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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.

    #![allow(missing_docs)]
    #![unstable(feature = "raw", issue = "27751")]

    //! Contains struct definitions for the layout of compiler built-in types.
    //!
    //! They can be used as targets of transmutes in unsafe code for manipulating
    //! the raw representations directly.
    //!
    //! Their definition should always match the ABI defined in `rustc::back::abi`.

    /// The representation of a trait object like `&SomeTrait`.
    ///
    /// This struct has the same layout as types like `&SomeTrait` and
    /// `Box<AnotherTrait>`. The [Trait Objects chapter of the
    /// Book][moreinfo] contains more details about the precise nature of
    /// these internals.
    ///
    /// [moreinfo]: ../../book/trait-objects.html#representation
    ///
    /// `TraitObject` is guaranteed to match layouts, but it is not the
    /// type of trait objects (e.g. the fields are not directly accessible
    /// on a `&SomeTrait`) nor does it control that layout (changing the
    /// definition will not change the layout of a `&SomeTrait`). It is
    /// only designed to be used by unsafe code that needs to manipulate
    /// the low-level details.
    ///
    /// There is no `Repr` implementation for `TraitObject` because there
    /// is no way to refer to all trait objects generically, so the only
    /// way to create values of this type is with functions like
    /// `std::mem::transmute`. Similarly, the only way to create a true
    /// trait object from a `TraitObject` value is with `transmute`.
    ///
    /// Synthesizing a trait object with mismatched types—one where the
    /// vtable does not correspond to the type of the value to which the
    /// data pointer points—is highly likely to lead to undefined
    /// behavior.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(raw)]
    ///
    /// use std::mem;
    /// use std::raw;
    ///
    /// // an example trait
    /// trait Foo {
    ///     fn bar(&self) -> i32;
    /// }
    /// impl Foo for i32 {
    ///     fn bar(&self) -> i32 {
    ///          *self + 1
    ///     }
    /// }
    ///
    /// let value: i32 = 123;
    ///
    /// // let the compiler make a trait object
    /// let object: &Foo = &value;
    ///
    /// // look at the raw representation
    /// let raw_object: raw::TraitObject = unsafe { mem::transmute(object) };
    ///
    /// // the data pointer is the address of `value`
    /// assert_eq!(raw_object.data as *const i32, &value as *const _);
    ///
    ///
    /// let other_value: i32 = 456;
    ///
    /// // construct a new object, pointing to a different `i32`, being
    /// // careful to use the `i32` vtable from `object`
    /// let synthesized: &Foo = unsafe {
    ///      mem::transmute(raw::TraitObject {
    ///          data: &other_value as *const _ as *mut (),
    ///          vtable: raw_object.vtable
    ///      })
    /// };
    ///
    /// // it should work just like we constructed a trait object out of
    /// // `other_value` directly
    /// assert_eq!(synthesized.bar(), 457);
    /// ```
    #[repr(C)]
    #[derive(Copy, Clone)]
    #[allow(missing_debug_implementations)]
    pub struct TraitObject {
        pub data: *mut (),
        pub vtable: *mut (),
    }

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