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| <h1 id="coerceunsized"><a class="header" href="#coerceunsized"><a href="https://doc.rust-lang.org/std/ops/trait.CoerceUnsized.html"><code>CoerceUnsized</code></a></a></h1> |
| <p><code>CoerceUnsized</code> is primarily concerned with data containers. When a struct |
| (typically, a smart pointer) implements <code>CoerceUnsized</code>, that means that the |
| data it points to is being unsized.</p> |
| <p>Some implementors of <code>CoerceUnsized</code> include:</p> |
| <ul> |
| <li><code>&T</code></li> |
| <li><code>Arc<T></code></li> |
| <li><code>Box<T></code></li> |
| </ul> |
| <p>This trait is (eventually) intended to be implemented by user-written smart |
| pointers, and there are rules about when a type is allowed to implement |
| <code>CoerceUnsized</code> that are explained in the trait's documentation.</p> |
| <h1 id="unsize"><a class="header" href="#unsize"><a href="https://doc.rust-lang.org/std/marker/trait.Unsize.html"><code>Unsize</code></a></a></h1> |
| <p>To contrast, the <code>Unsize</code> trait is concerned the actual types that are allowed |
| to be unsized.</p> |
| <p>This is not intended to be implemented by users ever, since <code>Unsize</code> does not |
| instruct the compiler (namely codegen) <em>how</em> to unsize a type, just whether it |
| is allowed to be unsized. This is paired somewhat intimately with codegen |
| which must understand how types are represented and unsized.</p> |
| <h2 id="primitive-unsizing-implementations"><a class="header" href="#primitive-unsizing-implementations">Primitive unsizing implementations</a></h2> |
| <p>Built-in implementations are provided for:</p> |
| <ul> |
| <li><code>T</code> -> <code>dyn Trait + 'a</code> when <code>T: Trait</code> (and <code>T: Sized + 'a</code>, and <code>Trait</code> |
| is dyn-compatible<sup class="footnote-reference" id="fr-2-1"><a href="#footnote-2">1</a></sup>).</li> |
| <li><code>[T; N]</code> -> <code>[T]</code></li> |
| </ul> |
| <h2 id="structural-implementations"><a class="header" href="#structural-implementations">Structural implementations</a></h2> |
| <p>There is one implementation of <code>Unsize</code> which can be thought of as |
| structural:</p> |
| <ul> |
| <li><code>Struct<.., Pi, .., Pj, ..>: Unsize<Struct<.., Ui, .., Uj, ..>></code> given |
| <code>TailField<Pi, .., Pj>: Unsize<Ui, .. Uj></code>, which allows the tail field of a |
| struct to be unsized if it is the only field that mentions generic parameters |
| <code>Pi</code>, .., <code>Pj</code> (which don't need to be contiguous).</li> |
| </ul> |
| <p>The rules for struct unsizing are slightly complicated, since they |
| may allow more than one parameter to be changed (not necessarily unsized) and |
| are best stated in terms of the tail field of the struct.</p> |
| <p>(Tuple unsizing was previously implemented behind the feature gate |
| <code>unsized_tuple_coercion</code>, but the implementation was removed by <a href="https://github.com/rust-lang/rust/pull/137728">#137728</a>.)</p> |
| <h2 id="upcasting-implementations"><a class="header" href="#upcasting-implementations">Upcasting implementations</a></h2> |
| <p>Two things are called "upcasting" internally:</p> |
| <ol> |
| <li>True upcasting <code>dyn SubTrait</code> -> <code>dyn SuperTrait</code> (this also allows |
| dropping auto traits and adjusting lifetimes, as below).</li> |
| <li>Dropping auto traits and adjusting the lifetimes of dyn trait |
| <em>without changing the principal<sup class="footnote-reference" id="fr-1-1"><a href="#footnote-1">2</a></sup></em>: |
| <code>dyn Trait + AutoTraits... + 'a</code> -> <code>dyn Trait + NewAutoTraits... + 'b</code> |
| when <code>AutoTraits</code> ⊇ <code>NewAutoTraits</code>, and <code>'a: 'b</code>.</li> |
| </ol> |
| <p>These may seem like different operations, since (1.) includes adjusting the |
| vtable of a dyn trait, while (2.) is a no-op. However, to the type system, |
| these are handled with much the same code.</p> |
| <p>This built-in implementation of <code>Unsize</code> is the most involved, particularly |
| after <a href="https://github.com/rust-lang/rust/pull/114036">it was reworked</a> to |
| support the complexities of associated types.</p> |
| <p>Specifically, the upcasting algorithm involves: For each supertrait of the |
| source dyn trait's principal (including itself)...</p> |
| <ol> |
| <li>Unify the super trait ref with the principal of the target (making sure |
| we only ever upcast to a true supertrait, and never <a href="https://github.com/rust-lang/rust/blob/f3457dbf84cd86d284454d12705861398ece76c3/tests/ui/traits/trait-upcasting/illegal-upcast-from-impl.rs#L19">via an impl</a>).</li> |
| <li>For every auto trait in the target, check that it's present in the source |
| (allowing us to drop auto traits, but never gain new ones).</li> |
| <li>For every projection in the target, check that it unifies with a single |
| projection in the source (since there may be more than one given |
| <code>trait Sub: Sup<.., A = i32> + Sup<.., A = u32></code>).</li> |
| </ol> |
| <p>Specifically, (3.) prevents a choice of projection bound to guide inference |
| unnecessarily, though it may guide inference when it is unambiguous.</p> |
| <hr> |
| <ol class="footnote-definition"><li id="footnote-2"> |
| <p>Formerly known as "object safe". <a href="#fr-2-1">↩</a></p> |
| </li> |
| <li id="footnote-1"> |
| <p>The principal is the one non-auto trait of a <code>dyn Trait</code>. <a href="#fr-1-1">↩</a></p> |
| </li> |
| </ol> |
| </main> |
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