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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"><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" class="active"><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="drop-flags.html#drop-flags" id="drop-flags"><h1>Drop Flags</h1></a>
<p>The examples in the previous section introduce an interesting problem for Rust.
We have seen that it's possible to conditionally initialize, deinitialize, and
reinitialize locations of memory totally safely. For Copy types, this isn't
particularly notable since they're just a random pile of bits. However types
with destructors are a different story: Rust needs to know whether to call a
destructor whenever a variable is assigned to, or a variable goes out of scope.
How can it do this with conditional initialization?</p>
<p>Note that this is not a problem that all assignments need worry about. In
particular, assigning through a dereference unconditionally drops, and assigning
in a <code>let</code> unconditionally doesn't drop:</p>
<pre><code>let mut x = Box::new(0); // let makes a fresh variable, so never need to drop
let y = &mut x;
*y = Box::new(1); // Deref assumes the referent is initialized, so always drops
</code></pre>
<p>This is only a problem when overwriting a previously initialized variable or
one of its subfields.</p>
<p>It turns out that Rust actually tracks whether a type should be dropped or not
<em>at runtime</em>. As a variable becomes initialized and uninitialized, a <em>drop flag</em>
for that variable is toggled. When a variable might need to be dropped, this
flag is evaluated to determine if it should be dropped.</p>
<p>Of course, it is often the case that a value's initialization state can be
statically known at every point in the program. If this is the case, then the
compiler can theoretically generate more efficient code! For instance, straight-
line code has such <em>static drop semantics</em>:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
let mut x = Box::new(0); // x was uninit; just overwrite.
let mut y = x; // y was uninit; just overwrite and make x uninit.
x = Box::new(0); // x was uninit; just overwrite.
y = x; // y was init; Drop y, overwrite it, and make x uninit!
// y goes out of scope; y was init; Drop y!
// x goes out of scope; x was uninit; do nothing.
#}</code></pre></pre>
<p>Similarly, branched code where all branches have the same behavior with respect
to initialization has static drop semantics:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
# let condition = true;
let mut x = Box::new(0); // x was uninit; just overwrite.
if condition {
drop(x) // x gets moved out; make x uninit.
} else {
println!("{}", x);
drop(x) // x gets moved out; make x uninit.
}
x = Box::new(0); // x was uninit; just overwrite.
// x goes out of scope; x was init; Drop x!
#}</code></pre></pre>
<p>However code like this <em>requires</em> runtime information to correctly Drop:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
# let condition = true;
let x;
if condition {
x = Box::new(0); // x was uninit; just overwrite.
println!("{}", x);
}
// x goes out of scope; x might be uninit;
// check the flag!
#}</code></pre></pre>
<p>Of course, in this case it's trivial to retrieve static drop semantics:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
# let condition = true;
if condition {
let x = Box::new(0);
println!("{}", x);
}
#}</code></pre></pre>
<p>The drop flags are tracked on the stack and no longer stashed in types that
implement drop.</p>
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