library/core/src/cmp.rs

   1 //! Utilities for comparing and ordering values.
   2 //!
   3 //! This module contains various tools for comparing and ordering values. In
   4 //! summary:
   5 //!
   6 //! * [`Eq`] and [`PartialEq`] are traits that allow you to define total and
   7 //!   partial equality between values, respectively. Implementing them overloads
   8 //!   the `==` and `!=` operators.
   9 //! * [`Ord`] and [`PartialOrd`] are traits that allow you to define total and
  10 //!   partial orderings between values, respectively. Implementing them overloads
  11 //!   the `<`, `<=`, `>`, and `>=` operators.
  12 //! * [`Ordering`] is an enum returned by the main functions of [`Ord`] and
  13 //!   [`PartialOrd`], and describes an ordering.
  14 //! * [`Reverse`] is a struct that allows you to easily reverse an ordering.
  15 //! * [`max`] and [`min`] are functions that build off of [`Ord`] and allow you
  16 //!   to find the maximum or minimum of two values.
  17 //!
  18 //! For more details, see the respective documentation of each item in the list.
  19 //!
  20 //! [`max`]: Ord::max
  21 //! [`min`]: Ord::min
  22
  23 #![stable(feature = "rust1", since = "1.0.0")]
  24
  25 use crate::const_closure::ConstFnMutClosure;
  26 use crate::marker::Destruct;
  27 #[cfg(bootstrap)]
  28 use crate::marker::StructuralPartialEq;
  29
  30 use self::Ordering::*;
  31
  32 /// Trait for equality comparisons.
  33 ///
  34 /// `x.eq(y)` can also be written `x == y`, and `x.ne(y)` can be written `x != y`.
  35 /// We use the easier-to-read infix notation in the remainder of this documentation.
  36 ///
  37 /// This trait allows for partial equality, for types that do not have a full
  38 /// equivalence relation. For example, in floating point numbers `NaN != NaN`,
  39 /// so floating point types implement `PartialEq` but not [`trait@Eq`].
  40 /// Formally speaking, when `Rhs == Self`, this trait corresponds to a [partial equivalence
  41 /// relation](https://en.wikipedia.org/wiki/Partial_equivalence_relation).
  42 ///
  43 /// Implementations must ensure that `eq` and `ne` are consistent with each other:
  44 ///
  45 /// - `a != b` if and only if `!(a == b)`.
  46 ///
  47 /// The default implementation of `ne` provides this consistency and is almost
  48 /// always sufficient. It should not be overridden without very good reason.
  49 ///
  50 /// If [`PartialOrd`] or [`Ord`] are also implemented for `Self` and `Rhs`, their methods must also
  51 /// be consistent with `PartialEq` (see the documentation of those traits for the exact
  52 /// requirements). It's easy to accidentally make them disagree by deriving some of the traits and
  53 /// manually implementing others.
  54 ///
  55 /// The equality relation `==` must satisfy the following conditions
  56 /// (for all `a`, `b`, `c` of type `A`, `B`, `C`):
  57 ///
  58 /// - **Symmetric**: if `A: PartialEq<B>` and `B: PartialEq<A>`, then **`a == b`
  59 ///   implies `b == a`**; and
  60 ///
  61 /// - **Transitive**: if `A: PartialEq<B>` and `B: PartialEq<C>` and `A:
  62 ///   PartialEq<C>`, then **`a == b` and `b == c` implies `a == c`**.
  63 ///
  64 /// Note that the `B: PartialEq<A>` (symmetric) and `A: PartialEq<C>`
  65 /// (transitive) impls are not forced to exist, but these requirements apply
  66 /// whenever they do exist.
  67 ///
  68 /// ## Derivable
  69 ///
  70 /// This trait can be used with `#[derive]`. When `derive`d on structs, two
  71 /// instances are equal if all fields are equal, and not equal if any fields
  72 /// are not equal. When `derive`d on enums, two instances are equal if they
  73 /// are the same variant and all fields are equal.
  74 ///
  75 /// ## How can I implement `PartialEq`?
  76 ///
  77 /// An example implementation for a domain in which two books are considered
  78 /// the same book if their ISBN matches, even if the formats differ:
  79 ///
  80 /// ```
  81 /// enum BookFormat {
  82 ///     Paperback,
  83 ///     Hardback,
  84 ///     Ebook,
  85 /// }
  86 ///
  87 /// struct Book {
  88 ///     isbn: i32,
  89 ///     format: BookFormat,
  90 /// }
  91 ///
  92 /// impl PartialEq for Book {
  93 ///     fn eq(&self, other: &Self) -> bool {
  94 ///         self.isbn == other.isbn
  95 ///     }
  96 /// }
  97 ///
  98 /// let b1 = Book { isbn: 3, format: BookFormat::Paperback };
  99 /// let b2 = Book { isbn: 3, format: BookFormat::Ebook };
 100 /// let b3 = Book { isbn: 10, format: BookFormat::Paperback };
 101 ///
 102 /// assert!(b1 == b2);
 103 /// assert!(b1 != b3);
 104 /// ```
 105 ///
 106 /// ## How can I compare two different types?
 107 ///
 108 /// The type you can compare with is controlled by `PartialEq`'s type parameter.
 109 /// For example, let's tweak our previous code a bit:
 110 ///
 111 /// ```
 112 /// // The derive implements <BookFormat> == <BookFormat> comparisons
 113 /// #[derive(PartialEq)]
 114 /// enum BookFormat {
 115 ///     Paperback,
 116 ///     Hardback,
 117 ///     Ebook,
 118 /// }
 119 ///
 120 /// struct Book {
 121 ///     isbn: i32,
 122 ///     format: BookFormat,
 123 /// }
 124 ///
 125 /// // Implement <Book> == <BookFormat> comparisons
 126 /// impl PartialEq<BookFormat> for Book {
 127 ///     fn eq(&self, other: &BookFormat) -> bool {
 128 ///         self.format == *other
 129 ///     }
 130 /// }
 131 ///
 132 /// // Implement <BookFormat> == <Book> comparisons
 133 /// impl PartialEq<Book> for BookFormat {
 134 ///     fn eq(&self, other: &Book) -> bool {
 135 ///         *self == other.format
 136 ///     }
 137 /// }
 138 ///
 139 /// let b1 = Book { isbn: 3, format: BookFormat::Paperback };
 140 ///
 141 /// assert!(b1 == BookFormat::Paperback);
 142 /// assert!(BookFormat::Ebook != b1);
 143 /// ```
 144 ///
 145 /// By changing `impl PartialEq for Book` to `impl PartialEq<BookFormat> for Book`,
 146 /// we allow `BookFormat`s to be compared with `Book`s.
 147 ///
 148 /// A comparison like the one above, which ignores some fields of the struct,
 149 /// can be dangerous. It can easily lead to an unintended violation of the
 150 /// requirements for a partial equivalence relation. For example, if we kept
 151 /// the above implementation of `PartialEq<Book>` for `BookFormat` and added an
 152 /// implementation of `PartialEq<Book>` for `Book` (either via a `#[derive]` or
 153 /// via the manual implementation from the first example) then the result would
 154 /// violate transitivity:
 155 ///
 156 /// ```should_panic
 157 /// #[derive(PartialEq)]
 158 /// enum BookFormat {
 159 ///     Paperback,
 160 ///     Hardback,
 161 ///     Ebook,
 162 /// }
 163 ///
 164 /// #[derive(PartialEq)]
 165 /// struct Book {
 166 ///     isbn: i32,
 167 ///     format: BookFormat,
 168 /// }
 169 ///
 170 /// impl PartialEq<BookFormat> for Book {
 171 ///     fn eq(&self, other: &BookFormat) -> bool {
 172 ///         self.format == *other
 173 ///     }
 174 /// }
 175 ///
 176 /// impl PartialEq<Book> for BookFormat {
 177 ///     fn eq(&self, other: &Book) -> bool {
 178 ///         *self == other.format
 179 ///     }
 180 /// }
 181 ///
 182 /// fn main() {
 183 ///     let b1 = Book { isbn: 1, format: BookFormat::Paperback };
 184 ///     let b2 = Book { isbn: 2, format: BookFormat::Paperback };
 185 ///
 186 ///     assert!(b1 == BookFormat::Paperback);
 187 ///     assert!(BookFormat::Paperback == b2);
 188 ///
 189 ///     // The following should hold by transitivity but doesn't.
 190 ///     assert!(b1 == b2); // <-- PANICS
 191 /// }
 192 /// ```
 193 ///
 194 /// # Examples
 195 ///
 196 /// ```
 197 /// let x: u32 = 0;
 198 /// let y: u32 = 1;
 199 ///
 200 /// assert_eq!(x == y, false);
 201 /// assert_eq!(x.eq(&y), false);
 202 /// ```
 203 ///
 204 /// [`eq`]: PartialEq::eq
 205 /// [`ne`]: PartialEq::ne
 206 #[lang = "eq"]
 207 #[stable(feature = "rust1", since = "1.0.0")]
 208 #[doc(alias = "==")]
 209 #[doc(alias = "!=")]
 210 #[rustc_on_unimplemented(
 211     message = "can't compare `{Self}` with `{Rhs}`",
 212     label = "no implementation for `{Self} == {Rhs}`",
 213     append_const_msg
 214 )]
 215 #[const_trait]
 216 #[rustc_diagnostic_item = "PartialEq"]
 217 pub trait PartialEq<Rhs: ?Sized = Self> {
 218     /// This method tests for `self` and `other` values to be equal, and is used
 219     /// by `==`.
 220     #[must_use]
 221     #[stable(feature = "rust1", since = "1.0.0")]
 222     fn eq(&self, other: &Rhs) -> bool;
 223
 224     /// This method tests for `!=`. The default implementation is almost always
 225     /// sufficient, and should not be overridden without very good reason.
 226     #[inline]
 227     #[must_use]
 228     #[stable(feature = "rust1", since = "1.0.0")]
 229     fn ne(&self, other: &Rhs) -> bool {
 230         !self.eq(other)
 231     }
 232 }
 233
 234 /// Derive macro generating an impl of the trait `PartialEq`.
 235 #[rustc_builtin_macro]
 236 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
 237 #[allow_internal_unstable(core_intrinsics, structural_match)]
 238 pub macro PartialEq($item:item) {
 239     /* compiler built-in */
 240 }
 241
 242 /// Trait for equality comparisons which are [equivalence relations](
 243 /// https://en.wikipedia.org/wiki/Equivalence_relation).
 244 ///
 245 /// This means, that in addition to `a == b` and `a != b` being strict inverses, the equality must
 246 /// be (for all `a`, `b` and `c`):
 247 ///
 248 /// - reflexive: `a == a`;
 249 /// - symmetric: `a == b` implies `b == a`; and
 250 /// - transitive: `a == b` and `b == c` implies `a == c`.
 251 ///
 252 /// This property cannot be checked by the compiler, and therefore `Eq` implies
 253 /// [`PartialEq`], and has no extra methods.
 254 ///
 255 /// ## Derivable
 256 ///
 257 /// This trait can be used with `#[derive]`. When `derive`d, because `Eq` has
 258 /// no extra methods, it is only informing the compiler that this is an
 259 /// equivalence relation rather than a partial equivalence relation. Note that
 260 /// the `derive` strategy requires all fields are `Eq`, which isn't
 261 /// always desired.
 262 ///
 263 /// ## How can I implement `Eq`?
 264 ///
 265 /// If you cannot use the `derive` strategy, specify that your type implements
 266 /// `Eq`, which has no methods:
 267 ///
 268 /// ```
 269 /// enum BookFormat { Paperback, Hardback, Ebook }
 270 /// struct Book {
 271 ///     isbn: i32,
 272 ///     format: BookFormat,
 273 /// }
 274 /// impl PartialEq for Book {
 275 ///     fn eq(&self, other: &Self) -> bool {
 276 ///         self.isbn == other.isbn
 277 ///     }
 278 /// }
 279 /// impl Eq for Book {}
 280 /// ```
 281 #[doc(alias = "==")]
 282 #[doc(alias = "!=")]
 283 #[stable(feature = "rust1", since = "1.0.0")]
 284 #[rustc_diagnostic_item = "Eq"]
 285 pub trait Eq: PartialEq<Self> {
 286     // this method is used solely by #[deriving] to assert
 287     // that every component of a type implements #[deriving]
 288     // itself, the current deriving infrastructure means doing this
 289     // assertion without using a method on this trait is nearly
 290     // impossible.
 291     //
 292     // This should never be implemented by hand.
 293     #[doc(hidden)]
 294     #[no_coverage] // rust-lang/rust#84605
 295     #[inline]
 296     #[stable(feature = "rust1", since = "1.0.0")]
 297     fn assert_receiver_is_total_eq(&self) {}
 298 }
 299
 300 /// Derive macro generating an impl of the trait `Eq`.
 301 #[rustc_builtin_macro]
 302 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
 303 #[allow_internal_unstable(core_intrinsics, derive_eq, structural_match, no_coverage)]
 304 pub macro Eq($item:item) {
 305     /* compiler built-in */
 306 }
 307
 308 // FIXME: this struct is used solely by #[derive] to
 309 // assert that every component of a type implements Eq.
 310 //
 311 // This struct should never appear in user code.
 312 #[doc(hidden)]
 313 #[allow(missing_debug_implementations)]
 314 #[unstable(feature = "derive_eq", reason = "deriving hack, should not be public", issue = "none")]
 315 pub struct AssertParamIsEq<T: Eq + ?Sized> {
 316     _field: crate::marker::PhantomData<T>,
 317 }
 318
 319 /// An `Ordering` is the result of a comparison between two values.
 320 ///
 321 /// # Examples
 322 ///
 323 /// ```
 324 /// use std::cmp::Ordering;
 325 ///
 326 /// let result = 1.cmp(&2);
 327 /// assert_eq!(Ordering::Less, result);
 328 ///
 329 /// let result = 1.cmp(&1);
 330 /// assert_eq!(Ordering::Equal, result);
 331 ///
 332 /// let result = 2.cmp(&1);
 333 /// assert_eq!(Ordering::Greater, result);
 334 /// ```
 335 #[derive(Clone, Copy, Eq, Debug, Hash)]
 336 #[cfg_attr(not(bootstrap), derive_const(PartialOrd, Ord, PartialEq))]
 337 #[stable(feature = "rust1", since = "1.0.0")]
 338 #[repr(i8)]
 339 pub enum Ordering {
 340     /// An ordering where a compared value is less than another.
 341     #[stable(feature = "rust1", since = "1.0.0")]
 342     Less = -1,
 343     /// An ordering where a compared value is equal to another.
 344     #[stable(feature = "rust1", since = "1.0.0")]
 345     Equal = 0,
 346     /// An ordering where a compared value is greater than another.
 347     #[stable(feature = "rust1", since = "1.0.0")]
 348     Greater = 1,
 349 }
 350
 351 impl Ordering {
 352     /// Returns `true` if the ordering is the `Equal` variant.
 353     ///
 354     /// # Examples
 355     ///
 356     /// ```
 357     /// use std::cmp::Ordering;
 358     ///
 359     /// assert_eq!(Ordering::Less.is_eq(), false);
 360     /// assert_eq!(Ordering::Equal.is_eq(), true);
 361     /// assert_eq!(Ordering::Greater.is_eq(), false);
 362     /// ```
 363     #[inline]
 364     #[must_use]
 365     #[rustc_const_stable(feature = "ordering_helpers", since = "1.53.0")]
 366     #[stable(feature = "ordering_helpers", since = "1.53.0")]
 367     pub const fn is_eq(self) -> bool {
 368         matches!(self, Equal)
 369     }
 370
 371     /// Returns `true` if the ordering is not the `Equal` variant.
 372     ///
 373     /// # Examples
 374     ///
 375     /// ```
 376     /// use std::cmp::Ordering;
 377     ///
 378     /// assert_eq!(Ordering::Less.is_ne(), true);
 379     /// assert_eq!(Ordering::Equal.is_ne(), false);
 380     /// assert_eq!(Ordering::Greater.is_ne(), true);
 381     /// ```
 382     #[inline]
 383     #[must_use]
 384     #[rustc_const_stable(feature = "ordering_helpers", since = "1.53.0")]
 385     #[stable(feature = "ordering_helpers", since = "1.53.0")]
 386     pub const fn is_ne(self) -> bool {
 387         !matches!(self, Equal)
 388     }
 389
 390     /// Returns `true` if the ordering is the `Less` variant.
 391     ///
 392     /// # Examples
 393     ///
 394     /// ```
 395     /// use std::cmp::Ordering;
 396     ///
 397     /// assert_eq!(Ordering::Less.is_lt(), true);
 398     /// assert_eq!(Ordering::Equal.is_lt(), false);
 399     /// assert_eq!(Ordering::Greater.is_lt(), false);
 400     /// ```
 401     #[inline]
 402     #[must_use]
 403     #[rustc_const_stable(feature = "ordering_helpers", since = "1.53.0")]
 404     #[stable(feature = "ordering_helpers", since = "1.53.0")]
 405     pub const fn is_lt(self) -> bool {
 406         matches!(self, Less)
 407     }
 408
 409     /// Returns `true` if the ordering is the `Greater` variant.
 410     ///
 411     /// # Examples
 412     ///
 413     /// ```
 414     /// use std::cmp::Ordering;
 415     ///
 416     /// assert_eq!(Ordering::Less.is_gt(), false);
 417     /// assert_eq!(Ordering::Equal.is_gt(), false);
 418     /// assert_eq!(Ordering::Greater.is_gt(), true);
 419     /// ```
 420     #[inline]
 421     #[must_use]
 422     #[rustc_const_stable(feature = "ordering_helpers", since = "1.53.0")]
 423     #[stable(feature = "ordering_helpers", since = "1.53.0")]
 424     pub const fn is_gt(self) -> bool {
 425         matches!(self, Greater)
 426     }
 427
 428     /// Returns `true` if the ordering is either the `Less` or `Equal` variant.
 429     ///
 430     /// # Examples
 431     ///
 432     /// ```
 433     /// use std::cmp::Ordering;
 434     ///
 435     /// assert_eq!(Ordering::Less.is_le(), true);
 436     /// assert_eq!(Ordering::Equal.is_le(), true);
 437     /// assert_eq!(Ordering::Greater.is_le(), false);
 438     /// ```
 439     #[inline]
 440     #[must_use]
 441     #[rustc_const_stable(feature = "ordering_helpers", since = "1.53.0")]
 442     #[stable(feature = "ordering_helpers", since = "1.53.0")]
 443     pub const fn is_le(self) -> bool {
 444         !matches!(self, Greater)
 445     }
 446
 447     /// Returns `true` if the ordering is either the `Greater` or `Equal` variant.
 448     ///
 449     /// # Examples
 450     ///
 451     /// ```
 452     /// use std::cmp::Ordering;
 453     ///
 454     /// assert_eq!(Ordering::Less.is_ge(), false);
 455     /// assert_eq!(Ordering::Equal.is_ge(), true);
 456     /// assert_eq!(Ordering::Greater.is_ge(), true);
 457     /// ```
 458     #[inline]
 459     #[must_use]
 460     #[rustc_const_stable(feature = "ordering_helpers", since = "1.53.0")]
 461     #[stable(feature = "ordering_helpers", since = "1.53.0")]
 462     pub const fn is_ge(self) -> bool {
 463         !matches!(self, Less)
 464     }
 465
 466     /// Reverses the `Ordering`.
 467     ///
 468     /// * `Less` becomes `Greater`.
 469     /// * `Greater` becomes `Less`.
 470     /// * `Equal` becomes `Equal`.
 471     ///
 472     /// # Examples
 473     ///
 474     /// Basic behavior:
 475     ///
 476     /// ```
 477     /// use std::cmp::Ordering;
 478     ///
 479     /// assert_eq!(Ordering::Less.reverse(), Ordering::Greater);
 480     /// assert_eq!(Ordering::Equal.reverse(), Ordering::Equal);
 481     /// assert_eq!(Ordering::Greater.reverse(), Ordering::Less);
 482     /// ```
 483     ///
 484     /// This method can be used to reverse a comparison:
 485     ///
 486     /// ```
 487     /// let data: &mut [_] = &mut [2, 10, 5, 8];
 488     ///
 489     /// // sort the array from largest to smallest.
 490     /// data.sort_by(|a, b| a.cmp(b).reverse());
 491     ///
 492     /// let b: &mut [_] = &mut [10, 8, 5, 2];
 493     /// assert!(data == b);
 494     /// ```
 495     #[inline]
 496     #[must_use]
 497     #[rustc_const_stable(feature = "const_ordering", since = "1.48.0")]
 498     #[stable(feature = "rust1", since = "1.0.0")]
 499     pub const fn reverse(self) -> Ordering {
 500         match self {
 501             Less => Greater,
 502             Equal => Equal,
 503             Greater => Less,
 504         }
 505     }
 506
 507     /// Chains two orderings.
 508     ///
 509     /// Returns `self` when it's not `Equal`. Otherwise returns `other`.
 510     ///
 511     /// # Examples
 512     ///
 513     /// ```
 514     /// use std::cmp::Ordering;
 515     ///
 516     /// let result = Ordering::Equal.then(Ordering::Less);
 517     /// assert_eq!(result, Ordering::Less);
 518     ///
 519     /// let result = Ordering::Less.then(Ordering::Equal);
 520     /// assert_eq!(result, Ordering::Less);
 521     ///
 522     /// let result = Ordering::Less.then(Ordering::Greater);
 523     /// assert_eq!(result, Ordering::Less);
 524     ///
 525     /// let result = Ordering::Equal.then(Ordering::Equal);
 526     /// assert_eq!(result, Ordering::Equal);
 527     ///
 528     /// let x: (i64, i64, i64) = (1, 2, 7);
 529     /// let y: (i64, i64, i64) = (1, 5, 3);
 530     /// let result = x.0.cmp(&y.0).then(x.1.cmp(&y.1)).then(x.2.cmp(&y.2));
 531     ///
 532     /// assert_eq!(result, Ordering::Less);
 533     /// ```
 534     #[inline]
 535     #[must_use]
 536     #[rustc_const_stable(feature = "const_ordering", since = "1.48.0")]
 537     #[stable(feature = "ordering_chaining", since = "1.17.0")]
 538     pub const fn then(self, other: Ordering) -> Ordering {
 539         match self {
 540             Equal => other,
 541             _ => self,
 542         }
 543     }
 544
 545     /// Chains the ordering with the given function.
 546     ///
 547     /// Returns `self` when it's not `Equal`. Otherwise calls `f` and returns
 548     /// the result.
 549     ///
 550     /// # Examples
 551     ///
 552     /// ```
 553     /// use std::cmp::Ordering;
 554     ///
 555     /// let result = Ordering::Equal.then_with(|| Ordering::Less);
 556     /// assert_eq!(result, Ordering::Less);
 557     ///
 558     /// let result = Ordering::Less.then_with(|| Ordering::Equal);
 559     /// assert_eq!(result, Ordering::Less);
 560     ///
 561     /// let result = Ordering::Less.then_with(|| Ordering::Greater);
 562     /// assert_eq!(result, Ordering::Less);
 563     ///
 564     /// let result = Ordering::Equal.then_with(|| Ordering::Equal);
 565     /// assert_eq!(result, Ordering::Equal);
 566     ///
 567     /// let x: (i64, i64, i64) = (1, 2, 7);
 568     /// let y: (i64, i64, i64) = (1, 5, 3);
 569     /// let result = x.0.cmp(&y.0).then_with(|| x.1.cmp(&y.1)).then_with(|| x.2.cmp(&y.2));
 570     ///
 571     /// assert_eq!(result, Ordering::Less);
 572     /// ```
 573     #[inline]
 574     #[must_use]
 575     #[stable(feature = "ordering_chaining", since = "1.17.0")]
 576     pub fn then_with<F: FnOnce() -> Ordering>(self, f: F) -> Ordering {
 577         match self {
 578             Equal => f(),
 579             _ => self,
 580         }
 581     }
 582 }
 583
 584 /// A helper struct for reverse ordering.
 585 ///
 586 /// This struct is a helper to be used with functions like [`Vec::sort_by_key`] and
 587 /// can be used to reverse order a part of a key.
 588 ///
 589 /// [`Vec::sort_by_key`]: ../../std/vec/struct.Vec.html#method.sort_by_key
 590 ///
 591 /// # Examples
 592 ///
 593 /// ```
 594 /// use std::cmp::Reverse;
 595 ///
 596 /// let mut v = vec![1, 2, 3, 4, 5, 6];
 597 /// v.sort_by_key(|&num| (num > 3, Reverse(num)));
 598 /// assert_eq!(v, vec![3, 2, 1, 6, 5, 4]);
 599 /// ```
 600 #[derive(PartialEq, Eq, Debug, Copy, Default, Hash)]
 601 #[stable(feature = "reverse_cmp_key", since = "1.19.0")]
 602 #[repr(transparent)]
 603 pub struct Reverse<T>(#[stable(feature = "reverse_cmp_key", since = "1.19.0")] pub T);
 604
 605 #[stable(feature = "reverse_cmp_key", since = "1.19.0")]
 606 #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
 607 impl<T: ~const PartialOrd> const PartialOrd for Reverse<T> {
 608     #[inline]
 609     fn partial_cmp(&self, other: &Reverse<T>) -> Option<Ordering> {
 610         other.0.partial_cmp(&self.0)
 611     }
 612
 613     #[inline]
 614     fn lt(&self, other: &Self) -> bool {
 615         other.0 < self.0
 616     }
 617     #[inline]
 618     fn le(&self, other: &Self) -> bool {
 619         other.0 <= self.0
 620     }
 621     #[inline]
 622     fn gt(&self, other: &Self) -> bool {
 623         other.0 > self.0
 624     }
 625     #[inline]
 626     fn ge(&self, other: &Self) -> bool {
 627         other.0 >= self.0
 628     }
 629 }
 630
 631 #[stable(feature = "reverse_cmp_key", since = "1.19.0")]
 632 impl<T: Ord> Ord for Reverse<T> {
 633     #[inline]
 634     fn cmp(&self, other: &Reverse<T>) -> Ordering {
 635         other.0.cmp(&self.0)
 636     }
 637 }
 638
 639 #[stable(feature = "reverse_cmp_key", since = "1.19.0")]
 640 impl<T: Clone> Clone for Reverse<T> {
 641     #[inline]
 642     fn clone(&self) -> Reverse<T> {
 643         Reverse(self.0.clone())
 644     }
 645
 646     #[inline]
 647     fn clone_from(&mut self, other: &Self) {
 648         self.0.clone_from(&other.0)
 649     }
 650 }
 651
 652 /// Trait for types that form a [total order](https://en.wikipedia.org/wiki/Total_order).
 653 ///
 654 /// Implementations must be consistent with the [`PartialOrd`] implementation, and ensure
 655 /// `max`, `min`, and `clamp` are consistent with `cmp`:
 656 ///
 657 /// - `partial_cmp(a, b) == Some(cmp(a, b))`.
 658 /// - `max(a, b) == max_by(a, b, cmp)` (ensured by the default implementation).
 659 /// - `min(a, b) == min_by(a, b, cmp)` (ensured by the default implementation).
 660 /// - For `a.clamp(min, max)`, see the [method docs](#method.clamp)
 661 ///   (ensured by the default implementation).
 662 ///
 663 /// It's easy to accidentally make `cmp` and `partial_cmp` disagree by
 664 /// deriving some of the traits and manually implementing others.
 665 ///
 666 /// ## Corollaries
 667 ///
 668 /// From the above and the requirements of `PartialOrd`, it follows that `<` defines a strict total order.
 669 /// This means that for all `a`, `b` and `c`:
 670 ///
 671 /// - exactly one of `a < b`, `a == b` or `a > b` is true; and
 672 /// - `<` is transitive: `a < b` and `b < c` implies `a < c`. The same must hold for both `==` and `>`.
 673 ///
 674 /// ## Derivable
 675 ///
 676 /// This trait can be used with `#[derive]`.
 677 ///
 678 /// When `derive`d on structs, it will produce a
 679 /// [lexicographic](https://en.wikipedia.org/wiki/Lexicographic_order) ordering
 680 /// based on the top-to-bottom declaration order of the struct's members.
 681 ///
 682 /// When `derive`d on enums, variants are ordered by their discriminants.
 683 /// By default, the discriminant is smallest for variants at the top, and
 684 /// largest for variants at the bottom. Here's an example:
 685 ///
 686 /// ```
 687 /// #[derive(PartialEq, Eq, PartialOrd, Ord)]
 688 /// enum E {
 689 ///     Top,
 690 ///     Bottom,
 691 /// }
 692 ///
 693 /// assert!(E::Top < E::Bottom);
 694 /// ```
 695 ///
 696 /// However, manually setting the discriminants can override this default
 697 /// behavior:
 698 ///
 699 /// ```
 700 /// #[derive(PartialEq, Eq, PartialOrd, Ord)]
 701 /// enum E {
 702 ///     Top = 2,
 703 ///     Bottom = 1,
 704 /// }
 705 ///
 706 /// assert!(E::Bottom < E::Top);
 707 /// ```
 708 ///
 709 /// ## Lexicographical comparison
 710 ///
 711 /// Lexicographical comparison is an operation with the following properties:
 712 ///  - Two sequences are compared element by element.
 713 ///  - The first mismatching element defines which sequence is lexicographically less or greater than the other.
 714 ///  - If one sequence is a prefix of another, the shorter sequence is lexicographically less than the other.
 715 ///  - If two sequence have equivalent elements and are of the same length, then the sequences are lexicographically equal.
 716 ///  - An empty sequence is lexicographically less than any non-empty sequence.
 717 ///  - Two empty sequences are lexicographically equal.
 718 ///
 719 /// ## How can I implement `Ord`?
 720 ///
 721 /// `Ord` requires that the type also be [`PartialOrd`] and [`Eq`] (which requires [`PartialEq`]).
 722 ///
 723 /// Then you must define an implementation for [`cmp`]. You may find it useful to use
 724 /// [`cmp`] on your type's fields.
 725 ///
 726 /// Here's an example where you want to sort people by height only, disregarding `id`
 727 /// and `name`:
 728 ///
 729 /// ```
 730 /// use std::cmp::Ordering;
 731 ///
 732 /// #[derive(Eq)]
 733 /// struct Person {
 734 ///     id: u32,
 735 ///     name: String,
 736 ///     height: u32,
 737 /// }
 738 ///
 739 /// impl Ord for Person {
 740 ///     fn cmp(&self, other: &Self) -> Ordering {
 741 ///         self.height.cmp(&other.height)
 742 ///     }
 743 /// }
 744 ///
 745 /// impl PartialOrd for Person {
 746 ///     fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
 747 ///         Some(self.cmp(other))
 748 ///     }
 749 /// }
 750 ///
 751 /// impl PartialEq for Person {
 752 ///     fn eq(&self, other: &Self) -> bool {
 753 ///         self.height == other.height
 754 ///     }
 755 /// }
 756 /// ```
 757 ///
 758 /// [`cmp`]: Ord::cmp
 759 #[doc(alias = "<")]
 760 #[doc(alias = ">")]
 761 #[doc(alias = "<=")]
 762 #[doc(alias = ">=")]
 763 #[stable(feature = "rust1", since = "1.0.0")]
 764 #[rustc_diagnostic_item = "Ord"]
 765 #[const_trait]
 766 pub trait Ord: Eq + PartialOrd<Self> {
 767     /// This method returns an [`Ordering`] between `self` and `other`.
 768     ///
 769     /// By convention, `self.cmp(&other)` returns the ordering matching the expression
 770     /// `self <operator> other` if true.
 771     ///
 772     /// # Examples
 773     ///
 774     /// ```
 775     /// use std::cmp::Ordering;
 776     ///
 777     /// assert_eq!(5.cmp(&10), Ordering::Less);
 778     /// assert_eq!(10.cmp(&5), Ordering::Greater);
 779     /// assert_eq!(5.cmp(&5), Ordering::Equal);
 780     /// ```
 781     #[must_use]
 782     #[stable(feature = "rust1", since = "1.0.0")]
 783     fn cmp(&self, other: &Self) -> Ordering;
 784
 785     /// Compares and returns the maximum of two values.
 786     ///
 787     /// Returns the second argument if the comparison determines them to be equal.
 788     ///
 789     /// # Examples
 790     ///
 791     /// ```
 792     /// assert_eq!(2, 1.max(2));
 793     /// assert_eq!(2, 2.max(2));
 794     /// ```
 795     #[stable(feature = "ord_max_min", since = "1.21.0")]
 796     #[inline]
 797     #[must_use]
 798     fn max(self, other: Self) -> Self
 799     where
 800         Self: Sized,
 801         Self: ~const Destruct,
 802     {
 803         // HACK(fee1-dead): go back to using `self.max_by(other, Ord::cmp)`
 804         // when trait methods are allowed to be used when a const closure is
 805         // expected.
 806         match self.cmp(&other) {
 807             Ordering::Less | Ordering::Equal => other,
 808             Ordering::Greater => self,
 809         }
 810     }
 811
 812     /// Compares and returns the minimum of two values.
 813     ///
 814     /// Returns the first argument if the comparison determines them to be equal.
 815     ///
 816     /// # Examples
 817     ///
 818     /// ```
 819     /// assert_eq!(1, 1.min(2));
 820     /// assert_eq!(2, 2.min(2));
 821     /// ```
 822     #[stable(feature = "ord_max_min", since = "1.21.0")]
 823     #[inline]
 824     #[must_use]
 825     fn min(self, other: Self) -> Self
 826     where
 827         Self: Sized,
 828         Self: ~const Destruct,
 829     {
 830         // HACK(fee1-dead): go back to using `self.min_by(other, Ord::cmp)`
 831         // when trait methods are allowed to be used when a const closure is
 832         // expected.
 833         match self.cmp(&other) {
 834             Ordering::Less | Ordering::Equal => self,
 835             Ordering::Greater => other,
 836         }
 837     }
 838
 839     /// Restrict a value to a certain interval.
 840     ///
 841     /// Returns `max` if `self` is greater than `max`, and `min` if `self` is
 842     /// less than `min`. Otherwise this returns `self`.
 843     ///
 844     /// # Panics
 845     ///
 846     /// Panics if `min > max`.
 847     ///
 848     /// # Examples
 849     ///
 850     /// ```
 851     /// assert!((-3).clamp(-2, 1) == -2);
 852     /// assert!(0.clamp(-2, 1) == 0);
 853     /// assert!(2.clamp(-2, 1) == 1);
 854     /// ```
 855     #[must_use]
 856     #[stable(feature = "clamp", since = "1.50.0")]
 857     fn clamp(self, min: Self, max: Self) -> Self
 858     where
 859         Self: Sized,
 860         Self: ~const Destruct,
 861         Self: ~const PartialOrd,
 862     {
 863         assert!(min <= max);
 864         if self < min {
 865             min
 866         } else if self > max {
 867             max
 868         } else {
 869             self
 870         }
 871     }
 872 }
 873
 874 /// Derive macro generating an impl of the trait `Ord`.
 875 #[rustc_builtin_macro]
 876 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
 877 #[allow_internal_unstable(core_intrinsics)]
 878 pub macro Ord($item:item) {
 879     /* compiler built-in */
 880 }
 881
 882 #[stable(feature = "rust1", since = "1.0.0")]
 883 #[cfg(bootstrap)]
 884 impl StructuralPartialEq for Ordering {}
 885
 886 #[stable(feature = "rust1", since = "1.0.0")]
 887 #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
 888 #[cfg(bootstrap)]
 889 impl const PartialEq for Ordering {
 890     #[inline]
 891     fn eq(&self, other: &Self) -> bool {
 892         (*self as i32).eq(&(*other as i32))
 893     }
 894 }
 895
 896 #[stable(feature = "rust1", since = "1.0.0")]
 897 #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
 898 #[cfg(bootstrap)]
 899 impl const Ord for Ordering {
 900     #[inline]
 901     fn cmp(&self, other: &Ordering) -> Ordering {
 902         (*self as i32).cmp(&(*other as i32))
 903     }
 904 }
 905
 906 #[stable(feature = "rust1", since = "1.0.0")]
 907 #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
 908 #[cfg(bootstrap)]
 909 impl const PartialOrd for Ordering {
 910     #[inline]
 911     fn partial_cmp(&self, other: &Ordering) -> Option<Ordering> {
 912         (*self as i32).partial_cmp(&(*other as i32))
 913     }
 914 }
 915
 916 /// Trait for types that form a [partial order](https://en.wikipedia.org/wiki/Partial_order).
 917 ///
 918 /// The `lt`, `le`, `gt`, and `ge` methods of this trait can be called using
 919 /// the `<`, `<=`, `>`, and `>=` operators, respectively.
 920 ///
 921 /// The methods of this trait must be consistent with each other and with those of [`PartialEq`].
 922 /// The following conditions must hold:
 923 ///
 924 /// 1. `a == b` if and only if `partial_cmp(a, b) == Some(Equal)`.
 925 /// 2. `a < b` if and only if `partial_cmp(a, b) == Some(Less)`
 926 /// 3. `a > b` if and only if `partial_cmp(a, b) == Some(Greater)`
 927 /// 4. `a <= b` if and only if `a < b || a == b`
 928 /// 5. `a >= b` if and only if `a > b || a == b`
 929 /// 6. `a != b` if and only if `!(a == b)`.
 930 ///
 931 /// Conditions 2–5 above are ensured by the default implementation.
 932 /// Condition 6 is already ensured by [`PartialEq`].
 933 ///
 934 /// If [`Ord`] is also implemented for `Self` and `Rhs`, it must also be consistent with
 935 /// `partial_cmp` (see the documentation of that trait for the exact requirements). It's
 936 /// easy to accidentally make them disagree by deriving some of the traits and manually
 937 /// implementing others.
 938 ///
 939 /// The comparison must satisfy, for all `a`, `b` and `c`:
 940 ///
 941 /// - transitivity: `a < b` and `b < c` implies `a < c`. The same must hold for both `==` and `>`.
 942 /// - duality: `a < b` if and only if `b > a`.
 943 ///
 944 /// Note that these requirements mean that the trait itself must be implemented symmetrically and
 945 /// transitively: if `T: PartialOrd<U>` and `U: PartialOrd<V>` then `U: PartialOrd<T>` and `T:
 946 /// PartialOrd<V>`.
 947 ///
 948 /// ## Corollaries
 949 ///
 950 /// The following corollaries follow from the above requirements:
 951 ///
 952 /// - irreflexivity of `<` and `>`: `!(a < a)`, `!(a > a)`
 953 /// - transitivity of `>`: if `a > b` and `b > c` then `a > c`
 954 /// - duality of `partial_cmp`: `partial_cmp(a, b) == partial_cmp(b, a).map(Ordering::reverse)`
 955 ///
 956 /// ## Derivable
 957 ///
 958 /// This trait can be used with `#[derive]`.
 959 ///
 960 /// When `derive`d on structs, it will produce a
 961 /// [lexicographic](https://en.wikipedia.org/wiki/Lexicographic_order) ordering
 962 /// based on the top-to-bottom declaration order of the struct's members.
 963 ///
 964 /// When `derive`d on enums, variants are ordered by their discriminants.
 965 /// By default, the discriminant is smallest for variants at the top, and
 966 /// largest for variants at the bottom. Here's an example:
 967 ///
 968 /// ```
 969 /// #[derive(PartialEq, PartialOrd)]
 970 /// enum E {
 971 ///     Top,
 972 ///     Bottom,
 973 /// }
 974 ///
 975 /// assert!(E::Top < E::Bottom);
 976 /// ```
 977 ///
 978 /// However, manually setting the discriminants can override this default
 979 /// behavior:
 980 ///
 981 /// ```
 982 /// #[derive(PartialEq, PartialOrd)]
 983 /// enum E {
 984 ///     Top = 2,
 985 ///     Bottom = 1,
 986 /// }
 987 ///
 988 /// assert!(E::Bottom < E::Top);
 989 /// ```
 990 ///
 991 /// ## How can I implement `PartialOrd`?
 992 ///
 993 /// `PartialOrd` only requires implementation of the [`partial_cmp`] method, with the others
 994 /// generated from default implementations.
 995 ///
 996 /// However it remains possible to implement the others separately for types which do not have a
 997 /// total order. For example, for floating point numbers, `NaN < 0 == false` and `NaN >= 0 ==
 998 /// false` (cf. IEEE 754-2008 section 5.11).
 999 ///
1000 /// `PartialOrd` requires your type to be [`PartialEq`].
1001 ///
1002 /// If your type is [`Ord`], you can implement [`partial_cmp`] by using [`cmp`]:
1003 ///
1004 /// ```
1005 /// use std::cmp::Ordering;
1006 ///
1007 /// #[derive(Eq)]
1008 /// struct Person {
1009 ///     id: u32,
1010 ///     name: String,
1011 ///     height: u32,
1012 /// }
1013 ///
1014 /// impl PartialOrd for Person {
1015 ///     fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
1016 ///         Some(self.cmp(other))
1017 ///     }
1018 /// }
1019 ///
1020 /// impl Ord for Person {
1021 ///     fn cmp(&self, other: &Self) -> Ordering {
1022 ///         self.height.cmp(&other.height)
1023 ///     }
1024 /// }
1025 ///
1026 /// impl PartialEq for Person {
1027 ///     fn eq(&self, other: &Self) -> bool {
1028 ///         self.height == other.height
1029 ///     }
1030 /// }
1031 /// ```
1032 ///
1033 /// You may also find it useful to use [`partial_cmp`] on your type's fields. Here
1034 /// is an example of `Person` types who have a floating-point `height` field that
1035 /// is the only field to be used for sorting:
1036 ///
1037 /// ```
1038 /// use std::cmp::Ordering;
1039 ///
1040 /// struct Person {
1041 ///     id: u32,
1042 ///     name: String,
1043 ///     height: f64,
1044 /// }
1045 ///
1046 /// impl PartialOrd for Person {
1047 ///     fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
1048 ///         self.height.partial_cmp(&other.height)
1049 ///     }
1050 /// }
1051 ///
1052 /// impl PartialEq for Person {
1053 ///     fn eq(&self, other: &Self) -> bool {
1054 ///         self.height == other.height
1055 ///     }
1056 /// }
1057 /// ```
1058 ///
1059 /// # Examples
1060 ///
1061 /// ```
1062 /// let x: u32 = 0;
1063 /// let y: u32 = 1;
1064 ///
1065 /// assert_eq!(x < y, true);
1066 /// assert_eq!(x.lt(&y), true);
1067 /// ```
1068 ///
1069 /// [`partial_cmp`]: PartialOrd::partial_cmp
1070 /// [`cmp`]: Ord::cmp
1071 #[lang = "partial_ord"]
1072 #[stable(feature = "rust1", since = "1.0.0")]
1073 #[doc(alias = ">")]
1074 #[doc(alias = "<")]
1075 #[doc(alias = "<=")]
1076 #[doc(alias = ">=")]
1077 #[rustc_on_unimplemented(
1078     message = "can't compare `{Self}` with `{Rhs}`",
1079     label = "no implementation for `{Self} < {Rhs}` and `{Self} > {Rhs}`",
1080     append_const_msg
1081 )]
1082 #[const_trait]
1083 #[rustc_diagnostic_item = "PartialOrd"]
1084 pub trait PartialOrd<Rhs: ?Sized = Self>: PartialEq<Rhs> {
1085     /// This method returns an ordering between `self` and `other` values if one exists.
1086     ///
1087     /// # Examples
1088     ///
1089     /// ```
1090     /// use std::cmp::Ordering;
1091     ///
1092     /// let result = 1.0.partial_cmp(&2.0);
1093     /// assert_eq!(result, Some(Ordering::Less));
1094     ///
1095     /// let result = 1.0.partial_cmp(&1.0);
1096     /// assert_eq!(result, Some(Ordering::Equal));
1097     ///
1098     /// let result = 2.0.partial_cmp(&1.0);
1099     /// assert_eq!(result, Some(Ordering::Greater));
1100     /// ```
1101     ///
1102     /// When comparison is impossible:
1103     ///
1104     /// ```
1105     /// let result = f64::NAN.partial_cmp(&1.0);
1106     /// assert_eq!(result, None);
1107     /// ```
1108     #[must_use]
1109     #[stable(feature = "rust1", since = "1.0.0")]
1110     fn partial_cmp(&self, other: &Rhs) -> Option<Ordering>;
1111
1112     /// This method tests less than (for `self` and `other`) and is used by the `<` operator.
1113     ///
1114     /// # Examples
1115     ///
1116     /// ```
1117     /// let result = 1.0 < 2.0;
1118     /// assert_eq!(result, true);
1119     ///
1120     /// let result = 2.0 < 1.0;
1121     /// assert_eq!(result, false);
1122     /// ```
1123     #[inline]
1124     #[must_use]
1125     #[stable(feature = "rust1", since = "1.0.0")]
1126     fn lt(&self, other: &Rhs) -> bool {
1127         matches!(self.partial_cmp(other), Some(Less))
1128     }
1129
1130     /// This method tests less than or equal to (for `self` and `other`) and is used by the `<=`
1131     /// operator.
1132     ///
1133     /// # Examples
1134     ///
1135     /// ```
1136     /// let result = 1.0 <= 2.0;
1137     /// assert_eq!(result, true);
1138     ///
1139     /// let result = 2.0 <= 2.0;
1140     /// assert_eq!(result, true);
1141     /// ```
1142     #[inline]
1143     #[must_use]
1144     #[stable(feature = "rust1", since = "1.0.0")]
1145     fn le(&self, other: &Rhs) -> bool {
1146         matches!(self.partial_cmp(other), Some(Less | Equal))
1147     }
1148
1149     /// This method tests greater than (for `self` and `other`) and is used by the `>` operator.
1150     ///
1151     /// # Examples
1152     ///
1153     /// ```
1154     /// let result = 1.0 > 2.0;
1155     /// assert_eq!(result, false);
1156     ///
1157     /// let result = 2.0 > 2.0;
1158     /// assert_eq!(result, false);
1159     /// ```
1160     #[inline]
1161     #[must_use]
1162     #[stable(feature = "rust1", since = "1.0.0")]
1163     fn gt(&self, other: &Rhs) -> bool {
1164         matches!(self.partial_cmp(other), Some(Greater))
1165     }
1166
1167     /// This method tests greater than or equal to (for `self` and `other`) and is used by the `>=`
1168     /// operator.
1169     ///
1170     /// # Examples
1171     ///
1172     /// ```
1173     /// let result = 2.0 >= 1.0;
1174     /// assert_eq!(result, true);
1175     ///
1176     /// let result = 2.0 >= 2.0;
1177     /// assert_eq!(result, true);
1178     /// ```
1179     #[inline]
1180     #[must_use]
1181     #[stable(feature = "rust1", since = "1.0.0")]
1182     fn ge(&self, other: &Rhs) -> bool {
1183         matches!(self.partial_cmp(other), Some(Greater | Equal))
1184     }
1185 }
1186
1187 /// Derive macro generating an impl of the trait `PartialOrd`.
1188 #[rustc_builtin_macro]
1189 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
1190 #[allow_internal_unstable(core_intrinsics)]
1191 pub macro PartialOrd($item:item) {
1192     /* compiler built-in */
1193 }
1194
1195 /// Compares and returns the minimum of two values.
1196 ///
1197 /// Returns the first argument if the comparison determines them to be equal.
1198 ///
1199 /// Internally uses an alias to [`Ord::min`].
1200 ///
1201 /// # Examples
1202 ///
1203 /// ```
1204 /// use std::cmp;
1205 ///
1206 /// assert_eq!(1, cmp::min(1, 2));
1207 /// assert_eq!(2, cmp::min(2, 2));
1208 /// ```
1209 #[inline]
1210 #[must_use]
1211 #[stable(feature = "rust1", since = "1.0.0")]
1212 #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
1213 #[cfg_attr(not(test), rustc_diagnostic_item = "cmp_min")]
1214 pub const fn min<T: ~const Ord + ~const Destruct>(v1: T, v2: T) -> T {
1215     v1.min(v2)
1216 }
1217
1218 /// Returns the minimum of two values with respect to the specified comparison function.
1219 ///
1220 /// Returns the first argument if the comparison determines them to be equal.
1221 ///
1222 /// # Examples
1223 ///
1224 /// ```
1225 /// use std::cmp;
1226 ///
1227 /// assert_eq!(cmp::min_by(-2, 1, |x: &i32, y: &i32| x.abs().cmp(&y.abs())), 1);
1228 /// assert_eq!(cmp::min_by(-2, 2, |x: &i32, y: &i32| x.abs().cmp(&y.abs())), -2);
1229 /// ```
1230 #[inline]
1231 #[must_use]
1232 #[stable(feature = "cmp_min_max_by", since = "1.53.0")]
1233 #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
1234 pub const fn min_by<T, F: ~const FnOnce(&T, &T) -> Ordering>(v1: T, v2: T, compare: F) -> T
1235 where
1236     T: ~const Destruct,
1237     F: ~const Destruct,
1238 {
1239     match compare(&v1, &v2) {
1240         Ordering::Less | Ordering::Equal => v1,
1241         Ordering::Greater => v2,
1242     }
1243 }
1244
1245 /// Returns the element that gives the minimum value from the specified function.
1246 ///
1247 /// Returns the first argument if the comparison determines them to be equal.
1248 ///
1249 /// # Examples
1250 ///
1251 /// ```
1252 /// use std::cmp;
1253 ///
1254 /// assert_eq!(cmp::min_by_key(-2, 1, |x: &i32| x.abs()), 1);
1255 /// assert_eq!(cmp::min_by_key(-2, 2, |x: &i32| x.abs()), -2);
1256 /// ```
1257 #[inline]
1258 #[must_use]
1259 #[stable(feature = "cmp_min_max_by", since = "1.53.0")]
1260 #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
1261 pub const fn min_by_key<T, F: ~const FnMut(&T) -> K, K: ~const Ord>(v1: T, v2: T, mut f: F) -> T
1262 where
1263     T: ~const Destruct,
1264     F: ~const Destruct,
1265     K: ~const Destruct,
1266 {
1267     const fn imp<T, F: ~const FnMut(&T) -> K, K: ~const Ord>(
1268         f: &mut F,
1269         (v1, v2): (&T, &T),
1270     ) -> Ordering
1271     where
1272         T: ~const Destruct,
1273         K: ~const Destruct,
1274     {
1275         f(v1).cmp(&f(v2))
1276     }
1277     min_by(v1, v2, ConstFnMutClosure::new(&mut f, imp))
1278 }
1279
1280 /// Compares and returns the maximum of two values.
1281 ///
1282 /// Returns the second argument if the comparison determines them to be equal.
1283 ///
1284 /// Internally uses an alias to [`Ord::max`].
1285 ///
1286 /// # Examples
1287 ///
1288 /// ```
1289 /// use std::cmp;
1290 ///
1291 /// assert_eq!(2, cmp::max(1, 2));
1292 /// assert_eq!(2, cmp::max(2, 2));
1293 /// ```
1294 #[inline]
1295 #[must_use]
1296 #[stable(feature = "rust1", since = "1.0.0")]
1297 #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
1298 #[cfg_attr(not(test), rustc_diagnostic_item = "cmp_max")]
1299 pub const fn max<T: ~const Ord + ~const Destruct>(v1: T, v2: T) -> T {
1300     v1.max(v2)
1301 }
1302
1303 /// Returns the maximum of two values with respect to the specified comparison function.
1304 ///
1305 /// Returns the second argument if the comparison determines them to be equal.
1306 ///
1307 /// # Examples
1308 ///
1309 /// ```
1310 /// use std::cmp;
1311 ///
1312 /// assert_eq!(cmp::max_by(-2, 1, |x: &i32, y: &i32| x.abs().cmp(&y.abs())), -2);
1313 /// assert_eq!(cmp::max_by(-2, 2, |x: &i32, y: &i32| x.abs().cmp(&y.abs())), 2);
1314 /// ```
1315 #[inline]
1316 #[must_use]
1317 #[stable(feature = "cmp_min_max_by", since = "1.53.0")]
1318 #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
1319 pub const fn max_by<T, F: ~const FnOnce(&T, &T) -> Ordering>(v1: T, v2: T, compare: F) -> T
1320 where
1321     T: ~const Destruct,
1322     F: ~const Destruct,
1323 {
1324     match compare(&v1, &v2) {
1325         Ordering::Less | Ordering::Equal => v2,
1326         Ordering::Greater => v1,
1327     }
1328 }
1329
1330 /// Returns the element that gives the maximum value from the specified function.
1331 ///
1332 /// Returns the second argument if the comparison determines them to be equal.
1333 ///
1334 /// # Examples
1335 ///
1336 /// ```
1337 /// use std::cmp;
1338 ///
1339 /// assert_eq!(cmp::max_by_key(-2, 1, |x: &i32| x.abs()), -2);
1340 /// assert_eq!(cmp::max_by_key(-2, 2, |x: &i32| x.abs()), 2);
1341 /// ```
1342 #[inline]
1343 #[must_use]
1344 #[stable(feature = "cmp_min_max_by", since = "1.53.0")]
1345 #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
1346 pub const fn max_by_key<T, F: ~const FnMut(&T) -> K, K: ~const Ord>(v1: T, v2: T, mut f: F) -> T
1347 where
1348     T: ~const Destruct,
1349     F: ~const Destruct,
1350     K: ~const Destruct,
1351 {
1352     const fn imp<T, F: ~const FnMut(&T) -> K, K: ~const Ord>(
1353         f: &mut F,
1354         (v1, v2): (&T, &T),
1355     ) -> Ordering
1356     where
1357         T: ~const Destruct,
1358         K: ~const Destruct,
1359     {
1360         f(v1).cmp(&f(v2))
1361     }
1362     max_by(v1, v2, ConstFnMutClosure::new(&mut f, imp))
1363 }
1364
1365 // Implementation of PartialEq, Eq, PartialOrd and Ord for primitive types
1366 mod impls {
1367     use crate::cmp::Ordering::{self, Equal, Greater, Less};
1368     use crate::hint::unreachable_unchecked;
1369
1370     macro_rules! partial_eq_impl {
1371         ($($t:ty)*) => ($(
1372             #[stable(feature = "rust1", since = "1.0.0")]
1373             #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
1374             impl const PartialEq for $t {
1375                 #[inline]
1376                 fn eq(&self, other: &$t) -> bool { (*self) == (*other) }
1377                 #[inline]
1378                 fn ne(&self, other: &$t) -> bool { (*self) != (*other) }
1379             }
1380         )*)
1381     }
1382
1383     #[stable(feature = "rust1", since = "1.0.0")]
1384     #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
1385     impl const PartialEq for () {
1386         #[inline]
1387         fn eq(&self, _other: &()) -> bool {
1388             true
1389         }
1390         #[inline]
1391         fn ne(&self, _other: &()) -> bool {
1392             false
1393         }
1394     }
1395
1396     partial_eq_impl! {
1397         bool char usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f32 f64
1398     }
1399
1400     macro_rules! eq_impl {
1401         ($($t:ty)*) => ($(
1402             #[stable(feature = "rust1", since = "1.0.0")]
1403             impl Eq for $t {}
1404         )*)
1405     }
1406
1407     eq_impl! { () bool char usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
1408
1409     macro_rules! partial_ord_impl {
1410         ($($t:ty)*) => ($(
1411             #[stable(feature = "rust1", since = "1.0.0")]
1412             #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
1413             impl const PartialOrd for $t {
1414                 #[inline]
1415                 fn partial_cmp(&self, other: &$t) -> Option<Ordering> {
1416                     match (*self <= *other, *self >= *other) {
1417                         (false, false) => None,
1418                         (false, true) => Some(Greater),
1419                         (true, false) => Some(Less),
1420                         (true, true) => Some(Equal),
1421                     }
1422                 }
1423                 #[inline]
1424                 fn lt(&self, other: &$t) -> bool { (*self) < (*other) }
1425                 #[inline]
1426                 fn le(&self, other: &$t) -> bool { (*self) <= (*other) }
1427                 #[inline]
1428                 fn ge(&self, other: &$t) -> bool { (*self) >= (*other) }
1429                 #[inline]
1430                 fn gt(&self, other: &$t) -> bool { (*self) > (*other) }
1431             }
1432         )*)
1433     }
1434
1435     #[stable(feature = "rust1", since = "1.0.0")]
1436     #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
1437     impl const PartialOrd for () {
1438         #[inline]
1439         fn partial_cmp(&self, _: &()) -> Option<Ordering> {
1440             Some(Equal)
1441         }
1442     }
1443
1444     #[stable(feature = "rust1", since = "1.0.0")]
1445     #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
1446     impl const PartialOrd for bool {
1447         #[inline]
1448         fn partial_cmp(&self, other: &bool) -> Option<Ordering> {
1449             Some(self.cmp(other))
1450         }
1451     }
1452
1453     partial_ord_impl! { f32 f64 }
1454
1455     macro_rules! ord_impl {
1456         ($($t:ty)*) => ($(
1457             #[stable(feature = "rust1", since = "1.0.0")]
1458             #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
1459             impl const PartialOrd for $t {
1460                 #[inline]
1461                 fn partial_cmp(&self, other: &$t) -> Option<Ordering> {
1462                     Some(self.cmp(other))
1463                 }
1464                 #[inline]
1465                 fn lt(&self, other: &$t) -> bool { (*self) < (*other) }
1466                 #[inline]
1467                 fn le(&self, other: &$t) -> bool { (*self) <= (*other) }
1468                 #[inline]
1469                 fn ge(&self, other: &$t) -> bool { (*self) >= (*other) }
1470                 #[inline]
1471                 fn gt(&self, other: &$t) -> bool { (*self) > (*other) }
1472             }
1473
1474             #[stable(feature = "rust1", since = "1.0.0")]
1475             #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
1476             impl const Ord for $t {
1477                 #[inline]
1478                 fn cmp(&self, other: &$t) -> Ordering {
1479                     // The order here is important to generate more optimal assembly.
1480                     // See <https://github.com/rust-lang/rust/issues/63758> for more info.
1481                     if *self < *other { Less }
1482                     else if *self == *other { Equal }
1483                     else { Greater }
1484                 }
1485             }
1486         )*)
1487     }
1488
1489     #[stable(feature = "rust1", since = "1.0.0")]
1490     #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
1491     impl const Ord for () {
1492         #[inline]
1493         fn cmp(&self, _other: &()) -> Ordering {
1494             Equal
1495         }
1496     }
1497
1498     #[stable(feature = "rust1", since = "1.0.0")]
1499     #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
1500     impl const Ord for bool {
1501         #[inline]
1502         fn cmp(&self, other: &bool) -> Ordering {
1503             // Casting to i8's and converting the difference to an Ordering generates
1504             // more optimal assembly.
1505             // See <https://github.com/rust-lang/rust/issues/66780> for more info.
1506             match (*self as i8) - (*other as i8) {
1507                 -1 => Less,
1508                 0 => Equal,
1509                 1 => Greater,
1510                 // SAFETY: bool as i8 returns 0 or 1, so the difference can't be anything else
1511                 _ => unsafe { unreachable_unchecked() },
1512             }
1513         }
1514     }
1515
1516     ord_impl! { char usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
1517
1518     #[unstable(feature = "never_type", issue = "35121")]
1519     #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
1520     impl const PartialEq for ! {
1521         fn eq(&self, _: &!) -> bool {
1522             *self
1523         }
1524     }
1525
1526     #[unstable(feature = "never_type", issue = "35121")]
1527     impl Eq for ! {}
1528
1529     #[unstable(feature = "never_type", issue = "35121")]
1530     #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
1531     impl const PartialOrd for ! {
1532         fn partial_cmp(&self, _: &!) -> Option<Ordering> {
1533             *self
1534         }
1535     }
1536
1537     #[unstable(feature = "never_type", issue = "35121")]
1538     #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
1539     impl const Ord for ! {
1540         fn cmp(&self, _: &!) -> Ordering {
1541             *self
1542         }
1543     }
1544
1545     // & pointers
1546
1547     #[stable(feature = "rust1", since = "1.0.0")]
1548     #[rustc_const_unstable(feature = "const_cmp", issue = "92391")]
1549     impl<A: ?Sized, B: ?Sized> const PartialEq<&B> for &A
1550     where
1551         A: ~const PartialEq<B>,
1552     {
1553         #[inline]
1554         fn eq(&self, other: &&B) -> bool {
1555             PartialEq::eq(*self, *other)
1556         }
1557         #[inline]
1558         fn ne(&self, other: &&B) -> bool {
1559             PartialEq::ne(*self, *other)
1560         }
1561     }
1562     #[stable(feature = "rust1", since = "1.0.0")]
1563     impl<A: ?Sized, B: ?Sized> PartialOrd<&B> for &A
1564     where
1565         A: PartialOrd<B>,
1566     {
1567         #[inline]
1568         fn partial_cmp(&self, other: &&B) -> Option<Ordering> {
1569             PartialOrd::partial_cmp(*self, *other)
1570         }
1571         #[inline]
1572         fn lt(&self, other: &&B) -> bool {
1573             PartialOrd::lt(*self, *other)
1574         }
1575         #[inline]
1576         fn le(&self, other: &&B) -> bool {
1577             PartialOrd::le(*self, *other)
1578         }
1579         #[inline]
1580         fn gt(&self, other: &&B) -> bool {
1581             PartialOrd::gt(*self, *other)
1582         }
1583         #[inline]
1584         fn ge(&self, other: &&B) -> bool {
1585             PartialOrd::ge(*self, *other)
1586         }
1587     }
1588     #[stable(feature = "rust1", since = "1.0.0")]
1589     impl<A: ?Sized> Ord for &A
1590     where
1591         A: Ord,
1592     {
1593         #[inline]
1594         fn cmp(&self, other: &Self) -> Ordering {
1595             Ord::cmp(*self, *other)
1596         }
1597     }
1598     #[stable(feature = "rust1", since = "1.0.0")]
1599     impl<A: ?Sized> Eq for &A where A: Eq {}
1600
1601     // &mut pointers
1602
1603     #[stable(feature = "rust1", since = "1.0.0")]
1604     impl<A: ?Sized, B: ?Sized> PartialEq<&mut B> for &mut A
1605     where
1606         A: PartialEq<B>,
1607     {
1608         #[inline]
1609         fn eq(&self, other: &&mut B) -> bool {
1610             PartialEq::eq(*self, *other)
1611         }
1612         #[inline]
1613         fn ne(&self, other: &&mut B) -> bool {
1614             PartialEq::ne(*self, *other)
1615         }
1616     }
1617     #[stable(feature = "rust1", since = "1.0.0")]
1618     impl<A: ?Sized, B: ?Sized> PartialOrd<&mut B> for &mut A
1619     where
1620         A: PartialOrd<B>,
1621     {
1622         #[inline]
1623         fn partial_cmp(&self, other: &&mut B) -> Option<Ordering> {
1624             PartialOrd::partial_cmp(*self, *other)
1625         }
1626         #[inline]
1627         fn lt(&self, other: &&mut B) -> bool {
1628             PartialOrd::lt(*self, *other)
1629         }
1630         #[inline]
1631         fn le(&self, other: &&mut B) -> bool {
1632             PartialOrd::le(*self, *other)
1633         }
1634         #[inline]
1635         fn gt(&self, other: &&mut B) -> bool {
1636             PartialOrd::gt(*self, *other)
1637         }
1638         #[inline]
1639         fn ge(&self, other: &&mut B) -> bool {
1640             PartialOrd::ge(*self, *other)
1641         }
1642     }
1643     #[stable(feature = "rust1", since = "1.0.0")]
1644     impl<A: ?Sized> Ord for &mut A
1645     where
1646         A: Ord,
1647     {
1648         #[inline]
1649         fn cmp(&self, other: &Self) -> Ordering {
1650             Ord::cmp(*self, *other)
1651         }
1652     }
1653     #[stable(feature = "rust1", since = "1.0.0")]
1654     impl<A: ?Sized> Eq for &mut A where A: Eq {}
1655
1656     #[stable(feature = "rust1", since = "1.0.0")]
1657     impl<A: ?Sized, B: ?Sized> PartialEq<&mut B> for &A
1658     where
1659         A: PartialEq<B>,
1660     {
1661         #[inline]
1662         fn eq(&self, other: &&mut B) -> bool {
1663             PartialEq::eq(*self, *other)
1664         }
1665         #[inline]
1666         fn ne(&self, other: &&mut B) -> bool {
1667             PartialEq::ne(*self, *other)
1668         }
1669     }
1670
1671     #[stable(feature = "rust1", since = "1.0.0")]
1672     impl<A: ?Sized, B: ?Sized> PartialEq<&B> for &mut A
1673     where
1674         A: PartialEq<B>,
1675     {
1676         #[inline]
1677         fn eq(&self, other: &&B) -> bool {
1678             PartialEq::eq(*self, *other)
1679         }
1680         #[inline]
1681         fn ne(&self, other: &&B) -> bool {
1682             PartialEq::ne(*self, *other)
1683         }
1684     }
1685 }