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| <h1 id="drop-check"><a class="header" href="#drop-check">Drop Check</a></h1> |
| <p>We generally require the type of locals to be well-formed whenever the |
| local is used. This includes proving the where-bounds of the local and |
| also requires all regions used by it to be live.</p> |
| <p>The only exception to this is when implicitly dropping values when they |
| go out of scope. This does not necessarily require the value to be live:</p> |
| <pre><pre class="playground"><code class="language-rust">fn main() { |
| let x = vec![]; |
| { |
| let y = String::from("I am temporary"); |
| x.push(&y); |
| } |
| // `x` goes out of scope here, after the reference to `y` |
| // is invalidated. This means that while dropping `x` its type |
| // is not well-formed as it contain regions which are not live. |
| }</code></pre></pre> |
| <p>This is only sound if dropping the value does not try to access any dead |
| region. We check this by requiring the type of the value to be |
| drop-live. |
| The requirements for which are computed in <code>fn dropck_outlives</code>.</p> |
| <p>The rest of this section uses the following type definition for a type |
| which requires its region parameter to be live:</p> |
| <pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>struct PrintOnDrop<'a>(&'a str); |
| impl<'a> Drop for PrintOnDrop<'_> { |
| fn drop(&mut self) { |
| println!("{}", self.0); |
| } |
| } |
| <span class="boring">}</span></code></pre></pre> |
| <h2 id="how-values-are-dropped"><a class="header" href="#how-values-are-dropped">How values are dropped</a></h2> |
| <p>At its core, a value of type <code>T</code> is dropped by executing its "drop |
| glue". Drop glue is compiler generated and first calls <code><T as Drop>::drop</code> and then recursively calls the drop glue of any recursively |
| owned values.</p> |
| <ul> |
| <li>If <code>T</code> has an explicit <code>Drop</code> impl, call <code><T as Drop>::drop</code>.</li> |
| <li>Regardless of whether <code>T</code> implements <code>Drop</code>, recurse into all values |
| <em>owned</em> by <code>T</code>: |
| <ul> |
| <li>references, raw pointers, function pointers, function items, trait |
| objects<sup class="footnote-reference" id="fr-traitobj-1"><a href="#footnote-traitobj">1</a></sup>, and scalars do not own anything.</li> |
| <li>tuples, slices, and arrays consider their elements to be owned. |
| For arrays of length zero we do not own any value of the element |
| type.</li> |
| <li>all fields (of all variants) of ADTs are considered owned. We |
| consider all variants for enums. The exception here is |
| <code>ManuallyDrop<U></code> which is not considered to own <code>U</code>. |
| <code>PhantomData<U></code> also does not own anything. |
| closures and generators own their captured upvars.</li> |
| </ul> |
| </li> |
| </ul> |
| <p>Whether a type has drop glue is returned by <a href="https://github.com/rust-lang/rust/blob/320b412f9c55bf480d26276ff0ab480e4ecb29c0/compiler/rustc_middle/src/ty/util.rs#L1086-L1108"><code>fn Ty::needs_drop</code></a>.</p> |
| <h3 id="partially-dropping-a-local"><a class="header" href="#partially-dropping-a-local">Partially dropping a local</a></h3> |
| <p>For types which do not implement <code>Drop</code> themselves, we can also |
| partially move parts of the value before dropping the rest. In this case |
| only the drop glue for the not-yet moved values is called, e.g.</p> |
| <pre><pre class="playground"><code class="language-rust">fn main() { |
| let mut x = (PrintOnDrop("third"), PrintOnDrop("first")); |
| drop(x.1); |
| println!("second") |
| }</code></pre></pre> |
| <p>During MIR building we assume that a local may get dropped whenever it |
| goes out of scope <em>as long as its type needs drop</em>. Computing the exact |
| drop glue for a variable happens <strong>after</strong> borrowck in the |
| <code>ElaborateDrops</code> pass. This means that even if some part of the local |
| have been dropped previously, dropck still requires this value to be |
| live. This is the case even if we completely moved a local.</p> |
| <pre><pre class="playground"><code class="language-rust">fn main() { |
| let mut x; |
| { |
| let temp = String::from("I am temporary"); |
| x = PrintOnDrop(&temp); |
| drop(x); |
| } |
| } //~ ERROR `temp` does not live long enough.</code></pre></pre> |
| <p>It should be possible to add some amount of drop elaboration before |
| borrowck, allowing this example to compile. There is an unstable feature |
| to move drop elaboration before const checking: |
| <a href="https://github.com/rust-lang/rust/issues/73255">#73255</a>. Such a feature |
| gate does not exist for doing some drop elaboration before borrowck, |
| although there's a <a href="https://github.com/rust-lang/compiler-team/issues/558">relevant |
| MCP</a>.</p> |
| <h3 id="dropck_outlives"><a class="header" href="#dropck_outlives"><code>dropck_outlives</code></a></h3> |
| <p>There are two distinct "liveness" computations that we perform:</p> |
| <ul> |
| <li>a value <code>v</code> is <em>use-live</em> at location <code>L</code> if it may be "used" later; a |
| <em>use</em> here is basically anything that is not a <em>drop</em></li> |
| <li>a value <code>v</code> is <em>drop-live</em> at location <code>L</code> if it maybe dropped later</li> |
| </ul> |
| <p>When things are <em>use-live</em>, their entire type must be valid at <code>L</code>. When |
| they are <em>drop-live</em>, all regions that are required by dropck must be |
| valid at <code>L</code>. The values dropped in the MIR are <em>places</em>.</p> |
| <p>The constraints computed by <code>dropck_outlives</code> for a type closely match |
| the generated drop glue for that type. Unlike drop glue, |
| <code>dropck_outlives</code> cares about the types of owned values, not the values |
| itself. For a value of type <code>T</code></p> |
| <ul> |
| <li>if <code>T</code> has an explicit <code>Drop</code>, require all generic arguments to be |
| live, unless they are marked with <code>#[may_dangle]</code> in which case they |
| are fully ignored</li> |
| <li>regardless of whether <code>T</code> has an explicit <code>Drop</code>, recurse into all |
| types <em>owned</em> by <code>T</code> |
| <ul> |
| <li>references, raw pointers, function pointers, function items, trait |
| objects<sup class="footnote-reference" id="fr-traitobj-2"><a href="#footnote-traitobj">1</a></sup>, and scalars do not own anything.</li> |
| <li>tuples, slices and arrays consider their element type to be owned. |
| <strong>For arrays we currently do not check whether their length is |
| zero</strong>.</li> |
| <li>all fields (of all variants) of ADTs are considered owned. The |
| exception here is <code>ManuallyDrop<U></code> which is not considered to own |
| <code>U</code>. <strong>We consider <code>PhantomData<U></code> to own <code>U</code></strong>.</li> |
| <li>closures and generators own their captured upvars.</li> |
| </ul> |
| </li> |
| </ul> |
| <p>The sections marked in bold are cases where <code>dropck_outlives</code> considers |
| types to be owned which are ignored by <code>Ty::needs_drop</code>. We only rely on |
| <code>dropck_outlives</code> if <code>Ty::needs_drop</code> for the containing local returned |
| <code>true</code>.This means liveness requirements can change depending on whether |
| a type is contained in a larger local. <strong>This is inconsistent, and |
| should be fixed: an example <a href="https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=8b5f5f005a03971b22edb1c20c5e6cbe">for |
| arrays</a> |
| and <a href="https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=44c6e2b1fae826329fd54c347603b6c8">for |
| <code>PhantomData</code></a>.</strong><sup class="footnote-reference" id="fr-core-1"><a href="#footnote-core">2</a></sup></p> |
| <p>One possible way these inconsistencies can be fixed is by MIR building |
| to be more pessimistic, probably by making <code>Ty::needs_drop</code> weaker, or |
| alternatively, changing <code>dropck_outlives</code> to be more precise, requiring |
| fewer regions to be live.</p> |
| <hr> |
| <ol class="footnote-definition"><li id="footnote-traitobj"> |
| <p>you can consider trait objects to have a builtin <code>Drop</code> |
| implementation which directly uses the <code>drop_in_place</code> provided by the |
| vtable. This <code>Drop</code> implementation requires all its generic parameters |
| to be live. <a href="#fr-traitobj-1">↩</a> <a href="#fr-traitobj-2">↩2</a></p> |
| </li> |
| <li id="footnote-core"> |
| <p>This is the core assumption of <a href="https://github.com/rust-lang/rust/issues/110288">#110288</a> and <a href="https://github.com/rust-lang/rfcs/pull/3417">RFC 3417</a>. <a href="#fr-core-1">↩</a></p> |
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