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