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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="kw">use</span> <span class="ident">marker</span>::<span class="ident">Unsize</span>;
<span class="doccomment">/// Trait that indicates that this is a pointer or a wrapper for one,</span>
<span class="doccomment">/// where unsizing can be performed on the pointee.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// See the [DST coercion RfC][dst-coerce] and [the nomicon entry on coercion][nomicon-coerce]</span>
<span class="doccomment">/// for more details.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// For builtin pointer types, pointers to `T` will coerce to pointers to `U` if `T: Unsize<U>`</span>
<span class="doccomment">/// by converting from a thin pointer to a fat pointer.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// For custom types, the coercion here works by coercing `Foo<T>` to `Foo<U>`</span>
<span class="doccomment">/// provided an impl of `CoerceUnsized<Foo<U>> for Foo<T>` exists.</span>
<span class="doccomment">/// Such an impl can only be written if `Foo<T>` has only a single non-phantomdata</span>
<span class="doccomment">/// field involving `T`. If the type of that field is `Bar<T>`, an implementation</span>
<span class="doccomment">/// of `CoerceUnsized<Bar<U>> for Bar<T>` must exist. The coercion will work by</span>
<span class="doccomment">/// coercing the `Bar<T>` field into `Bar<U>` and filling in the rest of the fields</span>
<span class="doccomment">/// from `Foo<T>` to create a `Foo<U>`. This will effectively drill down to a pointer</span>
<span class="doccomment">/// field and coerce that.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Generally, for smart pointers you will implement</span>
<span class="doccomment">/// `CoerceUnsized<Ptr<U>> for Ptr<T> where T: Unsize<U>, U: ?Sized`, with an</span>
<span class="doccomment">/// optional `?Sized` bound on `T` itself. For wrapper types that directly embed `T`</span>
<span class="doccomment">/// like `Cell<T>` and `RefCell<T>`, you</span>
<span class="doccomment">/// can directly implement `CoerceUnsized<Wrap<U>> for Wrap<T> where T: CoerceUnsized<U>`.</span>
<span class="doccomment">/// This will let coercions of types like `Cell<Box<T>>` work.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`Unsize`][unsize] is used to mark types which can be coerced to DSTs if behind</span>
<span class="doccomment">/// pointers. It is implemented automatically by the compiler.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [dst-coerce]: https://github.com/rust-lang/rfcs/blob/master/text/0982-dst-coercion.md</span>
<span class="doccomment">/// [unsize]: ../marker/trait.Unsize.html</span>
<span class="doccomment">/// [nomicon-coerce]: ../../nomicon/coercions.html</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"coerce_unsized"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"27732"</span>)]</span>
<span class="attribute">#[<span class="ident">lang</span> <span class="op">=</span> <span class="string">"coerce_unsized"</span>]</span>
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">CoerceUnsized</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="comment">// Empty.</span>
}
<span class="comment">// &mut T -> &mut U</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"coerce_unsized"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"27732"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">+</span><span class="ident">Unsize</span><span class="op"><</span><span class="ident">U</span><span class="op">></span>, <span class="ident">U</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">CoerceUnsized</span><span class="op"><</span><span class="kw-2">&</span><span class="lifetime">'a</span> <span class="kw-2">mut</span> <span class="ident">U</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="kw-2">mut</span> <span class="ident">T</span> {}
<span class="comment">// &mut T -> &U</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"coerce_unsized"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"27732"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="lifetime">'b</span>: <span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">+</span><span class="ident">Unsize</span><span class="op"><</span><span class="ident">U</span><span class="op">></span>, <span class="ident">U</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">CoerceUnsized</span><span class="op"><</span><span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">U</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'b</span> <span class="kw-2">mut</span> <span class="ident">T</span> {}
<span class="comment">// &mut T -> *mut U</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"coerce_unsized"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"27732"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">+</span><span class="ident">Unsize</span><span class="op"><</span><span class="ident">U</span><span class="op">></span>, <span class="ident">U</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">CoerceUnsized</span><span class="op"><</span><span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">U</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="kw-2">mut</span> <span class="ident">T</span> {}
<span class="comment">// &mut T -> *const U</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"coerce_unsized"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"27732"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">+</span><span class="ident">Unsize</span><span class="op"><</span><span class="ident">U</span><span class="op">></span>, <span class="ident">U</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">CoerceUnsized</span><span class="op"><</span><span class="kw-2">*</span><span class="kw">const</span> <span class="ident">U</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="kw-2">mut</span> <span class="ident">T</span> {}
<span class="comment">// &T -> &U</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"coerce_unsized"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"27732"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="lifetime">'b</span>: <span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">+</span><span class="ident">Unsize</span><span class="op"><</span><span class="ident">U</span><span class="op">></span>, <span class="ident">U</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">CoerceUnsized</span><span class="op"><</span><span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">U</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'b</span> <span class="ident">T</span> {}
<span class="comment">// &T -> *const U</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"coerce_unsized"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"27732"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">+</span><span class="ident">Unsize</span><span class="op"><</span><span class="ident">U</span><span class="op">></span>, <span class="ident">U</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">CoerceUnsized</span><span class="op"><</span><span class="kw-2">*</span><span class="kw">const</span> <span class="ident">U</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">T</span> {}
<span class="comment">// *mut T -> *mut U</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"coerce_unsized"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"27732"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">+</span><span class="ident">Unsize</span><span class="op"><</span><span class="ident">U</span><span class="op">></span>, <span class="ident">U</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">CoerceUnsized</span><span class="op"><</span><span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">U</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">T</span> {}
<span class="comment">// *mut T -> *const U</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"coerce_unsized"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"27732"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">+</span><span class="ident">Unsize</span><span class="op"><</span><span class="ident">U</span><span class="op">></span>, <span class="ident">U</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">CoerceUnsized</span><span class="op"><</span><span class="kw-2">*</span><span class="kw">const</span> <span class="ident">U</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">T</span> {}
<span class="comment">// *const T -> *const U</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"coerce_unsized"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"27732"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">+</span><span class="ident">Unsize</span><span class="op"><</span><span class="ident">U</span><span class="op">></span>, <span class="ident">U</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">CoerceUnsized</span><span class="op"><</span><span class="kw-2">*</span><span class="kw">const</span> <span class="ident">U</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">*</span><span class="kw">const</span> <span class="ident">T</span> {}
</pre>
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