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 #[cfg_attr(not(test), rustc_diagnostic_item = "AsRef")]
 156 pub trait AsRef<T: ?Sized> {
 157     /// Performs the conversion.
 158     #[stable(feature = "rust1", since = "1.0.0")]
 159     fn as_ref(&self) -> &T;
 160 }
 161
 162 /// Used to do a cheap mutable-to-mutable reference conversion.
 163 ///
 164 /// This trait is similar to [`AsRef`] but used for converting between mutable
 165 /// references. If you need to do a costly conversion it is better to
 166 /// implement [`From`] with type `&mut T` or write a custom function.
 167 ///
 168 /// **Note: This trait must not fail**. If the conversion can fail, use a
 169 /// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
 170 ///
 171 /// # Generic Implementations
 172 ///
 173 /// - `AsMut` auto-dereferences if the inner type is a mutable reference
 174 ///   (e.g.: `foo.as_mut()` will work the same if `foo` has type `&mut Foo`
 175 ///   or `&mut &mut Foo`)
 176 ///
 177 /// # Examples
 178 ///
 179 /// Using `AsMut` as trait bound for a generic function we can accept all mutable references
 180 /// that can be converted to type `&mut T`. Because [`Box<T>`] implements `AsMut<T>` we can
 181 /// write a function `add_one` that takes all arguments that can be converted to `&mut u64`.
 182 /// Because [`Box<T>`] implements `AsMut<T>`, `add_one` accepts arguments of type
 183 /// `&mut Box<u64>` as well:
 184 ///
 185 /// ```
 186 /// fn add_one<T: AsMut<u64>>(num: &mut T) {
 187 ///     *num.as_mut() += 1;
 188 /// }
 189 ///
 190 /// let mut boxed_num = Box::new(0);
 191 /// add_one(&mut boxed_num);
 192 /// assert_eq!(*boxed_num, 1);
 193 /// ```
 194 ///
 195 /// [`Box<T>`]: ../../std/boxed/struct.Box.html
 196 #[stable(feature = "rust1", since = "1.0.0")]
 197 #[cfg_attr(not(test), rustc_diagnostic_item = "AsMut")]
 198 pub trait AsMut<T: ?Sized> {
 199     /// Performs the conversion.
 200     #[stable(feature = "rust1", since = "1.0.0")]
 201     fn as_mut(&mut self) -> &mut T;
 202 }
 203
 204 /// A value-to-value conversion that consumes the input value. The
 205 /// opposite of [`From`].
 206 ///
 207 /// One should avoid implementing [`Into`] and implement [`From`] instead.
 208 /// Implementing [`From`] automatically provides one with an implementation of [`Into`]
 209 /// thanks to the blanket implementation in the standard library.
 210 ///
 211 /// Prefer using [`Into`] over [`From`] when specifying trait bounds on a generic function
 212 /// to ensure that types that only implement [`Into`] can be used as well.
 213 ///
 214 /// **Note: This trait must not fail**. If the conversion can fail, use [`TryInto`].
 215 ///
 216 /// # Generic Implementations
 217 ///
 218 /// - [`From`]`<T> for U` implies `Into<U> for T`
 219 /// - [`Into`] is reflexive, which means that `Into<T> for T` is implemented
 220 ///
 221 /// # Implementing [`Into`] for conversions to external types in old versions of Rust
 222 ///
 223 /// Prior to Rust 1.41, if the destination type was not part of the current crate
 224 /// then you couldn't implement [`From`] directly.
 225 /// For example, take this code:
 226 ///
 227 /// ```
 228 /// struct Wrapper<T>(Vec<T>);
 229 /// impl<T> From<Wrapper<T>> for Vec<T> {
 230 ///     fn from(w: Wrapper<T>) -> Vec<T> {
 231 ///         w.0
 232 ///     }
 233 /// }
 234 /// ```
 235 /// This will fail to compile in older versions of the language because Rust's orphaning rules
 236 /// used to be a little bit more strict. To bypass this, you could implement [`Into`] directly:
 237 ///
 238 /// ```
 239 /// struct Wrapper<T>(Vec<T>);
 240 /// impl<T> Into<Vec<T>> for Wrapper<T> {
 241 ///     fn into(self) -> Vec<T> {
 242 ///         self.0
 243 ///     }
 244 /// }
 245 /// ```
 246 ///
 247 /// It is important to understand that [`Into`] does not provide a [`From`] implementation
 248 /// (as [`From`] does with [`Into`]). Therefore, you should always try to implement [`From`]
 249 /// and then fall back to [`Into`] if [`From`] can't be implemented.
 250 ///
 251 /// # Examples
 252 ///
 253 /// [`String`] implements [`Into`]`<`[`Vec`]`<`[`u8`]`>>`:
 254 ///
 255 /// In order to express that we want a generic function to take all arguments that can be
 256 /// converted to a specified type `T`, we can use a trait bound of [`Into`]`<T>`.
 257 /// For example: The function `is_hello` takes all arguments that can be converted into a
 258 /// [`Vec`]`<`[`u8`]`>`.
 259 ///
 260 /// ```
 261 /// fn is_hello<T: Into<Vec<u8>>>(s: T) {
 262 ///    let bytes = b"hello".to_vec();
 263 ///    assert_eq!(bytes, s.into());
 264 /// }
 265 ///
 266 /// let s = "hello".to_string();
 267 /// is_hello(s);
 268 /// ```
 269 ///
 270 /// [`String`]: ../../std/string/struct.String.html
 271 /// [`Vec`]: ../../std/vec/struct.Vec.html
 272 #[rustc_diagnostic_item = "into_trait"]
 273 #[stable(feature = "rust1", since = "1.0.0")]
 274 pub trait Into<T>: Sized {
 275     /// Performs the conversion.
 276     #[stable(feature = "rust1", since = "1.0.0")]
 277     fn into(self) -> T;
 278 }
 279
 280 /// Used to do value-to-value conversions while consuming the input value. It is the reciprocal of
 281 /// [`Into`].
 282 ///
 283 /// One should always prefer implementing `From` over [`Into`]
 284 /// because implementing `From` automatically provides one with an implementation of [`Into`]
 285 /// thanks to the blanket implementation in the standard library.
 286 ///
 287 /// Only implement [`Into`] when targeting a version prior to Rust 1.41 and converting to a type
 288 /// outside the current crate.
 289 /// `From` was not able to do these types of conversions in earlier versions because of Rust's
 290 /// orphaning rules.
 291 /// See [`Into`] for more details.
 292 ///
 293 /// Prefer using [`Into`] over using `From` when specifying trait bounds on a generic function.
 294 /// This way, types that directly implement [`Into`] can be used as arguments as well.
 295 ///
 296 /// The `From` is also very useful when performing error handling. When constructing a function
 297 /// that is capable of failing, the return type will generally be of the form `Result<T, E>`.
 298 /// The `From` trait simplifies error handling by allowing a function to return a single error type
 299 /// that encapsulate multiple error types. See the "Examples" section and [the book][book] for more
 300 /// details.
 301 ///
 302 /// **Note: This trait must not fail**. If the conversion can fail, use [`TryFrom`].
 303 ///
 304 /// # Generic Implementations
 305 ///
 306 /// - `From<T> for U` implies [`Into`]`<U> for T`
 307 /// - `From` is reflexive, which means that `From<T> for T` is implemented
 308 ///
 309 /// # Examples
 310 ///
 311 /// [`String`] implements `From<&str>`:
 312 ///
 313 /// An explicit conversion from a `&str` to a String is done as follows:
 314 ///
 315 /// ```
 316 /// let string = "hello".to_string();
 317 /// let other_string = String::from("hello");
 318 ///
 319 /// assert_eq!(string, other_string);
 320 /// ```
 321 ///
 322 /// While performing error handling it is often useful to implement `From` for your own error type.
 323 /// By converting underlying error types to our own custom error type that encapsulates the
 324 /// underlying error type, we can return a single error type without losing information on the
 325 /// underlying cause. The '?' operator automatically converts the underlying error type to our
 326 /// custom error type by calling `Into<CliError>::into` which is automatically provided when
 327 /// implementing `From`. The compiler then infers which implementation of `Into` should be used.
 328 ///
 329 /// ```
 330 /// use std::fs;
 331 /// use std::io;
 332 /// use std::num;
 333 ///
 334 /// enum CliError {
 335 ///     IoError(io::Error),
 336 ///     ParseError(num::ParseIntError),
 337 /// }
 338 ///
 339 /// impl From<io::Error> for CliError {
 340 ///     fn from(error: io::Error) -> Self {
 341 ///         CliError::IoError(error)
 342 ///     }
 343 /// }
 344 ///
 345 /// impl From<num::ParseIntError> for CliError {
 346 ///     fn from(error: num::ParseIntError) -> Self {
 347 ///         CliError::ParseError(error)
 348 ///     }
 349 /// }
 350 ///
 351 /// fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {
 352 ///     let mut contents = fs::read_to_string(&file_name)?;
 353 ///     let num: i32 = contents.trim().parse()?;
 354 ///     Ok(num)
 355 /// }
 356 /// ```
 357 ///
 358 /// [`String`]: ../../std/string/struct.String.html
 359 /// [`from`]: From::from
 360 /// [book]: ../../book/ch09-00-error-handling.html
 361 #[rustc_diagnostic_item = "from_trait"]
 362 #[stable(feature = "rust1", since = "1.0.0")]
 363 #[rustc_on_unimplemented(on(
 364     all(_Self = "&str", T = "std::string::String"),
 365     note = "to coerce a `{T}` into a `{Self}`, use `&*` as a prefix",
 366 ))]
 367 pub trait From<T>: Sized {
 368     /// Performs the conversion.
 369     #[lang = "from"]
 370     #[stable(feature = "rust1", since = "1.0.0")]
 371     fn from(_: T) -> Self;
 372 }
 373
 374 /// An attempted conversion that consumes `self`, which may or may not be
 375 /// expensive.
 376 ///
 377 /// Library authors should usually not directly implement this trait,
 378 /// but should prefer implementing the [`TryFrom`] trait, which offers
 379 /// greater flexibility and provides an equivalent `TryInto`
 380 /// implementation for free, thanks to a blanket implementation in the
 381 /// standard library. For more information on this, see the
 382 /// documentation for [`Into`].
 383 ///
 384 /// # Implementing `TryInto`
 385 ///
 386 /// This suffers the same restrictions and reasoning as implementing
 387 /// [`Into`], see there for details.
 388 #[rustc_diagnostic_item = "try_into_trait"]
 389 #[stable(feature = "try_from", since = "1.34.0")]
 390 pub trait TryInto<T>: Sized {
 391     /// The type returned in the event of a conversion error.
 392     #[stable(feature = "try_from", since = "1.34.0")]
 393     type Error;
 394
 395     /// Performs the conversion.
 396     #[stable(feature = "try_from", since = "1.34.0")]
 397     fn try_into(self) -> Result<T, Self::Error>;
 398 }
 399
 400 /// Simple and safe type conversions that may fail in a controlled
 401 /// way under some circumstances. It is the reciprocal of [`TryInto`].
 402 ///
 403 /// This is useful when you are doing a type conversion that may
 404 /// trivially succeed but may also need special handling.
 405 /// For example, there is no way to convert an [`i64`] into an [`i32`]
 406 /// using the [`From`] trait, because an [`i64`] may contain a value
 407 /// that an [`i32`] cannot represent and so the conversion would lose data.
 408 /// This might be handled by truncating the [`i64`] to an [`i32`] (essentially
 409 /// giving the [`i64`]'s value modulo [`i32::MAX`]) or by simply returning
 410 /// [`i32::MAX`], or by some other method.  The [`From`] trait is intended
 411 /// for perfect conversions, so the `TryFrom` trait informs the
 412 /// programmer when a type conversion could go bad and lets them
 413 /// decide how to handle it.
 414 ///
 415 /// # Generic Implementations
 416 ///
 417 /// - `TryFrom<T> for U` implies [`TryInto`]`<U> for T`
 418 /// - [`try_from`] is reflexive, which means that `TryFrom<T> for T`
 419 /// is implemented and cannot fail -- the associated `Error` type for
 420 /// calling `T::try_from()` on a value of type `T` is [`Infallible`].
 421 /// When the [`!`] type is stabilized [`Infallible`] and [`!`] will be
 422 /// equivalent.
 423 ///
 424 /// `TryFrom<T>` can be implemented as follows:
 425 ///
 426 /// ```
 427 /// use std::convert::TryFrom;
 428 ///
 429 /// struct GreaterThanZero(i32);
 430 ///
 431 /// impl TryFrom<i32> for GreaterThanZero {
 432 ///     type Error = &'static str;
 433 ///
 434 ///     fn try_from(value: i32) -> Result<Self, Self::Error> {
 435 ///         if value <= 0 {
 436 ///             Err("GreaterThanZero only accepts value superior than zero!")
 437 ///         } else {
 438 ///             Ok(GreaterThanZero(value))
 439 ///         }
 440 ///     }
 441 /// }
 442 /// ```
 443 ///
 444 /// # Examples
 445 ///
 446 /// As described, [`i32`] implements `TryFrom<`[`i64`]`>`:
 447 ///
 448 /// ```
 449 /// use std::convert::TryFrom;
 450 ///
 451 /// let big_number = 1_000_000_000_000i64;
 452 /// // Silently truncates `big_number`, requires detecting
 453 /// // and handling the truncation after the fact.
 454 /// let smaller_number = big_number as i32;
 455 /// assert_eq!(smaller_number, -727379968);
 456 ///
 457 /// // Returns an error because `big_number` is too big to
 458 /// // fit in an `i32`.
 459 /// let try_smaller_number = i32::try_from(big_number);
 460 /// assert!(try_smaller_number.is_err());
 461 ///
 462 /// // Returns `Ok(3)`.
 463 /// let try_successful_smaller_number = i32::try_from(3);
 464 /// assert!(try_successful_smaller_number.is_ok());
 465 /// ```
 466 ///
 467 /// [`try_from`]: TryFrom::try_from
 468 #[rustc_diagnostic_item = "try_from_trait"]
 469 #[stable(feature = "try_from", since = "1.34.0")]
 470 pub trait TryFrom<T>: Sized {
 471     /// The type returned in the event of a conversion error.
 472     #[stable(feature = "try_from", since = "1.34.0")]
 473     type Error;
 474
 475     /// Performs the conversion.
 476     #[stable(feature = "try_from", since = "1.34.0")]
 477     fn try_from(value: T) -> Result<Self, Self::Error>;
 478 }
 479
 480 ////////////////////////////////////////////////////////////////////////////////
 481 // GENERIC IMPLS
 482 ////////////////////////////////////////////////////////////////////////////////
 483
 484 // As lifts over &
 485 #[stable(feature = "rust1", since = "1.0.0")]
 486 impl<T: ?Sized, U: ?Sized> AsRef<U> for &T
 487 where
 488     T: AsRef<U>,
 489 {
 490     fn as_ref(&self) -> &U {
 491         <T as AsRef<U>>::as_ref(*self)
 492     }
 493 }
 494
 495 // As lifts over &mut
 496 #[stable(feature = "rust1", since = "1.0.0")]
 497 impl<T: ?Sized, U: ?Sized> AsRef<U> for &mut T
 498 where
 499     T: AsRef<U>,
 500 {
 501     fn as_ref(&self) -> &U {
 502         <T as AsRef<U>>::as_ref(*self)
 503     }
 504 }
 505
 506 // FIXME (#45742): replace the above impls for &/&mut with the following more general one:
 507 // // As lifts over Deref
 508 // impl<D: ?Sized + Deref<Target: AsRef<U>>, U: ?Sized> AsRef<U> for D {
 509 //     fn as_ref(&self) -> &U {
 510 //         self.deref().as_ref()
 511 //     }
 512 // }
 513
 514 // AsMut lifts over &mut
 515 #[stable(feature = "rust1", since = "1.0.0")]
 516 impl<T: ?Sized, U: ?Sized> AsMut<U> for &mut T
 517 where
 518     T: AsMut<U>,
 519 {
 520     fn as_mut(&mut self) -> &mut U {
 521         (*self).as_mut()
 522     }
 523 }
 524
 525 // FIXME (#45742): replace the above impl for &mut with the following more general one:
 526 // // AsMut lifts over DerefMut
 527 // impl<D: ?Sized + Deref<Target: AsMut<U>>, U: ?Sized> AsMut<U> for D {
 528 //     fn as_mut(&mut self) -> &mut U {
 529 //         self.deref_mut().as_mut()
 530 //     }
 531 // }
 532
 533 // From implies Into
 534 #[stable(feature = "rust1", since = "1.0.0")]
 535 impl<T, U> Into<U> for T
 536 where
 537     U: From<T>,
 538 {
 539     fn into(self) -> U {
 540         U::from(self)
 541     }
 542 }
 543
 544 // From (and thus Into) is reflexive
 545 #[stable(feature = "rust1", since = "1.0.0")]
 546 impl<T> From<T> for T {
 547     fn from(t: T) -> T {
 548         t
 549     }
 550 }
 551
 552 /// **Stability note:** This impl does not yet exist, but we are
 553 /// "reserving space" to add it in the future. See
 554 /// [rust-lang/rust#64715][#64715] for details.
 555 ///
 556 /// [#64715]: https://github.com/rust-lang/rust/issues/64715
 557 #[stable(feature = "convert_infallible", since = "1.34.0")]
 558 #[allow(unused_attributes)] // FIXME(#58633): do a principled fix instead.
 559 #[rustc_reservation_impl = "permitting this impl would forbid us from adding \
 560                             `impl<T> From<!> for T` later; see rust-lang/rust#64715 for details"]
 561 impl<T> From<!> for T {
 562     fn from(t: !) -> T {
 563         t
 564     }
 565 }
 566
 567 // TryFrom implies TryInto
 568 #[stable(feature = "try_from", since = "1.34.0")]
 569 impl<T, U> TryInto<U> for T
 570 where
 571     U: TryFrom<T>,
 572 {
 573     type Error = U::Error;
 574
 575     fn try_into(self) -> Result<U, U::Error> {
 576         U::try_from(self)
 577     }
 578 }
 579
 580 // Infallible conversions are semantically equivalent to fallible conversions
 581 // with an uninhabited error type.
 582 #[stable(feature = "try_from", since = "1.34.0")]
 583 impl<T, U> TryFrom<U> for T
 584 where
 585     U: Into<T>,
 586 {
 587     type Error = Infallible;
 588
 589     fn try_from(value: U) -> Result<Self, Self::Error> {
 590         Ok(U::into(value))
 591     }
 592 }
 593
 594 ////////////////////////////////////////////////////////////////////////////////
 595 // CONCRETE IMPLS
 596 ////////////////////////////////////////////////////////////////////////////////
 597
 598 #[stable(feature = "rust1", since = "1.0.0")]
 599 impl<T> AsRef<[T]> for [T] {
 600     fn as_ref(&self) -> &[T] {
 601         self
 602     }
 603 }
 604
 605 #[stable(feature = "rust1", since = "1.0.0")]
 606 impl<T> AsMut<[T]> for [T] {
 607     fn as_mut(&mut self) -> &mut [T] {
 608         self
 609     }
 610 }
 611
 612 #[stable(feature = "rust1", since = "1.0.0")]
 613 impl AsRef<str> for str {
 614     #[inline]
 615     fn as_ref(&self) -> &str {
 616         self
 617     }
 618 }
 619
 620 #[stable(feature = "as_mut_str_for_str", since = "1.51.0")]
 621 impl AsMut<str> for str {
 622     #[inline]
 623     fn as_mut(&mut self) -> &mut str {
 624         self
 625     }
 626 }
 627
 628 ////////////////////////////////////////////////////////////////////////////////
 629 // THE NO-ERROR ERROR TYPE
 630 ////////////////////////////////////////////////////////////////////////////////
 631
 632 /// The error type for errors that can never happen.
 633 ///
 634 /// Since this enum has no variant, a value of this type can never actually exist.
 635 /// This can be useful for generic APIs that use [`Result`] and parameterize the error type,
 636 /// to indicate that the result is always [`Ok`].
 637 ///
 638 /// For example, the [`TryFrom`] trait (conversion that returns a [`Result`])
 639 /// has a blanket implementation for all types where a reverse [`Into`] implementation exists.
 640 ///
 641 /// ```ignore (illustrates std code, duplicating the impl in a doctest would be an error)
 642 /// impl<T, U> TryFrom<U> for T where U: Into<T> {
 643 ///     type Error = Infallible;
 644 ///
 645 ///     fn try_from(value: U) -> Result<Self, Infallible> {
 646 ///         Ok(U::into(value))  // Never returns `Err`
 647 ///     }
 648 /// }
 649 /// ```
 650 ///
 651 /// # Future compatibility
 652 ///
 653 /// This enum has the same role as [the `!` “never” type][never],
 654 /// which is unstable in this version of Rust.
 655 /// When `!` is stabilized, we plan to make `Infallible` a type alias to it:
 656 ///
 657 /// ```ignore (illustrates future std change)
 658 /// pub type Infallible = !;
 659 /// ```
 660 ///
 661 /// … and eventually deprecate `Infallible`.
 662 ///
 663 /// However there is one case where `!` syntax can be used
 664 /// before `!` is stabilized as a full-fledged type: in the position of a function’s return type.
 665 /// Specifically, it is possible implementations for two different function pointer types:
 666 ///
 667 /// ```
 668 /// trait MyTrait {}
 669 /// impl MyTrait for fn() -> ! {}
 670 /// impl MyTrait for fn() -> std::convert::Infallible {}
 671 /// ```
 672 ///
 673 /// With `Infallible` being an enum, this code is valid.
 674 /// However when `Infallible` becomes an alias for the never type,
 675 /// the two `impl`s will start to overlap
 676 /// and therefore will be disallowed by the language’s trait coherence rules.
 677 #[stable(feature = "convert_infallible", since = "1.34.0")]
 678 #[derive(Copy)]
 679 pub enum Infallible {}
 680
 681 #[stable(feature = "convert_infallible", since = "1.34.0")]
 682 impl Clone for Infallible {
 683     fn clone(&self) -> Infallible {
 684         match *self {}
 685     }
 686 }
 687
 688 #[stable(feature = "convert_infallible", since = "1.34.0")]
 689 impl fmt::Debug for Infallible {
 690     fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
 691         match *self {}
 692     }
 693 }
 694
 695 #[stable(feature = "convert_infallible", since = "1.34.0")]
 696 impl fmt::Display for Infallible {
 697     fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
 698         match *self {}
 699     }
 700 }
 701
 702 #[stable(feature = "convert_infallible", since = "1.34.0")]
 703 impl PartialEq for Infallible {
 704     fn eq(&self, _: &Infallible) -> bool {
 705         match *self {}
 706     }
 707 }
 708
 709 #[stable(feature = "convert_infallible", since = "1.34.0")]
 710 impl Eq for Infallible {}
 711
 712 #[stable(feature = "convert_infallible", since = "1.34.0")]
 713 impl PartialOrd for Infallible {
 714     fn partial_cmp(&self, _other: &Self) -> Option<crate::cmp::Ordering> {
 715         match *self {}
 716     }
 717 }
 718
 719 #[stable(feature = "convert_infallible", since = "1.34.0")]
 720 impl Ord for Infallible {
 721     fn cmp(&self, _other: &Self) -> crate::cmp::Ordering {
 722         match *self {}
 723     }
 724 }
 725
 726 #[stable(feature = "convert_infallible", since = "1.34.0")]
 727 impl From<!> for Infallible {
 728     fn from(x: !) -> Self {
 729         x
 730     }
 731 }
 732
 733 #[stable(feature = "convert_infallible_hash", since = "1.44.0")]
 734 impl Hash for Infallible {
 735     fn hash<H: Hasher>(&self, _: &mut H) {
 736         match *self {}
 737     }
 738 }