401 lines
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401 lines
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<h1 id="pattern-and-exhaustiveness-checking"><a class="header" href="#pattern-and-exhaustiveness-checking">Pattern and exhaustiveness checking</a></h1>
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<p>In Rust, pattern matching and bindings have a few very helpful properties. The
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compiler will check that bindings are irrefutable when made and that match arms
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are exhaustive.</p>
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<h2 id="pattern-usefulness"><a class="header" href="#pattern-usefulness">Pattern usefulness</a></h2>
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<p>The central question that usefulness checking answers is:
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"in this match expression, is that branch redundant?".
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More precisely, it boils down to computing whether,
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given a list of patterns we have already seen,
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a given new pattern might match any new value.</p>
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<p>For example, in the following match expression,
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we ask in turn whether each pattern might match something
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that wasn't matched by the patterns above it.
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Here we see the 4th pattern is redundant with the 1st;
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that branch will get an "unreachable" warning.
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The 3rd pattern may or may not be useful,
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depending on whether <code>Foo</code> has other variants than <code>Bar</code>.
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Finally, we can ask whether the whole match is exhaustive
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by asking whether the wildcard pattern (<code>_</code>)
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is useful relative to the list of all the patterns in that match.
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Here we can see that <code>_</code> is useful (it would catch <code>(false, None)</code>);
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this expression would therefore get a "non-exhaustive match" error.</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>// x: (bool, Option<Foo>)
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match x {
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(true, _) => {} // 1
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(false, Some(Foo::Bar)) => {} // 2
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(false, Some(_)) => {} // 3
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(true, None) => {} // 4
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}
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<span class="boring">}</span></code></pre></pre>
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<p>Thus usefulness is used for two purposes:
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detecting unreachable code (which is useful to the user),
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and ensuring that matches are exhaustive (which is important for soundness,
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because a match expression can return a value).</p>
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<h2 id="where-it-happens"><a class="header" href="#where-it-happens">Where it happens</a></h2>
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<p>This check is done anywhere you can write a pattern: <code>match</code> expressions, <code>if let</code>, <code>let else</code>,
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plain <code>let</code>, and function arguments.</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>// `match`
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// Usefulness can detect unreachable branches and forbid non-exhaustive matches.
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match foo() {
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Ok(x) => x,
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Err(_) => panic!(),
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}
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// `if let`
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// Usefulness can detect unreachable branches.
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if let Some(x) = foo() {
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// ...
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}
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// `while let`
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// Usefulness can detect infinite loops and dead loops.
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while let Some(x) = it.next() {
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// ...
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}
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// Destructuring `let`
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// Usefulness can forbid non-exhaustive patterns.
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let Foo::Bar(x, y) = foo();
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// Destructuring function arguments
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// Usefulness can forbid non-exhaustive patterns.
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fn foo(Foo { x, y }: Foo) {
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// ...
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}
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<span class="boring">}</span></code></pre></pre>
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<h2 id="the-algorithm"><a class="header" href="#the-algorithm">The algorithm</a></h2>
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<p>Exhaustiveness checking is run before MIR building in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_build/thir/pattern/check_match/index.html"><code>check_match</code></a>.
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It is implemented in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_pattern_analysis/index.html"><code>rustc_pattern_analysis</code></a> crate,
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with the core of the algorithm in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_pattern_analysis/usefulness/index.html"><code>usefulness</code></a> module.
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That file contains a detailed description of the algorithm.</p>
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<h2 id="important-concepts"><a class="header" href="#important-concepts">Important concepts</a></h2>
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<h3 id="constructors-and-fields"><a class="header" href="#constructors-and-fields">Constructors and fields</a></h3>
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<p>In the value <code>Pair(Some(0), true)</code>, <code>Pair</code> is called the constructor of the value, and <code>Some(0)</code> and
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<code>true</code> are its fields. Every matchable value can be decomposed in this way. Examples of
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constructors are: <code>Some</code>, <code>None</code>, <code>(,)</code> (the 2-tuple constructor), <code>Foo {..}</code> (the constructor for
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a struct <code>Foo</code>), and <code>2</code> (the constructor for the number <code>2</code>).</p>
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<p>Each constructor takes a fixed number of fields; this is called its arity. <code>Pair</code> and <code>(,)</code> have
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arity 2, <code>Some</code> has arity 1, <code>None</code> and <code>42</code> have arity 0. Each type has a known set of
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constructors. Some types have many constructors (like <code>u64</code>) or even an infinitely many (like <code>&str</code>
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and <code>&[T]</code>).</p>
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<p>Patterns are similar: <code>Pair(Some(_), _)</code> has constructor <code>Pair</code> and two fields. The difference is
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that we get some extra pattern-only constructors, namely: the wildcard <code>_</code>, variable bindings,
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integer ranges like <code>0..=10</code>, and variable-length slices like <code>[_, .., _]</code>. We treat or-patterns
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separately.</p>
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<p>Now to check if a value <code>v</code> matches a pattern <code>p</code>, we check if <code>v</code>'s constructor matches <code>p</code>'s
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constructor, then recursively compare their fields if necessary. A few representative examples:</p>
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<ul>
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<li><code>matches!(v, _) := true</code></li>
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<li><code>matches!((v0, v1), (p0, p1)) := matches!(v0, p0) && matches!(v1, p1)</code></li>
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<li><code>matches!(Foo { a: v0, b: v1 }, Foo { a: p0, b: p1 }) := matches!(v0, p0) && matches!(v1, p1)</code></li>
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<li><code>matches!(Ok(v0), Ok(p0)) := matches!(v0, p0)</code></li>
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<li><code>matches!(Ok(v0), Err(p0)) := false</code> (incompatible variants)</li>
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<li><code>matches!(v, 1..=100) := matches!(v, 1) || ... || matches!(v, 100)</code></li>
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<li><code>matches!([v0], [p0, .., p1]) := false</code> (incompatible lengths)</li>
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<li><code>matches!([v0, v1, v2], [p0, .., p1]) := matches!(v0, p0) && matches!(v2, p1)</code></li>
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</ul>
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<p>This concept is absolutely central to pattern analysis. The <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_pattern_analysis/constructor/index.html"><code>constructor</code></a> module provides
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functions to extract, list and manipulate constructors. This is a useful enough concept that
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variations of it can be found in other places of the compiler, like in the MIR-lowering of a match
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expression and in some clippy lints.</p>
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<h3 id="constructor-grouping-and-splitting"><a class="header" href="#constructor-grouping-and-splitting">Constructor grouping and splitting</a></h3>
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<p>The pattern-only constructors (<code>_</code>, ranges and variable-length slices) each stand for a set of
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normal constructors, e.g. <code>_: Option<T></code> stands for the set {<code>None</code>, <code>Some</code>} and <code>[_, .., _]</code> stands
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for the infinite set {<code>[,]</code>, <code>[,,]</code>, <code>[,,,]</code>, ...} of the slice constructors of arity >= 2.</p>
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<p>In order to manage these constructors, we keep them as grouped as possible. For example:</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>match (0, false) {
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(0 ..=100, true) => {}
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(50..=150, false) => {}
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(0 ..=200, _) => {}
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}
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<span class="boring">}</span></code></pre></pre>
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<p>In this example, all of <code>0</code>, <code>1</code>, .., <code>49</code> match the same arms, and thus can be treated as a group.
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In fact, in this match, the only ranges we need to consider are: <code>0..50</code>, <code>50..=100</code>,
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<code>101..=150</code>,<code>151..=200</code> and <code>201..</code>. Similarly:</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>enum Direction { North, South, East, West }
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<span class="boring">let wind = (Direction::North, 0u8);
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</span>match wind {
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(Direction::North, 50..) => {}
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(_, _) => {}
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}
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<span class="boring">}</span></code></pre></pre>
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<p>Here we can treat all the non-<code>North</code> constructors as a group, giving us only two cases to handle:
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<code>North</code>, and everything else.</p>
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<p>This is called "constructor splitting" and is crucial to having exhaustiveness run in reasonable
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time.</p>
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<h3 id="usefulness-vs-reachability-in-the-presence-of-empty-types"><a class="header" href="#usefulness-vs-reachability-in-the-presence-of-empty-types">Usefulness vs reachability in the presence of empty types</a></h3>
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<p>This is likely the subtlest aspect of exhaustiveness. To be fully precise, a match doesn't operate
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on a value, it operates on a place. In certain unsafe circumstances, it is possible for a place to
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not contain valid data for its type. This has subtle consequences for empty types. Take the
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following:</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>enum Void {}
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let x: u8 = 0;
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let ptr: *const Void = &x as *const u8 as *const Void;
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unsafe {
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match *ptr {
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_ => println!("Reachable!"),
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}
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}
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<span class="boring">}</span></code></pre></pre>
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<p>In this example, <code>ptr</code> is a valid pointer pointing to a place with invalid data. The <code>_</code> pattern
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does not look at the contents of the place <code>*ptr</code>, so this code is ok and the arm is taken. In other
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words, despite the place we are inspecting being of type <code>Void</code>, there is a reachable arm. If the
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arm had a binding however:</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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|
</span><span class="boring">fn main() {
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</span><span class="boring">#[derive(Copy, Clone)]
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</span><span class="boring">enum Void {}
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</span><span class="boring">let x: u8 = 0;
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</span><span class="boring">let ptr: *const Void = &x as *const u8 as *const Void;
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</span><span class="boring">unsafe {
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</span>match *ptr {
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_a => println!("Unreachable!"),
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}
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<span class="boring">}
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</span><span class="boring">}</span></code></pre></pre>
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<p>Here the binding loads the value of type <code>Void</code> from the <code>*ptr</code> place. In this example, this causes
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UB since the data is not valid. In the general case, this asserts validity of the data at <code>*ptr</code>.
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Either way, this arm will never be taken.</p>
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<p>Finally, let's consider the empty match <code>match *ptr {}</code>. If we consider this exhaustive, then
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having invalid data at <code>*ptr</code> is invalid. In other words, the empty match is semantically
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equivalent to the <code>_a => ...</code> match. In the interest of explicitness, we prefer the case with an
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arm, hence we won't tell the user to remove the <code>_a</code> arm. In other words, the <code>_a</code> arm is
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unreachable yet not redundant. This is why we lint on redundant arms rather than unreachable
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arms, despite the fact that the lint says "unreachable".</p>
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<p>These considerations only affects certain places, namely those that can contain non-valid data
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without UB. These are: pointer dereferences, reference dereferences, and union field accesses. We
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track during exhaustiveness checking whether a given place is known to contain valid data.</p>
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<p>Having said all that, the current implementation of exhaustiveness checking does not follow the
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above considerations. On stable, empty types are for the most part treated as non-empty. The
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<a href="https://github.com/rust-lang/rust/issues/51085"><code>exhaustive_patterns</code></a> feature errs on the other end: it allows omitting arms that could be
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reachable in unsafe situations. The <a href="https://github.com/rust-lang/rust/issues/118155"><code>never_patterns</code></a> experimental feature aims to fix this and
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permit the correct behavior of empty types in patterns.</p>
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</main>
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