library/core/src/convert/mod.rs

   1 //! Traits for conversions between types.
   2 //!
   3 //! The traits in this module provide a way to convert from one type to another type.
   4 //! Each trait serves a different purpose:
   5 //!
   6 //! - Implement the [`AsRef`] trait for cheap reference-to-reference conversions
   7 //! - Implement the [`AsMut`] trait for cheap mutable-to-mutable conversions
   8 //! - Implement the [`From`] trait for consuming value-to-value conversions
   9 //! - Implement the [`Into`] trait for consuming value-to-value conversions to types
  10 //!   outside the current crate
  11 //! - The [`TryFrom`] and [`TryInto`] traits behave like [`From`] and [`Into`],
  12 //!   but should be implemented when the conversion can fail.
  13 //!
  14 //! The traits in this module are often used as trait bounds for generic functions such that to
  15 //! arguments of multiple types are supported. See the documentation of each trait for examples.
  16 //!
  17 //! As a library author, you should always prefer implementing [`From<T>`][`From`] or
  18 //! [`TryFrom<T>`][`TryFrom`] rather than [`Into<U>`][`Into`] or [`TryInto<U>`][`TryInto`],
  19 //! as [`From`] and [`TryFrom`] provide greater flexibility and offer
  20 //! equivalent [`Into`] or [`TryInto`] implementations for free, thanks to a
  21 //! blanket implementation in the standard library. When targeting a version prior to Rust 1.41, it
  22 //! may be necessary to implement [`Into`] or [`TryInto`] directly when converting to a type
  23 //! outside the current crate.
  24 //!
  25 //! # Generic Implementations
  26 //!
  27 //! - [`AsRef`] and [`AsMut`] auto-dereference if the inner type is a reference
  28 //! - [`From`]`<U> for T` implies [`Into`]`<T> for U`
  29 //! - [`TryFrom`]`<U> for T` implies [`TryInto`]`<T> for U`
  30 //! - [`From`] and [`Into`] are reflexive, which means that all types can
  31 //!   `into` themselves and `from` themselves
  32 //!
  33 //! See each trait for usage examples.
  34
  35 #![stable(feature = "rust1", since = "1.0.0")]
  36
  37 use crate::fmt;
  38 use crate::hash::{Hash, Hasher};
  39
  40 mod num;
  41
  42 #[unstable(feature = "convert_float_to_int", issue = "67057")]
  43 pub use num::FloatToInt;
  44
  45 /// The identity function.
  46 ///
  47 /// Two things are important to note about this function:
  48 ///
  49 /// - It is not always equivalent to a closure like `|x| x`, since the
  50 ///   closure may coerce `x` into a different type.
  51 ///
  52 /// - It moves the input `x` passed to the function.
  53 ///
  54 /// While it might seem strange to have a function that just returns back the
  55 /// input, there are some interesting uses.
  56 ///
  57 /// # Examples
  58 ///
  59 /// Using `identity` to do nothing in a sequence of other, interesting,
  60 /// functions:
  61 ///
  62 /// ```rust
  63 /// use std::convert::identity;
  64 ///
  65 /// fn manipulation(x: u32) -> u32 {
  66 ///     // Let's pretend that adding one is an interesting function.
  67 ///     x + 1
  68 /// }
  69 ///
  70 /// let _arr = &[identity, manipulation];
  71 /// ```
  72 ///
  73 /// Using `identity` as a "do nothing" base case in a conditional:
  74 ///
  75 /// ```rust
  76 /// use std::convert::identity;
  77 ///
  78 /// # let condition = true;
  79 /// #
  80 /// # fn manipulation(x: u32) -> u32 { x + 1 }
  81 /// #
  82 /// let do_stuff = if condition { manipulation } else { identity };
  83 ///
  84 /// // Do more interesting stuff...
  85 ///
  86 /// let _results = do_stuff(42);
  87 /// ```
  88 ///
  89 /// Using `identity` to keep the `Some` variants of an iterator of `Option<T>`:
  90 ///
  91 /// ```rust
  92 /// use std::convert::identity;
  93 ///
  94 /// let iter = vec![Some(1), None, Some(3)].into_iter();
  95 /// let filtered = iter.filter_map(identity).collect::<Vec<_>>();
  96 /// assert_eq!(vec![1, 3], filtered);
  97 /// ```
  98 #[stable(feature = "convert_id", since = "1.33.0")]
  99 #[rustc_const_stable(feature = "const_identity", since = "1.33.0")]
 100 #[inline]
 101 pub const fn identity<T>(x: T) -> T {
 102     x
 103 }
 104
 105 /// Used to do a cheap reference-to-reference conversion.
 106 ///
 107 /// This trait is similar to [`AsMut`] which is used for converting between mutable references.
 108 /// If you need to do a costly conversion it is better to implement [`From`] with type
 109 /// `&T` or write a custom function.
 110 ///
 111 /// `AsRef` has the same signature as [`Borrow`], but [`Borrow`] is different in few aspects:
 112 ///
 113 /// - Unlike `AsRef`, [`Borrow`] has a blanket impl for any `T`, and can be used to accept either
 114 ///   a reference or a value.
 115 /// - [`Borrow`] also requires that [`Hash`], [`Eq`] and [`Ord`] for borrowed value are
 116 ///   equivalent to those of the owned value. For this reason, if you want to
 117 ///   borrow only a single field of a struct you can implement `AsRef`, but not [`Borrow`].
 118 ///
 119 /// **Note: This trait must not fail**. If the conversion can fail, use a
 120 /// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
 121 ///
 122 /// # Generic Implementations
 123 ///
 124 /// - `AsRef` auto-dereferences if the inner type is a reference or a mutable
 125 ///   reference (e.g.: `foo.as_ref()` will work the same if `foo` has type
 126 ///   `&mut Foo` or `&&mut Foo`)
 127 ///
 128 /// # Examples
 129 ///
 130 /// By using trait bounds we can accept arguments of different types as long as they can be
 131 /// converted to the specified type `T`.
 132 ///
 133 /// For example: By creating a generic function that takes an `AsRef<str>` we express that we
 134 /// want to accept all references that can be converted to [`&str`] as an argument.
 135 /// Since both [`String`] and [`&str`] implement `AsRef<str>` we can accept both as input argument.
 136 ///
 137 /// [`&str`]: primitive@str
 138 /// [`Borrow`]: crate::borrow::Borrow
 139 /// [`Eq`]: crate::cmp::Eq
 140 /// [`Ord`]: crate::cmp::Ord
 141 /// [`String`]: ../../std/string/struct.String.html
 142 ///
 143 /// ```
 144 /// fn is_hello<T: AsRef<str>>(s: T) {
 145 ///    assert_eq!("hello", s.as_ref());
 146 /// }
 147 ///
 148 /// let s = "hello";
 149 /// is_hello(s);
 150 ///
 151 /// let s = "hello".to_string();
 152 /// is_hello(s);
 153 /// ```
 154 #[stable(feature = "rust1", since = "1.0.0")]
 155 pub trait AsRef<T: ?Sized> {
 156     /// Performs the conversion.
 157     #[stable(feature = "rust1", since = "1.0.0")]
 158     fn as_ref(&self) -> &T;
 159 }
 160
 161 /// Used to do a cheap mutable-to-mutable reference conversion.
 162 ///
 163 /// This trait is similar to [`AsRef`] but used for converting between mutable
 164 /// references. If you need to do a costly conversion it is better to
 165 /// implement [`From`] with type `&mut T` or write a custom function.
 166 ///
 167 /// **Note: This trait must not fail**. If the conversion can fail, use a
 168 /// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
 169 ///
 170 /// # Generic Implementations
 171 ///
 172 /// - `AsMut` auto-dereferences if the inner type is a mutable reference
 173 ///   (e.g.: `foo.as_mut()` will work the same if `foo` has type `&mut Foo`
 174 ///   or `&mut &mut Foo`)
 175 ///
 176 /// # Examples
 177 ///
 178 /// Using `AsMut` as trait bound for a generic function we can accept all mutable references
 179 /// that can be converted to type `&mut T`. Because [`Box<T>`] implements `AsMut<T>` we can
 180 /// write a function `add_one` that takes all arguments that can be converted to `&mut u64`.
 181 /// Because [`Box<T>`] implements `AsMut<T>`, `add_one` accepts arguments of type
 182 /// `&mut Box<u64>` as well:
 183 ///
 184 /// ```
 185 /// fn add_one<T: AsMut<u64>>(num: &mut T) {
 186 ///     *num.as_mut() += 1;
 187 /// }
 188 ///
 189 /// let mut boxed_num = Box::new(0);
 190 /// add_one(&mut boxed_num);
 191 /// assert_eq!(*boxed_num, 1);
 192 /// ```
 193 ///
 194 /// [`Box<T>`]: ../../std/boxed/struct.Box.html
 195 #[stable(feature = "rust1", since = "1.0.0")]
 196 pub trait AsMut<T: ?Sized> {
 197     /// Performs the conversion.
 198     #[stable(feature = "rust1", since = "1.0.0")]
 199     fn as_mut(&mut self) -> &mut T;
 200 }
 201
 202 /// A value-to-value conversion that consumes the input value. The
 203 /// opposite of [`From`].
 204 ///
 205 /// One should avoid implementing [`Into`] and implement [`From`] instead.
 206 /// Implementing [`From`] automatically provides one with an implementation of [`Into`]
 207 /// thanks to the blanket implementation in the standard library.
 208 ///
 209 /// Prefer using [`Into`] over [`From`] when specifying trait bounds on a generic function
 210 /// to ensure that types that only implement [`Into`] can be used as well.
 211 ///
 212 /// **Note: This trait must not fail**. If the conversion can fail, use [`TryInto`].
 213 ///
 214 /// # Generic Implementations
 215 ///
 216 /// - [`From`]`<T> for U` implies `Into<U> for T`
 217 /// - [`Into`] is reflexive, which means that `Into<T> for T` is implemented
 218 ///
 219 /// # Implementing [`Into`] for conversions to external types in old versions of Rust
 220 ///
 221 /// Prior to Rust 1.41, if the destination type was not part of the current crate
 222 /// then you couldn't implement [`From`] directly.
 223 /// For example, take this code:
 224 ///
 225 /// ```
 226 /// struct Wrapper<T>(Vec<T>);
 227 /// impl<T> From<Wrapper<T>> for Vec<T> {
 228 ///     fn from(w: Wrapper<T>) -> Vec<T> {
 229 ///         w.0
 230 ///     }
 231 /// }
 232 /// ```
 233 /// This will fail to compile in older versions of the language because Rust's orphaning rules
 234 /// used to be a little bit more strict. To bypass this, you could implement [`Into`] directly:
 235 ///
 236 /// ```
 237 /// struct Wrapper<T>(Vec<T>);
 238 /// impl<T> Into<Vec<T>> for Wrapper<T> {
 239 ///     fn into(self) -> Vec<T> {
 240 ///         self.0
 241 ///     }
 242 /// }
 243 /// ```
 244 ///
 245 /// It is important to understand that [`Into`] does not provide a [`From`] implementation
 246 /// (as [`From`] does with [`Into`]). Therefore, you should always try to implement [`From`]
 247 /// and then fall back to [`Into`] if [`From`] can't be implemented.
 248 ///
 249 /// # Examples
 250 ///
 251 /// [`String`] implements [`Into`]`<`[`Vec`]`<`[`u8`]`>>`:
 252 ///
 253 /// In order to express that we want a generic function to take all arguments that can be
 254 /// converted to a specified type `T`, we can use a trait bound of [`Into`]`<T>`.
 255 /// For example: The function `is_hello` takes all arguments that can be converted into a
 256 /// [`Vec`]`<`[`u8`]`>`.
 257 ///
 258 /// ```
 259 /// fn is_hello<T: Into<Vec<u8>>>(s: T) {
 260 ///    let bytes = b"hello".to_vec();
 261 ///    assert_eq!(bytes, s.into());
 262 /// }
 263 ///
 264 /// let s = "hello".to_string();
 265 /// is_hello(s);
 266 /// ```
 267 ///
 268 /// [`String`]: ../../std/string/struct.String.html
 269 /// [`Vec`]: ../../std/vec/struct.Vec.html
 270 #[rustc_diagnostic_item = "into_trait"]
 271 #[stable(feature = "rust1", since = "1.0.0")]
 272 pub trait Into<T>: Sized {
 273     /// Performs the conversion.
 274     #[stable(feature = "rust1", since = "1.0.0")]
 275     fn into(self) -> T;
 276 }
 277
 278 /// Used to do value-to-value conversions while consuming the input value. It is the reciprocal of
 279 /// [`Into`].
 280 ///
 281 /// One should always prefer implementing `From` over [`Into`]
 282 /// because implementing `From` automatically provides one with an implementation of [`Into`]
 283 /// thanks to the blanket implementation in the standard library.
 284 ///
 285 /// Only implement [`Into`] when targeting a version prior to Rust 1.41 and converting to a type
 286 /// outside the current crate.
 287 /// `From` was not able to do these types of conversions in earlier versions because of Rust's
 288 /// orphaning rules.
 289 /// See [`Into`] for more details.
 290 ///
 291 /// Prefer using [`Into`] over using `From` when specifying trait bounds on a generic function.
 292 /// This way, types that directly implement [`Into`] can be used as arguments as well.
 293 ///
 294 /// The `From` is also very useful when performing error handling. When constructing a function
 295 /// that is capable of failing, the return type will generally be of the form `Result<T, E>`.
 296 /// The `From` trait simplifies error handling by allowing a function to return a single error type
 297 /// that encapsulate multiple error types. See the "Examples" section and [the book][book] for more
 298 /// details.
 299 ///
 300 /// **Note: This trait must not fail**. If the conversion can fail, use [`TryFrom`].
 301 ///
 302 /// # Generic Implementations
 303 ///
 304 /// - `From<T> for U` implies [`Into`]`<U> for T`
 305 /// - `From` is reflexive, which means that `From<T> for T` is implemented
 306 ///
 307 /// # Examples
 308 ///
 309 /// [`String`] implements `From<&str>`:
 310 ///
 311 /// An explicit conversion from a `&str` to a String is done as follows:
 312 ///
 313 /// ```
 314 /// let string = "hello".to_string();
 315 /// let other_string = String::from("hello");
 316 ///
 317 /// assert_eq!(string, other_string);
 318 /// ```
 319 ///
 320 /// While performing error handling it is often useful to implement `From` for your own error type.
 321 /// By converting underlying error types to our own custom error type that encapsulates the
 322 /// underlying error type, we can return a single error type without losing information on the
 323 /// underlying cause. The '?' operator automatically converts the underlying error type to our
 324 /// custom error type by calling `Into<CliError>::into` which is automatically provided when
 325 /// implementing `From`. The compiler then infers which implementation of `Into` should be used.
 326 ///
 327 /// ```
 328 /// use std::fs;
 329 /// use std::io;
 330 /// use std::num;
 331 ///
 332 /// enum CliError {
 333 ///     IoError(io::Error),
 334 ///     ParseError(num::ParseIntError),
 335 /// }
 336 ///
 337 /// impl From<io::Error> for CliError {
 338 ///     fn from(error: io::Error) -> Self {
 339 ///         CliError::IoError(error)
 340 ///     }
 341 /// }
 342 ///
 343 /// impl From<num::ParseIntError> for CliError {
 344 ///     fn from(error: num::ParseIntError) -> Self {
 345 ///         CliError::ParseError(error)
 346 ///     }
 347 /// }
 348 ///
 349 /// fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {
 350 ///     let mut contents = fs::read_to_string(&file_name)?;
 351 ///     let num: i32 = contents.trim().parse()?;
 352 ///     Ok(num)
 353 /// }
 354 /// ```
 355 ///
 356 /// [`String`]: ../../std/string/struct.String.html
 357 /// [`from`]: From::from
 358 /// [book]: ../../book/ch09-00-error-handling.html
 359 #[rustc_diagnostic_item = "from_trait"]
 360 #[stable(feature = "rust1", since = "1.0.0")]
 361 #[rustc_on_unimplemented(on(
 362     all(_Self = "&str", T = "std::string::String"),
 363     note = "to coerce a `{T}` into a `{Self}`, use `&*` as a prefix",
 364 ))]
 365 pub trait From<T>: Sized {
 366     /// Performs the conversion.
 367     #[lang = "from"]
 368     #[stable(feature = "rust1", since = "1.0.0")]
 369     fn from(_: T) -> Self;
 370 }
 371
 372 /// An attempted conversion that consumes `self`, which may or may not be
 373 /// expensive.
 374 ///
 375 /// Library authors should usually not directly implement this trait,
 376 /// but should prefer implementing the [`TryFrom`] trait, which offers
 377 /// greater flexibility and provides an equivalent `TryInto`
 378 /// implementation for free, thanks to a blanket implementation in the
 379 /// standard library. For more information on this, see the
 380 /// documentation for [`Into`].
 381 ///
 382 /// # Implementing `TryInto`
 383 ///
 384 /// This suffers the same restrictions and reasoning as implementing
 385 /// [`Into`], see there for details.
 386 #[rustc_diagnostic_item = "try_into_trait"]
 387 #[stable(feature = "try_from", since = "1.34.0")]
 388 pub trait TryInto<T>: Sized {
 389     /// The type returned in the event of a conversion error.
 390     #[stable(feature = "try_from", since = "1.34.0")]
 391     type Error;
 392
 393     /// Performs the conversion.
 394     #[stable(feature = "try_from", since = "1.34.0")]
 395     fn try_into(self) -> Result<T, Self::Error>;
 396 }
 397
 398 /// Simple and safe type conversions that may fail in a controlled
 399 /// way under some circumstances. It is the reciprocal of [`TryInto`].
 400 ///
 401 /// This is useful when you are doing a type conversion that may
 402 /// trivially succeed but may also need special handling.
 403 /// For example, there is no way to convert an [`i64`] into an [`i32`]
 404 /// using the [`From`] trait, because an [`i64`] may contain a value
 405 /// that an [`i32`] cannot represent and so the conversion would lose data.
 406 /// This might be handled by truncating the [`i64`] to an [`i32`] (essentially
 407 /// giving the [`i64`]'s value modulo [`i32::MAX`]) or by simply returning
 408 /// [`i32::MAX`], or by some other method.  The [`From`] trait is intended
 409 /// for perfect conversions, so the `TryFrom` trait informs the
 410 /// programmer when a type conversion could go bad and lets them
 411 /// decide how to handle it.
 412 ///
 413 /// # Generic Implementations
 414 ///
 415 /// - `TryFrom<T> for U` implies [`TryInto`]`<U> for T`
 416 /// - [`try_from`] is reflexive, which means that `TryFrom<T> for T`
 417 /// is implemented and cannot fail -- the associated `Error` type for
 418 /// calling `T::try_from()` on a value of type `T` is [`Infallible`].
 419 /// When the [`!`] type is stabilized [`Infallible`] and [`!`] will be
 420 /// equivalent.
 421 ///
 422 /// `TryFrom<T>` can be implemented as follows:
 423 ///
 424 /// ```
 425 /// use std::convert::TryFrom;
 426 ///
 427 /// struct GreaterThanZero(i32);
 428 ///
 429 /// impl TryFrom<i32> for GreaterThanZero {
 430 ///     type Error = &'static str;
 431 ///
 432 ///     fn try_from(value: i32) -> Result<Self, Self::Error> {
 433 ///         if value <= 0 {
 434 ///             Err("GreaterThanZero only accepts value superior than zero!")
 435 ///         } else {
 436 ///             Ok(GreaterThanZero(value))
 437 ///         }
 438 ///     }
 439 /// }
 440 /// ```
 441 ///
 442 /// # Examples
 443 ///
 444 /// As described, [`i32`] implements `TryFrom<`[`i64`]`>`:
 445 ///
 446 /// ```
 447 /// use std::convert::TryFrom;
 448 ///
 449 /// let big_number = 1_000_000_000_000i64;
 450 /// // Silently truncates `big_number`, requires detecting
 451 /// // and handling the truncation after the fact.
 452 /// let smaller_number = big_number as i32;
 453 /// assert_eq!(smaller_number, -727379968);
 454 ///
 455 /// // Returns an error because `big_number` is too big to
 456 /// // fit in an `i32`.
 457 /// let try_smaller_number = i32::try_from(big_number);
 458 /// assert!(try_smaller_number.is_err());
 459 ///
 460 /// // Returns `Ok(3)`.
 461 /// let try_successful_smaller_number = i32::try_from(3);
 462 /// assert!(try_successful_smaller_number.is_ok());
 463 /// ```
 464 ///
 465 /// [`try_from`]: TryFrom::try_from
 466 #[rustc_diagnostic_item = "try_from_trait"]
 467 #[stable(feature = "try_from", since = "1.34.0")]
 468 pub trait TryFrom<T>: Sized {
 469     /// The type returned in the event of a conversion error.
 470     #[stable(feature = "try_from", since = "1.34.0")]
 471     type Error;
 472
 473     /// Performs the conversion.
 474     #[stable(feature = "try_from", since = "1.34.0")]
 475     fn try_from(value: T) -> Result<Self, Self::Error>;
 476 }
 477
 478 ////////////////////////////////////////////////////////////////////////////////
 479 // GENERIC IMPLS
 480 ////////////////////////////////////////////////////////////////////////////////
 481
 482 // As lifts over &
 483 #[stable(feature = "rust1", since = "1.0.0")]
 484 impl<T: ?Sized, U: ?Sized> AsRef<U> for &T
 485 where
 486     T: AsRef<U>,
 487 {
 488     fn as_ref(&self) -> &U {
 489         <T as AsRef<U>>::as_ref(*self)
 490     }
 491 }
 492
 493 // As lifts over &mut
 494 #[stable(feature = "rust1", since = "1.0.0")]
 495 impl<T: ?Sized, U: ?Sized> AsRef<U> for &mut T
 496 where
 497     T: AsRef<U>,
 498 {
 499     fn as_ref(&self) -> &U {
 500         <T as AsRef<U>>::as_ref(*self)
 501     }
 502 }
 503
 504 // FIXME (#45742): replace the above impls for &/&mut with the following more general one:
 505 // // As lifts over Deref
 506 // impl<D: ?Sized + Deref<Target: AsRef<U>>, U: ?Sized> AsRef<U> for D {
 507 //     fn as_ref(&self) -> &U {
 508 //         self.deref().as_ref()
 509 //     }
 510 // }
 511
 512 // AsMut lifts over &mut
 513 #[stable(feature = "rust1", since = "1.0.0")]
 514 impl<T: ?Sized, U: ?Sized> AsMut<U> for &mut T
 515 where
 516     T: AsMut<U>,
 517 {
 518     fn as_mut(&mut self) -> &mut U {
 519         (*self).as_mut()
 520     }
 521 }
 522
 523 // FIXME (#45742): replace the above impl for &mut with the following more general one:
 524 // // AsMut lifts over DerefMut
 525 // impl<D: ?Sized + Deref<Target: AsMut<U>>, U: ?Sized> AsMut<U> for D {
 526 //     fn as_mut(&mut self) -> &mut U {
 527 //         self.deref_mut().as_mut()
 528 //     }
 529 // }
 530
 531 // From implies Into
 532 #[stable(feature = "rust1", since = "1.0.0")]
 533 impl<T, U> Into<U> for T
 534 where
 535     U: From<T>,
 536 {
 537     fn into(self) -> U {
 538         U::from(self)
 539     }
 540 }
 541
 542 // From (and thus Into) is reflexive
 543 #[stable(feature = "rust1", since = "1.0.0")]
 544 impl<T> From<T> for T {
 545     fn from(t: T) -> T {
 546         t
 547     }
 548 }
 549
 550 /// **Stability note:** This impl does not yet exist, but we are
 551 /// "reserving space" to add it in the future. See
 552 /// [rust-lang/rust#64715][#64715] for details.
 553 ///
 554 /// [#64715]: https://github.com/rust-lang/rust/issues/64715
 555 #[stable(feature = "convert_infallible", since = "1.34.0")]
 556 #[allow(unused_attributes)] // FIXME(#58633): do a principled fix instead.
 557 #[rustc_reservation_impl = "permitting this impl would forbid us from adding \
 558                             `impl<T> From<!> for T` later; see rust-lang/rust#64715 for details"]
 559 impl<T> From<!> for T {
 560     fn from(t: !) -> T {
 561         t
 562     }
 563 }
 564
 565 // TryFrom implies TryInto
 566 #[stable(feature = "try_from", since = "1.34.0")]
 567 impl<T, U> TryInto<U> for T
 568 where
 569     U: TryFrom<T>,
 570 {
 571     type Error = U::Error;
 572
 573     fn try_into(self) -> Result<U, U::Error> {
 574         U::try_from(self)
 575     }
 576 }
 577
 578 // Infallible conversions are semantically equivalent to fallible conversions
 579 // with an uninhabited error type.
 580 #[stable(feature = "try_from", since = "1.34.0")]
 581 impl<T, U> TryFrom<U> for T
 582 where
 583     U: Into<T>,
 584 {
 585     type Error = Infallible;
 586
 587     fn try_from(value: U) -> Result<Self, Self::Error> {
 588         Ok(U::into(value))
 589     }
 590 }
 591
 592 ////////////////////////////////////////////////////////////////////////////////
 593 // CONCRETE IMPLS
 594 ////////////////////////////////////////////////////////////////////////////////
 595
 596 #[stable(feature = "rust1", since = "1.0.0")]
 597 impl<T> AsRef<[T]> for [T] {
 598     fn as_ref(&self) -> &[T] {
 599         self
 600     }
 601 }
 602
 603 #[stable(feature = "rust1", since = "1.0.0")]
 604 impl<T> AsMut<[T]> for [T] {
 605     fn as_mut(&mut self) -> &mut [T] {
 606         self
 607     }
 608 }
 609
 610 #[stable(feature = "rust1", since = "1.0.0")]
 611 impl AsRef<str> for str {
 612     #[inline]
 613     fn as_ref(&self) -> &str {
 614         self
 615     }
 616 }
 617
 618 #[stable(feature = "as_mut_str_for_str", since = "1.51.0")]
 619 impl AsMut<str> for str {
 620     #[inline]
 621     fn as_mut(&mut self) -> &mut str {
 622         self
 623     }
 624 }
 625
 626 ////////////////////////////////////////////////////////////////////////////////
 627 // THE NO-ERROR ERROR TYPE
 628 ////////////////////////////////////////////////////////////////////////////////
 629
 630 /// The error type for errors that can never happen.
 631 ///
 632 /// Since this enum has no variant, a value of this type can never actually exist.
 633 /// This can be useful for generic APIs that use [`Result`] and parameterize the error type,
 634 /// to indicate that the result is always [`Ok`].
 635 ///
 636 /// For example, the [`TryFrom`] trait (conversion that returns a [`Result`])
 637 /// has a blanket implementation for all types where a reverse [`Into`] implementation exists.
 638 ///
 639 /// ```ignore (illustrates std code, duplicating the impl in a doctest would be an error)
 640 /// impl<T, U> TryFrom<U> for T where U: Into<T> {
 641 ///     type Error = Infallible;
 642 ///
 643 ///     fn try_from(value: U) -> Result<Self, Infallible> {
 644 ///         Ok(U::into(value))  // Never returns `Err`
 645 ///     }
 646 /// }
 647 /// ```
 648 ///
 649 /// # Future compatibility
 650 ///
 651 /// This enum has the same role as [the `!` “never” type][never],
 652 /// which is unstable in this version of Rust.
 653 /// When `!` is stabilized, we plan to make `Infallible` a type alias to it:
 654 ///
 655 /// ```ignore (illustrates future std change)
 656 /// pub type Infallible = !;
 657 /// ```
 658 ///
 659 /// … and eventually deprecate `Infallible`.
 660 ///
 661 /// However there is one case where `!` syntax can be used
 662 /// before `!` is stabilized as a full-fledged type: in the position of a function’s return type.
 663 /// Specifically, it is possible implementations for two different function pointer types:
 664 ///
 665 /// ```
 666 /// trait MyTrait {}
 667 /// impl MyTrait for fn() -> ! {}
 668 /// impl MyTrait for fn() -> std::convert::Infallible {}
 669 /// ```
 670 ///
 671 /// With `Infallible` being an enum, this code is valid.
 672 /// However when `Infallible` becomes an alias for the never type,
 673 /// the two `impl`s will start to overlap
 674 /// and therefore will be disallowed by the language’s trait coherence rules.
 675 #[stable(feature = "convert_infallible", since = "1.34.0")]
 676 #[derive(Copy)]
 677 pub enum Infallible {}
 678
 679 #[stable(feature = "convert_infallible", since = "1.34.0")]
 680 impl Clone for Infallible {
 681     fn clone(&self) -> Infallible {
 682         match *self {}
 683     }
 684 }
 685
 686 #[stable(feature = "convert_infallible", since = "1.34.0")]
 687 impl fmt::Debug for Infallible {
 688     fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
 689         match *self {}
 690     }
 691 }
 692
 693 #[stable(feature = "convert_infallible", since = "1.34.0")]
 694 impl fmt::Display for Infallible {
 695     fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
 696         match *self {}
 697     }
 698 }
 699
 700 #[stable(feature = "convert_infallible", since = "1.34.0")]
 701 impl PartialEq for Infallible {
 702     fn eq(&self, _: &Infallible) -> bool {
 703         match *self {}
 704     }
 705 }
 706
 707 #[stable(feature = "convert_infallible", since = "1.34.0")]
 708 impl Eq for Infallible {}
 709
 710 #[stable(feature = "convert_infallible", since = "1.34.0")]
 711 impl PartialOrd for Infallible {
 712     fn partial_cmp(&self, _other: &Self) -> Option<crate::cmp::Ordering> {
 713         match *self {}
 714     }
 715 }
 716
 717 #[stable(feature = "convert_infallible", since = "1.34.0")]
 718 impl Ord for Infallible {
 719     fn cmp(&self, _other: &Self) -> crate::cmp::Ordering {
 720         match *self {}
 721     }
 722 }
 723
 724 #[stable(feature = "convert_infallible", since = "1.34.0")]
 725 impl From<!> for Infallible {
 726     fn from(x: !) -> Self {
 727         x
 728     }
 729 }
 730
 731 #[stable(feature = "convert_infallible_hash", since = "1.44.0")]
 732 impl Hash for Infallible {
 733     fn hash<H: Hasher>(&self, _: &mut H) {
 734         match *self {}
 735     }
 736 }