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    // Copyright 2013 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.

    //! The `Default` trait for types which may have meaningful default values.
    //!
    //! Sometimes, you want to fall back to some kind of default value, and
    //! don't particularly care what it is. This comes up often with `struct`s
    //! that define a set of options:
    //!
    //! ```
    //! # #[allow(dead_code)]
    //! struct SomeOptions {
    //!     foo: i32,
    //!     bar: f32,
    //! }
    //! ```
    //!
    //! How can we define some default values? You can use `Default`:
    //!
    //! ```
    //! # #[allow(dead_code)]
    //! #[derive(Default)]
    //! struct SomeOptions {
    //!     foo: i32,
    //!     bar: f32,
    //! }
    //!
    //!
    //! fn main() {
    //!     let options: SomeOptions = Default::default();
    //! }
    //! ```
    //!
    //! Now, you get all of the default values. Rust implements `Default` for various primitives types.
    //! If you have your own type, you need to implement `Default` yourself:
    //!
    //! ```
    //! # #![allow(dead_code)]
    //! enum Kind {
    //!     A,
    //!     B,
    //!     C,
    //! }
    //!
    //! impl Default for Kind {
    //!     fn default() -> Kind { Kind::A }
    //! }
    //!
    //! #[derive(Default)]
    //! struct SomeOptions {
    //!     foo: i32,
    //!     bar: f32,
    //!     baz: Kind,
    //! }
    //!
    //!
    //! fn main() {
    //!     let options: SomeOptions = Default::default();
    //! }
    //! ```
    //!
    //! If you want to override a particular option, but still retain the other defaults:
    //!
    //! ```
    //! # #[allow(dead_code)]
    //! # #[derive(Default)]
    //! # struct SomeOptions {
    //! #     foo: i32,
    //! #     bar: f32,
    //! # }
    //! fn main() {
    //!     let options = SomeOptions { foo: 42, ..Default::default() };
    //! }
    //! ```

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

    use marker::Sized;

    /// A trait for giving a type a useful default value.
    ///
    /// A struct can derive default implementations of `Default` for basic types using
    /// `#[derive(Default)]`.
    ///
    /// # Examples
    ///
    /// ```
    /// # #[allow(dead_code)]
    /// #[derive(Default)]
    /// struct SomeOptions {
    ///     foo: i32,
    ///     bar: f32,
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait Default: Sized {
        /// Returns the "default value" for a type.
        ///
        /// Default values are often some kind of initial value, identity value, or anything else that
        /// may make sense as a default.
        ///
        /// # Examples
        ///
        /// Using built-in default values:
        ///
        /// ```
        /// let i: i8 = Default::default();
        /// let (x, y): (Option<String>, f64) = Default::default();
        /// let (a, b, (c, d)): (i32, u32, (bool, bool)) = Default::default();
        /// ```
        ///
        /// Making your own:
        ///
        /// ```
        /// # #[allow(dead_code)]
        /// enum Kind {
        ///     A,
        ///     B,
        ///     C,
        /// }
        ///
        /// impl Default for Kind {
        ///     fn default() -> Kind { Kind::A }
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn default() -> Self;
    }

    macro_rules! default_impl {
        ($t:ty, $v:expr) => {
            #[stable(feature = "rust1", since = "1.0.0")]
            impl Default for $t {
                #[inline]
                fn default() -> $t { $v }
            }
        }
    }

    default_impl! { (), () }
    default_impl! { bool, false }
    default_impl! { char, '\x00' }

    default_impl! { usize, 0 }
    default_impl! { u8, 0 }
    default_impl! { u16, 0 }
    default_impl! { u32, 0 }
    default_impl! { u64, 0 }

    default_impl! { isize, 0 }
    default_impl! { i8, 0 }
    default_impl! { i16, 0 }
    default_impl! { i32, 0 }
    default_impl! { i64, 0 }

    default_impl! { f32, 0.0f32 }
    default_impl! { f64, 0.0f64 }

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