      1
      2
      3
      4
      5
      6
      7
      8
      9
     10
     11
     12
     13
     14
     15
     16
     17
     18
     19
     20
     21
     22
     23
     24
     25
     26
     27
     28
     29
     30
     31
     32
     33
     34
     35
     36
     37
     38
     39
     40
     41
     42
     43
     44
     45
     46
     47
     48
     49
     50
     51
     52
     53
     54
     55
     56
     57
     58
     59
     60
     61
     62
     63
     64
     65
     66
     67
     68
     69
     70
     71
     72
     73
     74
     75
     76
     77
     78
     79
     80
     81
     82
     83
     84
     85
     86
     87
     88
     89
     90
     91
     92
     93
     94
     95
     96
     97
     98
     99
    100
    101
    102
    103
    104
    105
    106
    107
    108
    109
    110
    111
    112
    113
    114
    115
    116
    117
    118
    119
    120
    121
    122
    123
    124
    125
    126
    127
    128
    129
    130
    131
    132
    133
    134
    135
    136
    137
    138
    139
    140
    141
    142
    143
    144
    145
    146
    147
    148
    149
    150
    151
    152
    153
    154
    155
    156
    157
    158
    159
    160
    161
    162
    163
    164
    165
    166
    167
    168
    169
    170
    171
    172
    173
    174
    175
    176
    177
    178
    179
    180
    181
    182
    183
    184
    185
    186
    187
    188
    189
    190
    191
    192
    193
    194
    195
    196
    197
    198
    199
    200
    201
    202
    203
    204
    205
    206
    207
    208
    209
    210
    211
    212
    213
    214
    215
    216
    217
    218
    219
    220
    221
    222
    223
    224
    225
    226
    227
    228
    229
    230
    231
    232
    233
    234
    235
    236
    237
    238
    239
    240
    241
    242
    243
    244
    245
    246
    247
    248
    249
    250
    251
    252
    253
    254
    255
    256
    257
    258
    259
    260
    261
    262
    263
    264
    265
    266
    267
    268
    269
    270
    271
    272
    273
    274
    275
    276
    277
    278
    279
    280
    281
    282
    283
    284
    285
    286
    287
    288
    289
    290
    291
    292
    293
    294
    295
    296
    297
    298
    299
    300
    301
    302
    303
    304
    305
    306
    307
    308
    309
    310
    311
    312
    313
    314
    315
    316
    317
    318
    319
    320
    321
    322
    323
    324
    325
    326
    327
    328
    329
    330
    331
    332
    333
    334
    335
    336
    337
    338
    339
    340
    341
    342
    343
    344
    345
    346
    347
    348
    349
    350
    351
    352
    353
    354
    355
    356
    357
    358
    359
    360
    361
    362
    363
    364
    365
    366
    367
    368
    369
    370
    371
    372
    373
    374
    375
    376
    377
    378
    379
    380
    381
    382
    383
    384
    385
    386
    387
    388
    389
    390
    391
    392
    393
    394
    395
    396
    397
    398
    399
    400
    401
    402
    403
    404
    405
    406
    407
    408
    409
    410
    411
    412
    413
    414
    415
    416
    417
    418
    419
    420
    421
    422
    423
    424
    425
    426
    427
    428
    429
    430
    431
    432
    433
    434
    435
    436
    437
    438
    439
    440
    441
    442
    443
    444
    445
    446
    447
    448
    449
    450
    451
    452
    453
    454
    455
    456
    457
    458
    459
    460
    461
    462
    463
    464
    465
    466
    467
    468
    469
    470
    471
    472

    // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! Traits for conversions between types.
    //!
    //! The traits in this module provide a general way to talk about conversions
    //! from one type to another. They follow the standard Rust conventions of
    //! `as`/`into`/`from`.
    //!
    //! Like many traits, these are often used as bounds for generic functions, to
    //! support arguments of multiple types.
    //!
    //! - Implement the `As*` traits for reference-to-reference conversions
    //! - Implement the [`Into`] trait when you want to consume the value in the conversion
    //! - The [`From`] trait is the most flexible, useful for value _and_ reference conversions
    //! - The [`TryFrom`] and [`TryInto`] traits behave like [`From`] and [`Into`], but allow for the
    //!   conversion to fail
    //!
    //! As a library author, you should prefer implementing [`From<T>`][`From`] or
    //! [`TryFrom<T>`][`TryFrom`] rather than [`Into<U>`][`Into`] or [`TryInto<U>`][`TryInto`],
    //! as [`From`] and [`TryFrom`] provide greater flexibility and offer
    //! equivalent [`Into`] or [`TryInto`] implementations for free, thanks to a
    //! blanket implementation in the standard library.
    //!
    //! # Generic Implementations
    //!
    //! - [`AsRef`] and [`AsMut`] auto-dereference if the inner type is a reference
    //! - [`From`]`<U> for T` implies [`Into`]`<T> for U`
    //! - [`TryFrom`]`<U> for T` implies [`TryInto`]`<T> for U`
    //! - [`From`] and [`Into`] are reflexive, which means that all types can
    //!   `into` themselves and `from` themselves
    //!
    //! See each trait for usage examples.
    //!
    //! [`Into`]: trait.Into.html
    //! [`From`]: trait.From.html
    //! [`TryFrom`]: trait.TryFrom.html
    //! [`TryInto`]: trait.TryInto.html
    //! [`AsRef`]: trait.AsRef.html
    //! [`AsMut`]: trait.AsMut.html

    #![stable(feature = "rust1", since = "1.0.0")]

    use fmt;

    /// A type used as the error type for implementations of fallible conversion
    /// traits in cases where conversions cannot actually fail.
    ///
    /// Because `Infallible` has no variants, a value of this type can never exist.
    /// It is used only to satisfy trait signatures that expect an error type, and
    /// signals to both the compiler and the user that the error case is impossible.
    #[unstable(feature = "try_from", issue = "33417")]
    #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
    pub enum Infallible {}

    #[unstable(feature = "try_from", issue = "33417")]
    impl fmt::Display for Infallible {
        fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
            match *self {
            }
        }
    }
    /// A cheap reference-to-reference conversion. Used to convert a value to a
    /// reference value within generic code.
    ///
    /// `AsRef` is very similar to, but serves a slightly different purpose than,
    /// [`Borrow`].
    ///
    /// `AsRef` is to be used when wishing to convert to a reference of another
    /// type.
    /// `Borrow` is more related to the notion of taking the reference. It is
    /// useful when wishing to abstract over the type of reference
    /// (`&T`, `&mut T`) or allow both the referenced and owned type to be treated
    /// in the same manner.
    ///
    /// The key difference between the two traits is the intention:
    ///
    /// - Use `AsRef` when goal is to simply convert into a reference
    /// - Use `Borrow` when goal is related to writing code that is agnostic to the
    ///   type of borrow and if is reference or value
    ///
    /// See [the book][book] for a more detailed comparison.
    ///
    /// [book]: ../../book/first-edition/borrow-and-asref.html
    /// [`Borrow`]: ../../std/borrow/trait.Borrow.html
    ///
    /// **Note: this trait must not fail**. If the conversion can fail, use a
    /// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
    ///
    /// [`Option<T>`]: ../../std/option/enum.Option.html
    /// [`Result<T, E>`]: ../../std/result/enum.Result.html
    ///
    /// # Generic Implementations
    ///
    /// - `AsRef` auto-dereferences if the inner type is a reference or a mutable
    ///   reference (e.g.: `foo.as_ref()` will work the same if `foo` has type
    ///   `&mut Foo` or `&&mut Foo`)
    ///
    /// # Examples
    ///
    /// Both [`String`] and `&str` implement `AsRef<str>`:
    ///
    /// [`String`]: ../../std/string/struct.String.html
    ///
    /// ```
    /// fn is_hello<T: AsRef<str>>(s: T) {
    ///    assert_eq!("hello", s.as_ref());
    /// }
    ///
    /// let s = "hello";
    /// is_hello(s);
    ///
    /// let s = "hello".to_string();
    /// is_hello(s);
    /// ```
    ///
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait AsRef<T: ?Sized> {
        /// Performs the conversion.
        #[stable(feature = "rust1", since = "1.0.0")]
        fn as_ref(&self) -> &T;
    }

    /// A cheap, mutable reference-to-mutable reference conversion.
    ///
    /// This trait is similar to `AsRef` but used for converting between mutable
    /// references.
    ///
    /// **Note: this trait must not fail**. If the conversion can fail, use a
    /// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
    ///
    /// [`Option<T>`]: ../../std/option/enum.Option.html
    /// [`Result<T, E>`]: ../../std/result/enum.Result.html
    ///
    /// # Generic Implementations
    ///
    /// - `AsMut` auto-dereferences if the inner type is a mutable reference
    ///   (e.g.: `foo.as_mut()` will work the same if `foo` has type `&mut Foo`
    ///   or `&mut &mut Foo`)
    ///
    /// # Examples
    ///
    /// [`Box<T>`] implements `AsMut<T>`:
    ///
    /// [`Box<T>`]: ../../std/boxed/struct.Box.html
    ///
    /// ```
    /// fn add_one<T: AsMut<u64>>(num: &mut T) {
    ///     *num.as_mut() += 1;
    /// }
    ///
    /// let mut boxed_num = Box::new(0);
    /// add_one(&mut boxed_num);
    /// assert_eq!(*boxed_num, 1);
    /// ```
    ///
    ///
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait AsMut<T: ?Sized> {
        /// Performs the conversion.
        #[stable(feature = "rust1", since = "1.0.0")]
        fn as_mut(&mut self) -> &mut T;
    }

    /// A conversion that consumes `self`, which may or may not be expensive. The
    /// reciprocal of [`From`][From].
    ///
    /// **Note: this trait must not fail**. If the conversion can fail, use
    /// [`TryInto`] or a dedicated method which returns an [`Option<T>`] or a
    /// [`Result<T, E>`].
    ///
    /// Library authors should not directly implement this trait, but should prefer
    /// implementing the [`From`][From] trait, which offers greater flexibility and
    /// provides an equivalent `Into` implementation for free, thanks to a blanket
    /// implementation in the standard library.
    ///
    /// # Generic Implementations
    ///
    /// - [`From<T>`][From]` for U` implies `Into<U> for T`
    /// - [`into`] is reflexive, which means that `Into<T> for T` is implemented
    ///
    /// # Implementing `Into`
    ///
    /// There is one exception to implementing `Into`, and it's kind of esoteric.
    /// If the destination type is not part of the current crate, and it uses a
    /// generic variable, then you can't implement `From` directly.  For example,
    /// take this crate:
    ///
    /// ```compile_fail
    /// struct Wrapper<T>(Vec<T>);
    /// impl<T> From<Wrapper<T>> for Vec<T> {
    ///     fn from(w: Wrapper<T>) -> Vec<T> {
    ///         w.0
    ///     }
    /// }
    /// ```
    ///
    /// To fix this, you can implement `Into` directly:
    ///
    /// ```
    /// struct Wrapper<T>(Vec<T>);
    /// impl<T> Into<Vec<T>> for Wrapper<T> {
    ///     fn into(self) -> Vec<T> {
    ///         self.0
    ///     }
    /// }
    /// ```
    ///
    /// This won't always allow the conversion: for example, `try!` and `?`
    /// always use `From`. However, in most cases, people use `Into` to do the
    /// conversions, and this will allow that.
    ///
    /// In almost all cases, you should try to implement `From`, then fall back
    /// to `Into` if `From` can't be implemented.
    ///
    /// # Examples
    ///
    /// [`String`] implements `Into<Vec<u8>>`:
    ///
    /// ```
    /// fn is_hello<T: Into<Vec<u8>>>(s: T) {
    ///    let bytes = b"hello".to_vec();
    ///    assert_eq!(bytes, s.into());
    /// }
    ///
    /// let s = "hello".to_string();
    /// is_hello(s);
    /// ```
    ///
    /// [`TryInto`]: trait.TryInto.html
    /// [`Option<T>`]: ../../std/option/enum.Option.html
    /// [`Result<T, E>`]: ../../std/result/enum.Result.html
    /// [`String`]: ../../std/string/struct.String.html
    /// [From]: trait.From.html
    /// [`into`]: trait.Into.html#tymethod.into
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait Into<T>: Sized {
        /// Performs the conversion.
        #[stable(feature = "rust1", since = "1.0.0")]
        fn into(self) -> T;
    }

    /// Simple and safe type conversions in to `Self`. It is the reciprocal of
    /// `Into`.
    ///
    /// This trait is useful when performing error handling as described by
    /// [the book][book] and is closely related to the `?` operator.
    ///
    /// When constructing a function that is capable of failing the return type
    /// will generally be of the form `Result<T, E>`.
    ///
    /// The `From` trait allows for simplification of error handling by providing a
    /// means of returning a single error type that encapsulates numerous possible
    /// erroneous situations.
    ///
    /// This trait is not limited to error handling, rather the general case for
    /// this trait would be in any type conversions to have an explicit definition
    /// of how they are performed.
    ///
    /// **Note: this trait must not fail**. If the conversion can fail, use
    /// [`TryFrom`] or a dedicated method which returns an [`Option<T>`] or a
    /// [`Result<T, E>`].
    ///
    /// # Generic Implementations
    ///
    /// - `From<T> for U` implies [`Into<U>`]` for T`
    /// - [`from`] is reflexive, which means that `From<T> for T` is implemented
    ///
    /// # Examples
    ///
    /// [`String`] implements `From<&str>`:
    ///
    /// ```
    /// let string = "hello".to_string();
    /// let other_string = String::from("hello");
    ///
    /// assert_eq!(string, other_string);
    /// ```
    ///
    /// An example usage for error handling:
    ///
    /// ```
    /// use std::io::{self, Read};
    /// use std::num;
    ///
    /// enum CliError {
    ///     IoError(io::Error),
    ///     ParseError(num::ParseIntError),
    /// }
    ///
    /// impl From<io::Error> for CliError {
    ///     fn from(error: io::Error) -> Self {
    ///         CliError::IoError(error)
    ///     }
    /// }
    ///
    /// impl From<num::ParseIntError> for CliError {
    ///     fn from(error: num::ParseIntError) -> Self {
    ///         CliError::ParseError(error)
    ///     }
    /// }
    ///
    /// fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {
    ///     let mut file = std::fs::File::open("test")?;
    ///     let mut contents = String::new();
    ///     file.read_to_string(&mut contents)?;
    ///     let num: i32 = contents.trim().parse()?;
    ///     Ok(num)
    /// }
    /// ```
    ///
    /// [`TryFrom`]: trait.TryFrom.html
    /// [`Option<T>`]: ../../std/option/enum.Option.html
    /// [`Result<T, E>`]: ../../std/result/enum.Result.html
    /// [`String`]: ../../std/string/struct.String.html
    /// [`Into<U>`]: trait.Into.html
    /// [`from`]: trait.From.html#tymethod.from
    /// [book]: ../../book/first-edition/error-handling.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait From<T>: Sized {
        /// Performs the conversion.
        #[stable(feature = "rust1", since = "1.0.0")]
        fn from(_: T) -> Self;
    }

    /// An attempted conversion that consumes `self`, which may or may not be
    /// expensive.
    ///
    /// Library authors should not directly implement this trait, but should prefer
    /// implementing the [`TryFrom`] trait, which offers greater flexibility and
    /// provides an equivalent `TryInto` implementation for free, thanks to a
    /// blanket implementation in the standard library. For more information on this,
    /// see the documentation for [`Into`].
    ///
    /// [`TryFrom`]: trait.TryFrom.html
    /// [`Into`]: trait.Into.html
    #[unstable(feature = "try_from", issue = "33417")]
    pub trait TryInto<T>: Sized {
        /// The type returned in the event of a conversion error.
        type Error;

        /// Performs the conversion.
        fn try_into(self) -> Result<T, Self::Error>;
    }

    /// Attempt to construct `Self` via a conversion.
    #[unstable(feature = "try_from", issue = "33417")]
    pub trait TryFrom<T>: Sized {
        /// The type returned in the event of a conversion error.
        type Error;

        /// Performs the conversion.
        fn try_from(value: T) -> Result<Self, Self::Error>;
    }

    ////////////////////////////////////////////////////////////////////////////////
    // GENERIC IMPLS
    ////////////////////////////////////////////////////////////////////////////////

    // As lifts over &
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T: ?Sized, U: ?Sized> AsRef<U> for &'a T where T: AsRef<U>
    {
        fn as_ref(&self) -> &U {
            <T as AsRef<U>>::as_ref(*self)
        }
    }

    // As lifts over &mut
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T: ?Sized, U: ?Sized> AsRef<U> for &'a mut T where T: AsRef<U>
    {
        fn as_ref(&self) -> &U {
            <T as AsRef<U>>::as_ref(*self)
        }
    }

    // FIXME (#23442): replace the above impls for &/&mut with the following more general one:
    // // As lifts over Deref
    // impl<D: ?Sized + Deref, U: ?Sized> AsRef<U> for D where D::Target: AsRef<U> {
    //     fn as_ref(&self) -> &U {
    //         self.deref().as_ref()
    //     }
    // }

    // AsMut lifts over &mut
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T: ?Sized, U: ?Sized> AsMut<U> for &'a mut T where T: AsMut<U>
    {
        fn as_mut(&mut self) -> &mut U {
            (*self).as_mut()
        }
    }

    // FIXME (#23442): replace the above impl for &mut with the following more general one:
    // // AsMut lifts over DerefMut
    // impl<D: ?Sized + Deref, U: ?Sized> AsMut<U> for D where D::Target: AsMut<U> {
    //     fn as_mut(&mut self) -> &mut U {
    //         self.deref_mut().as_mut()
    //     }
    // }

    // From implies Into
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T, U> Into<U> for T where U: From<T>
    {
        fn into(self) -> U {
            U::from(self)
        }
    }

    // From (and thus Into) is reflexive
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> From<T> for T {
        fn from(t: T) -> T { t }
    }

    // TryFrom implies TryInto
    #[unstable(feature = "try_from", issue = "33417")]
    impl<T, U> TryInto<U> for T where U: TryFrom<T>
    {
        type Error = U::Error;

        fn try_into(self) -> Result<U, U::Error> {
            U::try_from(self)
        }
    }

    // Infallible conversions are semantically equivalent to fallible conversions
    // with an uninhabited error type.
    #[unstable(feature = "try_from", issue = "33417")]
    impl<T, U> TryFrom<U> for T where T: From<U> {
        type Error = Infallible;

        fn try_from(value: U) -> Result<Self, Self::Error> {
            Ok(T::from(value))
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // CONCRETE IMPLS
    ////////////////////////////////////////////////////////////////////////////////

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> AsRef<[T]> for [T] {
        fn as_ref(&self) -> &[T] {
            self
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> AsMut<[T]> for [T] {
        fn as_mut(&mut self) -> &mut [T] {
            self
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl AsRef<str> for str {
        #[inline]
        fn as_ref(&self) -> &str {
            self
        }
    }

# Help

## Keyboard Shortcuts

`?`
  Show this help dialog
`S`
  Focus the search field
`↑`
  Move up in search results
`↓`
  Move down in search results
`↹`
  Switch tab
`⏎`
  Go to active search result
`+`
  Expand all sections
`-`
  Collapse all sections

## Search Tricks

Prefix searches with a type followed by a colon (e.g. `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g. `vec -> usize` or `* -> vec`)