq<[U; N]> for [T; N]
where T: PartialEq<U>,

1.0.0 · source§

impl<T, U, const N: usize> PartialEq<[U; N]> for [T]
where T: PartialEq<U>,

1.0.0 · source§

impl<T, U, const N: usize> PartialEq<[U]> for [T; N]
where T: PartialEq<U>,

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impl<T, const N: usize> PartialEq for Mask<T, N>

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impl<T, const N: usize> PartialEq for Simd<T, N>

1.0.0 · source§

impl<T: PartialEq + Copy> PartialEq for Cell<T>

1.20.0 · source§

impl<T: PartialEq + ?Sized> PartialEq for ManuallyDrop<T>

1.17.0 · source§

impl<T: PartialEq> PartialEq for Bound<T>

1.0.0 · source§

impl<T: PartialEq> PartialEq for Option<T>

1.36.0 · source§

impl<T: PartialEq> PartialEq for Poll<T>

1.70.0 · source§

impl<T: PartialEq> PartialEq for OnceCell<T>

1.74.0 · source§

impl<T: PartialEq> PartialEq for Saturating<T>

1.0.0 · source§

impl<T: PartialEq> PartialEq for Wrapping<T>

1.19.0 · source§

impl<T: PartialEq> PartialEq for Reverse<T>

1.0.0 · source§

impl<T: PartialEq, E: PartialEq> PartialEq for Result<T, E>

1.0.0 · source§

impl<T: ?Sized + PartialEq> PartialEq for RefCell<T>

1.0.0 · source§

impl<T: ?Sized> PartialEq for *const T

1.0.0 · source§

impl<T: ?Sized> PartialEq for *mut T

1.0.0 · source§

impl<T: ?Sized> PartialEq for PhantomData<T>

1.25.0 · source§

impl<T: ?Sized> PartialEq for NonNull<T>

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impl<Y: PartialEq, R: PartialEq> PartialEq for CoroutineState<Y, R>