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</pre><pre class="rust ">
<span class="comment">// Copyright 2012 The Rust Project Developers. See the COPYRIGHT</span>
<span class="comment">// file at the top-level directory of this distribution and at</span>
<span class="comment">// http://rust-lang.org/COPYRIGHT.</span>
<span class="comment">//</span>
<span class="comment">// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or</span>
<span class="comment">// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license</span>
<span class="comment">// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your</span>
<span class="comment">// option. This file may not be copied, modified, or distributed</span>
<span class="comment">// except according to those terms.</span>
<span class="doccomment">/// Both `PLACE <- EXPR` and `box EXPR` desugar into expressions</span>
<span class="doccomment">/// that allocate an intermediate "place" that holds uninitialized</span>
<span class="doccomment">/// state. The desugaring evaluates EXPR, and writes the result at</span>
<span class="doccomment">/// the address returned by the `pointer` method of this trait.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// A `Place` can be thought of as a special representation for a</span>
<span class="doccomment">/// hypothetical `&uninit` reference (which Rust cannot currently</span>
<span class="doccomment">/// express directly). That is, it represents a pointer to</span>
<span class="doccomment">/// uninitialized storage.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The client is responsible for two steps: First, initializing the</span>
<span class="doccomment">/// payload (it can access its address via `pointer`). Second,</span>
<span class="doccomment">/// converting the agent to an instance of the owning pointer, via the</span>
<span class="doccomment">/// appropriate `finalize` method (see the `InPlace`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// If evaluating EXPR fails, then it is up to the destructor for the</span>
<span class="doccomment">/// implementation of Place to clean up any intermediate state</span>
<span class="doccomment">/// (e.g. deallocate box storage, pop a stack, etc).</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"placement_new_protocol"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"27779"</span>)]</span>
<span class="kw">pub</span> <span class="kw">unsafe</span> <span class="kw">trait</span> <span class="ident">Place</span><span class="op"><</span><span class="ident">Data</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> {
<span class="doccomment">/// Returns the address where the input value will be written.</span>
<span class="doccomment">/// Note that the data at this address is generally uninitialized,</span>
<span class="doccomment">/// and thus one should use `ptr::write` for initializing it.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This function must return a pointer through which a value</span>
<span class="doccomment">/// of type `Data` can be written.</span>
<span class="kw">fn</span> <span class="ident">pointer</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">Data</span>;
}
<span class="doccomment">/// Interface to implementations of `PLACE <- EXPR`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// `PLACE <- EXPR` effectively desugars into:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// # #![feature(placement_new_protocol, box_heap)]</span>
<span class="doccomment">/// # use std::ops::{Placer, Place, InPlace};</span>
<span class="doccomment">/// # #[allow(non_snake_case)]</span>
<span class="doccomment">/// # fn main() {</span>
<span class="doccomment">/// # let PLACE = std::boxed::HEAP;</span>
<span class="doccomment">/// # let EXPR = 1;</span>
<span class="doccomment">/// let p = PLACE;</span>
<span class="doccomment">/// let mut place = Placer::make_place(p);</span>
<span class="doccomment">/// let raw_place = Place::pointer(&mut place);</span>
<span class="doccomment">/// let value = EXPR;</span>
<span class="doccomment">/// unsafe {</span>
<span class="doccomment">/// std::ptr::write(raw_place, value);</span>
<span class="doccomment">/// InPlace::finalize(place)</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// # ; }</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The type of `PLACE <- EXPR` is derived from the type of `PLACE`;</span>
<span class="doccomment">/// if the type of `PLACE` is `P`, then the final type of the whole</span>
<span class="doccomment">/// expression is `P::Place::Owner` (see the `InPlace` and `Boxed`</span>
<span class="doccomment">/// traits).</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Values for types implementing this trait usually are transient</span>
<span class="doccomment">/// intermediate values (e.g. the return value of `Vec::emplace_back`)</span>
<span class="doccomment">/// or `Copy`, since the `make_place` method takes `self` by value.</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"placement_new_protocol"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"27779"</span>)]</span>
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">Placer</span><span class="op"><</span><span class="ident">Data</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> {
<span class="doccomment">/// `Place` is the intermediate agent guarding the</span>
<span class="doccomment">/// uninitialized state for `Data`.</span>
<span class="kw">type</span> <span class="ident">Place</span>: <span class="ident">InPlace</span><span class="op"><</span><span class="ident">Data</span><span class="op">></span>;
<span class="doccomment">/// Creates a fresh place from `self`.</span>
<span class="kw">fn</span> <span class="ident">make_place</span>(<span class="self">self</span>) <span class="op">-></span> <span class="self">Self</span>::<span class="ident">Place</span>;
}
<span class="doccomment">/// Specialization of `Place` trait supporting `PLACE <- EXPR`.</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"placement_new_protocol"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"27779"</span>)]</span>
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">InPlace</span><span class="op"><</span><span class="ident">Data</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span>: <span class="ident">Place</span><span class="op"><</span><span class="ident">Data</span><span class="op">></span> {
<span class="doccomment">/// `Owner` is the type of the end value of `PLACE <- EXPR`</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Note that when `PLACE <- EXPR` is solely used for</span>
<span class="doccomment">/// side-effecting an existing data-structure,</span>
<span class="doccomment">/// e.g. `Vec::emplace_back`, then `Owner` need not carry any</span>
<span class="doccomment">/// information at all (e.g. it can be the unit type `()` in that</span>
<span class="doccomment">/// case).</span>
<span class="kw">type</span> <span class="ident">Owner</span>;
<span class="doccomment">/// Converts self into the final value, shifting</span>
<span class="doccomment">/// deallocation/cleanup responsibilities (if any remain), over to</span>
<span class="doccomment">/// the returned instance of `Owner` and forgetting self.</span>
<span class="kw">unsafe</span> <span class="kw">fn</span> <span class="ident">finalize</span>(<span class="self">self</span>) <span class="op">-></span> <span class="self">Self</span>::<span class="ident">Owner</span>;
}
<span class="doccomment">/// Core trait for the `box EXPR` form.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// `box EXPR` effectively desugars into:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// # #![feature(placement_new_protocol)]</span>
<span class="doccomment">/// # use std::ops::{BoxPlace, Place, Boxed};</span>
<span class="doccomment">/// # #[allow(non_snake_case)]</span>
<span class="doccomment">/// # fn main() {</span>
<span class="doccomment">/// # let EXPR = 1;</span>
<span class="doccomment">/// let mut place = BoxPlace::make_place();</span>
<span class="doccomment">/// let raw_place = Place::pointer(&mut place);</span>
<span class="doccomment">/// let value = EXPR;</span>
<span class="doccomment">/// # let _: Box<_> =</span>
<span class="doccomment">/// unsafe {</span>
<span class="doccomment">/// ::std::ptr::write(raw_place, value);</span>
<span class="doccomment">/// Boxed::finalize(place)</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// # ; }</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The type of `box EXPR` is supplied from its surrounding</span>
<span class="doccomment">/// context; in the above expansion, the result type `T` is used</span>
<span class="doccomment">/// to determine which implementation of `Boxed` to use, and that</span>
<span class="doccomment">/// `<T as Boxed>` in turn dictates determines which</span>
<span class="doccomment">/// implementation of `BoxPlace` to use, namely:</span>
<span class="doccomment">/// `<<T as Boxed>::Place as BoxPlace>`.</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"placement_new_protocol"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"27779"</span>)]</span>
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">Boxed</span> {
<span class="doccomment">/// The kind of data that is stored in this kind of box.</span>
<span class="kw">type</span> <span class="ident">Data</span>; <span class="comment">/* (`Data` unused b/c cannot yet express below bound.) */</span>
<span class="doccomment">/// The place that will negotiate the storage of the data.</span>
<span class="kw">type</span> <span class="ident">Place</span>: <span class="ident">BoxPlace</span><span class="op"><</span><span class="self">Self</span>::<span class="ident">Data</span><span class="op">></span>;
<span class="doccomment">/// Converts filled place into final owning value, shifting</span>
<span class="doccomment">/// deallocation/cleanup responsibilities (if any remain), over to</span>
<span class="doccomment">/// returned instance of `Self` and forgetting `filled`.</span>
<span class="kw">unsafe</span> <span class="kw">fn</span> <span class="ident">finalize</span>(<span class="ident">filled</span>: <span class="self">Self</span>::<span class="ident">Place</span>) <span class="op">-></span> <span class="self">Self</span>;
}
<span class="doccomment">/// Specialization of `Place` trait supporting `box EXPR`.</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"placement_new_protocol"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"27779"</span>)]</span>
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">BoxPlace</span><span class="op"><</span><span class="ident">Data</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> : <span class="ident">Place</span><span class="op"><</span><span class="ident">Data</span><span class="op">></span> {
<span class="doccomment">/// Creates a globally fresh place.</span>
<span class="kw">fn</span> <span class="ident">make_place</span>() <span class="op">-></span> <span class="self">Self</span>;
}
</pre>
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