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    // Copyright 2012 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.

    /// Both `PLACE <- EXPR` and `box EXPR` desugar into expressions
    /// that allocate an intermediate "place" that holds uninitialized
    /// state.  The desugaring evaluates EXPR, and writes the result at
    /// the address returned by the `pointer` method of this trait.
    ///
    /// A `Place` can be thought of as a special representation for a
    /// hypothetical `&uninit` reference (which Rust cannot currently
    /// express directly). That is, it represents a pointer to
    /// uninitialized storage.
    ///
    /// The client is responsible for two steps: First, initializing the
    /// payload (it can access its address via `pointer`). Second,
    /// converting the agent to an instance of the owning pointer, via the
    /// appropriate `finalize` method (see the `InPlace`.
    ///
    /// If evaluating EXPR fails, then it is up to the destructor for the
    /// implementation of Place to clean up any intermediate state
    /// (e.g. deallocate box storage, pop a stack, etc).
    #[unstable(feature = "placement_new_protocol", issue = "27779")]
    pub unsafe trait Place<Data: ?Sized> {
        /// Returns the address where the input value will be written.
        /// Note that the data at this address is generally uninitialized,
        /// and thus one should use `ptr::write` for initializing it.
        ///
        /// This function must return a pointer through which a value
        /// of type `Data` can be written.
        fn pointer(&mut self) -> *mut Data;
    }

    /// Interface to implementations of  `PLACE <- EXPR`.
    ///
    /// `PLACE <- EXPR` effectively desugars into:
    ///
    /// ```
    /// # #![feature(placement_new_protocol, box_heap)]
    /// # use std::ops::{Placer, Place, InPlace};
    /// # #[allow(non_snake_case)]
    /// # fn main() {
    /// # let PLACE = std::boxed::HEAP;
    /// # let EXPR = 1;
    /// let p = PLACE;
    /// let mut place = Placer::make_place(p);
    /// let raw_place = Place::pointer(&mut place);
    /// let value = EXPR;
    /// unsafe {
    ///     std::ptr::write(raw_place, value);
    ///     InPlace::finalize(place)
    /// }
    /// # ; }
    /// ```
    ///
    /// The type of `PLACE <- EXPR` is derived from the type of `PLACE`;
    /// if the type of `PLACE` is `P`, then the final type of the whole
    /// expression is `P::Place::Owner` (see the `InPlace` and `Boxed`
    /// traits).
    ///
    /// Values for types implementing this trait usually are transient
    /// intermediate values (e.g. the return value of `Vec::emplace_back`)
    /// or `Copy`, since the `make_place` method takes `self` by value.
    #[unstable(feature = "placement_new_protocol", issue = "27779")]
    pub trait Placer<Data: ?Sized> {
        /// `Place` is the intermediate agent guarding the
        /// uninitialized state for `Data`.
        type Place: InPlace<Data>;

        /// Creates a fresh place from `self`.
        fn make_place(self) -> Self::Place;
    }

    /// Specialization of `Place` trait supporting `PLACE <- EXPR`.
    #[unstable(feature = "placement_new_protocol", issue = "27779")]
    pub trait InPlace<Data: ?Sized>: Place<Data> {
        /// `Owner` is the type of the end value of `PLACE <- EXPR`
        ///
        /// Note that when `PLACE <- EXPR` is solely used for
        /// side-effecting an existing data-structure,
        /// e.g. `Vec::emplace_back`, then `Owner` need not carry any
        /// information at all (e.g. it can be the unit type `()` in that
        /// case).
        type Owner;

        /// Converts self into the final value, shifting
        /// deallocation/cleanup responsibilities (if any remain), over to
        /// the returned instance of `Owner` and forgetting self.
        unsafe fn finalize(self) -> Self::Owner;
    }

    /// Core trait for the `box EXPR` form.
    ///
    /// `box EXPR` effectively desugars into:
    ///
    /// ```
    /// # #![feature(placement_new_protocol)]
    /// # use std::ops::{BoxPlace, Place, Boxed};
    /// # #[allow(non_snake_case)]
    /// # fn main() {
    /// # let EXPR = 1;
    /// let mut place = BoxPlace::make_place();
    /// let raw_place = Place::pointer(&mut place);
    /// let value = EXPR;
    /// # let _: Box<_> =
    /// unsafe {
    ///     ::std::ptr::write(raw_place, value);
    ///     Boxed::finalize(place)
    /// }
    /// # ; }
    /// ```
    ///
    /// The type of `box EXPR` is supplied from its surrounding
    /// context; in the above expansion, the result type `T` is used
    /// to determine which implementation of `Boxed` to use, and that
    /// `<T as Boxed>` in turn dictates determines which
    /// implementation of `BoxPlace` to use, namely:
    /// `<<T as Boxed>::Place as BoxPlace>`.
    #[unstable(feature = "placement_new_protocol", issue = "27779")]
    pub trait Boxed {
        /// The kind of data that is stored in this kind of box.
        type Data;  /* (`Data` unused b/c cannot yet express below bound.) */
        /// The place that will negotiate the storage of the data.
        type Place: BoxPlace<Self::Data>;

        /// Converts filled place into final owning value, shifting
        /// deallocation/cleanup responsibilities (if any remain), over to
        /// returned instance of `Self` and forgetting `filled`.
        unsafe fn finalize(filled: Self::Place) -> Self;
    }

    /// Specialization of `Place` trait supporting `box EXPR`.
    #[unstable(feature = "placement_new_protocol", issue = "27779")]
    pub trait BoxPlace<Data: ?Sized> : Place<Data> {
        /// Creates a globally fresh place.
        fn make_place() -> Self;
    }

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