Trait std::cmp::PartialEq

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

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

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);

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);

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
}

Examples

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

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

Required Methods

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

Tests for self and other values to be equal, and is used by ==.

Provided Methods

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

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

Implementors

impl PartialEq for AsciiChar

impl PartialEq for BacktraceStatus

impl PartialEq for TryReserveErrorKind

impl PartialEq for Infallible

impl PartialEq for VarError

impl PartialEq for std::fmt::Alignment

impl PartialEq for ErrorKind

impl PartialEq for SeekFrom

impl PartialEq for IpAddr

impl PartialEq for Ipv6MulticastScope

impl PartialEq for Shutdown

impl PartialEq for SocketAddr

impl PartialEq for FpCategory

impl PartialEq for IntErrorKind

impl PartialEq for BacktraceStyle

impl PartialEq for SearchStep

impl PartialEq for std::sync::atomic::Ordering

impl PartialEq for RecvTimeoutError

impl PartialEq for TryRecvError

impl PartialEq for std::cmp::Ordering

impl PartialEq for bool

impl PartialEq for char

impl PartialEq for f16

impl PartialEq for f32

impl PartialEq for f64

impl PartialEq for f128

impl PartialEq for i8

impl PartialEq for i16

impl PartialEq for i32

impl PartialEq for i64

impl PartialEq for i128

impl PartialEq for isize

impl PartialEq for !

impl PartialEq for str

impl PartialEq for u8

impl PartialEq for u16

impl PartialEq for u32

impl PartialEq for u64

impl PartialEq for u128

impl PartialEq for ()

impl PartialEq for usize

impl PartialEq for CpuidResult

impl PartialEq for AllocError

impl PartialEq for Layout

impl PartialEq for LayoutError

impl PartialEq for TypeId

impl PartialEq for CharTryFromError

impl PartialEq for DecodeUtf16Error

impl PartialEq for ParseCharError

impl PartialEq for TryFromCharError

impl PartialEq for UnorderedKeyError

impl PartialEq for TryReserveError

impl PartialEq for FromBytesUntilNulError

impl PartialEq for FromBytesWithNulError

impl PartialEq for FromVecWithNulError

impl PartialEq for IntoStringError

impl PartialEq for NulError

impl PartialEq for CStr

impl PartialEq for CString

impl PartialEq for OsStr

impl PartialEq for OsString

impl PartialEq for Error

impl PartialEq for FileType

impl PartialEq for Permissions

impl PartialEq for PhantomPinned

impl PartialEq for Assume

impl PartialEq for AddrParseError

impl PartialEq for Ipv4Addr

impl PartialEq for Ipv6Addr

impl PartialEq for SocketAddrV4

impl PartialEq for SocketAddrV6

impl PartialEq for ParseFloatError

impl PartialEq for ParseIntError

impl PartialEq for TryFromIntError

impl PartialEq for RangeFull

impl PartialEq for UCred

Available on Unix only.

impl PartialEq for InvalidHandleError

Available on Windows only.

impl PartialEq for NullHandleError

Available on Windows only.

impl PartialEq for Path

impl PartialEq for PathBuf

impl PartialEq for StripPrefixError

impl PartialEq for ExitStatus

impl PartialEq for ExitStatusError

impl PartialEq for Output

impl PartialEq for std::ptr::Alignment

impl PartialEq for ParseBoolError

impl PartialEq for Utf8Error

impl PartialEq for FromUtf8Error

impl PartialEq for String

impl PartialEq for RecvError

impl PartialEq for WaitTimeoutResult

impl PartialEq for RawWaker

impl PartialEq for RawWakerVTable

impl PartialEq for AccessError

impl PartialEq for ThreadId

impl PartialEq for Duration

impl PartialEq for Instant

impl PartialEq for SystemTime

impl PartialEq for TryFromFloatSecsError

impl PartialEq<&str> for OsString

impl PartialEq<IpAddr> for Ipv4Addr

impl PartialEq<IpAddr> for Ipv6Addr

impl PartialEq<str> for OsStr

impl PartialEq<str> for OsString

impl PartialEq<OsStr> for str

impl PartialEq<OsStr> for Path

impl PartialEq<OsStr> for PathBuf

impl PartialEq<OsString> for str

impl PartialEq<OsString> for Path

impl PartialEq<OsString> for PathBuf

impl PartialEq<Ipv4Addr> for IpAddr

impl PartialEq<Ipv6Addr> for IpAddr

impl PartialEq<Path> for OsStr

impl PartialEq<Path> for OsString

impl PartialEq<Path> for PathBuf

impl PartialEq<PathBuf> for OsStr

impl PartialEq<PathBuf> for OsString

impl PartialEq<PathBuf> for Path

impl<'a> PartialEq for Component<'a>

impl<'a> PartialEq for Prefix<'a>

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

impl<'a> PartialEq for Components<'a>

impl<'a> PartialEq for PrefixComponent<'a>

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

impl<'a> PartialEq<&'a OsStr> for Path

impl<'a> PartialEq<&'a OsStr> for PathBuf

impl<'a> PartialEq<&'a Path> for OsStr

impl<'a> PartialEq<&'a Path> for OsString

impl<'a> PartialEq<&'a Path> for PathBuf

impl<'a> PartialEq<Cow<'a, OsStr>> for Path

impl<'a> PartialEq<Cow<'a, OsStr>> for PathBuf

impl<'a> PartialEq<Cow<'a, Path>> for OsStr

impl<'a> PartialEq<Cow<'a, Path>> for OsString

impl<'a> PartialEq<Cow<'a, Path>> for Path

impl<'a> PartialEq<Cow<'a, Path>> for PathBuf

impl<'a> PartialEq<OsStr> for &'a Path

impl<'a> PartialEq<OsStr> for Cow<'a, Path>

impl<'a> PartialEq<OsString> for &'a str

impl<'a> PartialEq<OsString> for &'a Path

impl<'a> PartialEq<OsString> for Cow<'a, Path>

impl<'a> PartialEq<Path> for &'a OsStr

impl<'a> PartialEq<Path> for Cow<'a, OsStr>

impl<'a> PartialEq<Path> for Cow<'a, Path>

impl<'a> PartialEq<PathBuf> for &'a OsStr

impl<'a> PartialEq<PathBuf> for &'a Path

impl<'a> PartialEq<PathBuf> for Cow<'a, OsStr>

impl<'a> PartialEq<PathBuf> for Cow<'a, Path>

impl<'a, 'b> PartialEq<&'a str> for String

impl<'a, 'b> PartialEq<&'a OsStr> for OsString

impl<'a, 'b> PartialEq<&'a Path> for Cow<'b, OsStr>

impl<'a, 'b> PartialEq<&'b str> for Cow<'a, str>

impl<'a, 'b> PartialEq<&'b OsStr> for Cow<'a, OsStr>

impl<'a, 'b> PartialEq<&'b OsStr> for Cow<'a, Path>

impl<'a, 'b> PartialEq<&'b Path> for Cow<'a, Path>

impl<'a, 'b> PartialEq<Cow<'a, str>> for &'b str

impl<'a, 'b> PartialEq<Cow<'a, str>> for str

impl<'a, 'b> PartialEq<Cow<'a, str>> for String

impl<'a, 'b> PartialEq<Cow<'a, OsStr>> for &'b OsStr

impl<'a, 'b> PartialEq<Cow<'a, OsStr>> for OsStr

impl<'a, 'b> PartialEq<Cow<'a, OsStr>> for OsString

impl<'a, 'b> PartialEq<Cow<'a, Path>> for &'b OsStr

impl<'a, 'b> PartialEq<Cow<'a, Path>> for &'b Path

impl<'a, 'b> PartialEq<Cow<'b, OsStr>> for &'a Path

impl<'a, 'b> PartialEq<str> for Cow<'a, str>

impl<'a, 'b> PartialEq<str> for String

impl<'a, 'b> PartialEq<OsStr> for Cow<'a, OsStr>

impl<'a, 'b> PartialEq<OsStr> for OsString

impl<'a, 'b> PartialEq<OsString> for &'a OsStr

impl<'a, 'b> PartialEq<OsString> for Cow<'a, OsStr>

impl<'a, 'b> PartialEq<OsString> for OsStr

impl<'a, 'b> PartialEq<String> for &'a str

impl<'a, 'b> PartialEq<String> for Cow<'a, str>

impl<'a, 'b> PartialEq<String> for str

impl<'a, 'b, B, C> PartialEq<Cow<'b, C>> for Cow<'a, B>

where B: PartialEq<C> + ToOwned + ?Sized, C: ToOwned + ?Sized,

impl<A, B> PartialEq<&B> for &A

where A: PartialEq<B> + ?Sized, B: ?Sized,

impl<A, B> PartialEq<&B> for &mut A

where A: PartialEq<B> + ?Sized, B: ?Sized,

impl<A, B> PartialEq<&mut B> for &A

where A: PartialEq<B> + ?Sized, B: ?Sized,

impl<A, B> PartialEq<&mut B> for &mut A

where A: PartialEq<B> + ?Sized, B: ?Sized,

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

where B: PartialEq, C: PartialEq,

impl<Dyn> PartialEq for DynMetadata<Dyn>

where Dyn: ?Sized,

impl<F> PartialEq for F

where F: FnPtr,

impl<H> PartialEq for BuildHasherDefault<H>

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

where Idx: PartialEq,

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

where Idx: PartialEq,

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

where Idx: PartialEq,

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

where Idx: PartialEq,

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

where Idx: PartialEq,

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

where Idx: PartialEq,

impl<Idx> PartialEq for RangeTo<Idx>

where Idx: PartialEq,

impl<Idx> PartialEq for RangeToInclusive<Idx>

where Idx: PartialEq,

impl<K, V, A> PartialEq for BTreeMap<K, V, A>

where K: PartialEq, V: PartialEq, A: Allocator + Clone,

impl<K, V, S> PartialEq for HashMap<K, V, S>

where K: Eq + Hash, V: PartialEq, S: BuildHasher,

impl<Ptr, Q> PartialEq<Pin<Q>> for Pin<Ptr>

where Ptr: Deref, Q: Deref, <Ptr as Deref>::Target: PartialEq<<Q as Deref>::Target>,

impl<T> PartialEq for Bound<T>

where T: PartialEq,

impl<T> PartialEq for Option<T>

where T: PartialEq,

impl<T> PartialEq for Poll<T>

where T: PartialEq,

impl<T> PartialEq for *const T

where T: ?Sized,

impl<T> PartialEq for *mut T

where T: ?Sized,

impl<T> PartialEq for (T₁, T₂, …, Tₙ)

where T: PartialEq + ?Sized,

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

impl<T> PartialEq for Cell<T>

where T: PartialEq + Copy,

impl<T> PartialEq for OnceCell<T>

where T: PartialEq,

impl<T> PartialEq for RefCell<T>

where T: PartialEq + ?Sized,

impl<T> PartialEq for PhantomData<T>

where T: ?Sized,

impl<T> PartialEq for Discriminant<T>

impl<T> PartialEq for ManuallyDrop<T>

where T: PartialEq + ?Sized,

impl<T> PartialEq for NonZero<T>

where T: ZeroablePrimitive + PartialEq,

impl<T> PartialEq for Saturating<T>

where T: PartialEq,

impl<T> PartialEq for Wrapping<T>

where T: PartialEq,

impl<T> PartialEq for NonNull<T>

where T: ?Sized,

impl<T> PartialEq for Reverse<T>

where T: PartialEq,

impl<T, A> PartialEq for Box<T, A>

where T: PartialEq + ?Sized, A: Allocator,

impl<T, A> PartialEq for BTreeSet<T, A>

where T: PartialEq, A: Allocator + Clone,

impl<T, A> PartialEq for LinkedList<T, A>

where T: PartialEq, A: Allocator,

impl<T, A> PartialEq for VecDeque<T, A>

where T: PartialEq, A: Allocator,

impl<T, A> PartialEq for Rc<T, A>

where T: PartialEq + ?Sized, A: Allocator,

impl<T, A> PartialEq for Arc<T, A>

where T: PartialEq + ?Sized, A: Allocator,

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

where T: PartialEq, E: PartialEq,

impl<T, S> PartialEq for HashSet<T, S>

where T: Eq + Hash, S: BuildHasher,

impl<T, U> PartialEq<&[U]> for Cow<'_, [T]>

where T: PartialEq<U> + Clone,

impl<T, U> PartialEq<&mut [U]> for Cow<'_, [T]>

where T: PartialEq<U> + Clone,

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

where T: PartialEq<U>,

impl<T, U, A1, A2> PartialEq<Vec<U, A2>> for Vec<T, A1>

where A1: Allocator, A2: Allocator, T: PartialEq<U>,

impl<T, U, A> PartialEq<&[U]> for VecDeque<T, A>

where A: Allocator, T: PartialEq<U>,

impl<T, U, A> PartialEq<&[U]> for Vec<T, A>

where A: Allocator, T: PartialEq<U>,

impl<T, U, A> PartialEq<&mut [U]> for VecDeque<T, A>

where A: Allocator, T: PartialEq<U>,

impl<T, U, A> PartialEq<&mut [U]> for Vec<T, A>

where A: Allocator, T: PartialEq<U>,

impl<T, U, A> PartialEq<[U]> for Vec<T, A>

where A: Allocator, T: PartialEq<U>,

impl<T, U, A> PartialEq<Vec<U, A>> for &[T]

where A: Allocator, T: PartialEq<U>,

impl<T, U, A> PartialEq<Vec<U, A>> for &mut [T]

where A: Allocator, T: PartialEq<U>,

impl<T, U, A> PartialEq<Vec<U, A>> for Cow<'_, [T]>

where A: Allocator, T: PartialEq<U> + Clone,

impl<T, U, A> PartialEq<Vec<U, A>> for [T]

where A: Allocator, T: PartialEq<U>,

impl<T, U, A> PartialEq<Vec<U, A>> for VecDeque<T, A>

where A: Allocator, T: PartialEq<U>,

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

where A: Allocator, T: PartialEq<U>,

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

where A: Allocator, T: PartialEq<U>,

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

where A: Allocator, T: PartialEq<U>,

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

where A: Allocator, T: PartialEq<U>,

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

where A: Allocator, T: PartialEq<U>,

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

where T: PartialEq<U>,

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

where T: PartialEq<U>,

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

where T: PartialEq<U>,

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

where T: PartialEq<U>,

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

where T: PartialEq<U>,

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

where T: PartialEq<U>,

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

where T: PartialEq<U>,

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

where T: MaskElement + PartialEq, LaneCount<N>: SupportedLaneCount,

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

where LaneCount<N>: SupportedLaneCount, T: SimdElement + PartialEq,

impl<T: PartialEq> PartialEq for TrySendError<T>

impl<T: PartialEq> PartialEq for Cursor<T>

impl<T: PartialEq> PartialEq for SendError<T>

impl<T: PartialEq> PartialEq for OnceLock<T>

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

where Y: PartialEq, R: PartialEq,