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<ol class="chapter"><li class="affix"><a href="README.html">Introduction</a></li><li><a href="meet-safe-and-unsafe.html"><strong aria-hidden="true">1.</strong> Meet Safe and Unsafe</a></li><li><ol class="section"><li><a href="safe-unsafe-meaning.html"><strong aria-hidden="true">1.1.</strong> How Safe and Unsafe Interact</a></li><li><a href="what-unsafe-does.html"><strong aria-hidden="true">1.2.</strong> What Unsafe Can Do</a></li><li><a href="working-with-unsafe.html"><strong aria-hidden="true">1.3.</strong> Working with Unsafe</a></li></ol></li><li><a href="data.html"><strong aria-hidden="true">2.</strong> Data Layout</a></li><li><ol class="section"><li><a href="repr-rust.html"><strong aria-hidden="true">2.1.</strong> repr(Rust)</a></li><li><a href="exotic-sizes.html"><strong aria-hidden="true">2.2.</strong> Exotically Sized Types</a></li><li><a href="other-reprs.html"><strong aria-hidden="true">2.3.</strong> Other reprs</a></li></ol></li><li><a href="ownership.html"><strong aria-hidden="true">3.</strong> Ownership</a></li><li><ol class="section"><li><a href="references.html"><strong aria-hidden="true">3.1.</strong> References</a></li><li><a href="aliasing.html"><strong aria-hidden="true">3.2.</strong> Aliasing</a></li><li><a href="lifetimes.html"><strong aria-hidden="true">3.3.</strong> Lifetimes</a></li><li><a href="lifetime-mismatch.html"><strong aria-hidden="true">3.4.</strong> Limits of Lifetimes</a></li><li><a href="lifetime-elision.html"><strong aria-hidden="true">3.5.</strong> Lifetime Elision</a></li><li><a href="unbounded-lifetimes.html"><strong aria-hidden="true">3.6.</strong> Unbounded Lifetimes</a></li><li><a href="hrtb.html"><strong aria-hidden="true">3.7.</strong> Higher-Rank Trait Bounds</a></li><li><a href="subtyping.html"><strong aria-hidden="true">3.8.</strong> Subtyping and Variance</a></li><li><a href="dropck.html"><strong aria-hidden="true">3.9.</strong> Drop Check</a></li><li><a href="phantom-data.html"><strong aria-hidden="true">3.10.</strong> PhantomData</a></li><li><a href="borrow-splitting.html"><strong aria-hidden="true">3.11.</strong> Splitting Borrows</a></li></ol></li><li><a href="conversions.html"><strong aria-hidden="true">4.</strong> Type Conversions</a></li><li><ol class="section"><li><a href="coercions.html" class="active"><strong aria-hidden="true">4.1.</strong> Coercions</a></li><li><a href="dot-operator.html"><strong aria-hidden="true">4.2.</strong> The Dot Operator</a></li><li><a href="casts.html"><strong aria-hidden="true">4.3.</strong> Casts</a></li><li><a href="transmutes.html"><strong aria-hidden="true">4.4.</strong> Transmutes</a></li></ol></li><li><a href="uninitialized.html"><strong aria-hidden="true">5.</strong> Uninitialized Memory</a></li><li><ol class="section"><li><a href="checked-uninit.html"><strong aria-hidden="true">5.1.</strong> Checked</a></li><li><a href="drop-flags.html"><strong aria-hidden="true">5.2.</strong> Drop Flags</a></li><li><a href="unchecked-uninit.html"><strong aria-hidden="true">5.3.</strong> Unchecked</a></li></ol></li><li><a href="obrm.html"><strong aria-hidden="true">6.</strong> Ownership Based Resource Management</a></li><li><ol class="section"><li><a href="constructors.html"><strong aria-hidden="true">6.1.</strong> Constructors</a></li><li><a href="destructors.html"><strong aria-hidden="true">6.2.</strong> Destructors</a></li><li><a href="leaking.html"><strong aria-hidden="true">6.3.</strong> Leaking</a></li></ol></li><li><a href="unwinding.html"><strong aria-hidden="true">7.</strong> Unwinding</a></li><li><ol class="section"><li><a href="exception-safety.html"><strong aria-hidden="true">7.1.</strong> Exception Safety</a></li><li><a href="poisoning.html"><strong aria-hidden="true">7.2.</strong> Poisoning</a></li></ol></li><li><a href="concurrency.html"><strong aria-hidden="true">8.</strong> Concurrency</a></li><li><ol class="section"><li><a href="races.html"><strong aria-hidden="true">8.1.</strong> Races</a></li><li><a href="send-and-sync.html"><strong aria-hidden="true">8.2.</strong> Send and Sync</a></li><li><a href="atomics.html"><strong aria-hidden="true">8.3.</strong> Atomics</a></li></ol></li><li><a href="vec.html"><strong aria-hidden="true">9.</strong> Implementing Vec</a></li><li><ol class="section"><li><a href="vec-layout.html"><strong aria-hidden="true">9.1.</strong> Layout</a></li><li><a href="vec-alloc.html"><strong aria-hidden="true">9.2.</strong> Allocating</a></li><li><a href="vec-push-pop.html"><strong aria-hidden="true">9.3.</strong> Push and Pop</a></li><li><a href="vec-dealloc.html"><strong aria-hidden="true">9.4.</strong> Deallocating</a></li><li><a href="vec-deref.html"><strong aria-hidden="true">9.5.</strong> Deref</a></li><li><a href="vec-insert-remove.html"><strong aria-hidden="true">9.6.</strong> Insert and Remove</a></li><li><a href="vec-into-iter.html"><strong aria-hidden="true">9.7.</strong> IntoIter</a></li><li><a href="vec-raw.html"><strong aria-hidden="true">9.8.</strong> RawVec</a></li><li><a href="vec-drain.html"><strong aria-hidden="true">9.9.</strong> Drain</a></li><li><a href="vec-zsts.html"><strong aria-hidden="true">9.10.</strong> Handling Zero-Sized Types</a></li><li><a href="vec-final.html"><strong aria-hidden="true">9.11.</strong> Final Code</a></li></ol></li><li><a href="arc-and-mutex.html"><strong aria-hidden="true">10.</strong> Implementing Arc and Mutex</a></li><li><a href="ffi.html"><strong aria-hidden="true">11.</strong> FFI</a></li></ol>
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<a class="header" href="coercions.html#coercions" id="coercions"><h1>Coercions</h1></a>
<p>Types can implicitly be coerced to change in certain contexts. These changes are
generally just <em>weakening</em> of types, largely focused around pointers and
lifetimes. They mostly exist to make Rust "just work" in more cases, and are
largely harmless.</p>
<p>Here's all the kinds of coercion:</p>
<p>Coercion is allowed between the following types:</p>
<ul>
<li>Transitivity: <code>T_1</code> to <code>T_3</code> where <code>T_1</code> coerces to <code>T_2</code> and <code>T_2</code> coerces to
<code>T_3</code></li>
<li>Pointer Weakening:
<ul>
<li><code>&mut T</code> to <code>&T</code></li>
<li><code>*mut T</code> to <code>*const T</code></li>
<li><code>&T</code> to <code>*const T</code></li>
<li><code>&mut T</code> to <code>*mut T</code></li>
</ul>
</li>
<li>Unsizing: <code>T</code> to <code>U</code> if <code>T</code> implements <code>CoerceUnsized<U></code></li>
<li>Deref coercion: Expression <code>&x</code> of type <code>&T</code> to <code>&*x</code> of type <code>&U</code> if <code>T</code> derefs to <code>U</code> (i.e. <code>T: Deref<Target=U></code>)</li>
</ul>
<p><code>CoerceUnsized<Pointer<U>> for Pointer<T> where T: Unsize<U></code> is implemented
for all pointer types (including smart pointers like Box and Rc). Unsize is
only implemented automatically, and enables the following transformations:</p>
<ul>
<li><code>[T; n]</code> => <code>[T]</code></li>
<li><code>T</code> => <code>Trait</code> where <code>T: Trait</code></li>
<li><code>Foo<..., T, ...></code> => <code>Foo<..., U, ...></code> where:
<ul>
<li><code>T: Unsize<U></code></li>
<li><code>Foo</code> is a struct</li>
<li>Only the last field of <code>Foo</code> has type involving <code>T</code></li>
<li><code>T</code> is not part of the type of any other fields</li>
<li><code>Bar<T>: Unsize<Bar<U>></code>, if the last field of <code>Foo</code> has type <code>Bar<T></code></li>
</ul>
</li>
</ul>
<p>Coercions occur at a <em>coercion site</em>. Any location that is explicitly typed
will cause a coercion to its type. If inference is necessary, the coercion will
not be performed. Exhaustively, the coercion sites for an expression <code>e</code> to
type <code>U</code> are:</p>
<ul>
<li>let statements, statics, and consts: <code>let x: U = e</code></li>
<li>Arguments to functions: <code>takes_a_U(e)</code></li>
<li>Any expression that will be returned: <code>fn foo() -> U { e }</code></li>
<li>Struct literals: <code>Foo { some_u: e }</code></li>
<li>Array literals: <code>let x: [U; 10] = [e, ..]</code></li>
<li>Tuple literals: <code>let x: (U, ..) = (e, ..)</code></li>
<li>The last expression in a block: <code>let x: U = { ..; e }</code></li>
</ul>
<p>Note that we do not perform coercions when matching traits (except for
receivers, see below). If there is an impl for some type <code>U</code> and <code>T</code> coerces to
<code>U</code>, that does not constitute an implementation for <code>T</code>. For example, the
following will not type check, even though it is OK to coerce <code>t</code> to <code>&T</code> and
there is an impl for <code>&T</code>:</p>
<pre><code class="language-rust ignore">trait Trait {}
fn foo<X: Trait>(t: X) {}
impl<'a> Trait for &'a i32 {}
fn main() {
let t: &mut i32 = &mut 0;
foo(t);
}
</code></pre>
<pre><code class="language-text"><anon>:10:5: 10:8 error: the trait bound `&mut i32 : Trait` is not satisfied [E0277]
<anon>:10 foo(t);
^~~
</code></pre>
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