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| <h1 id="closure-capture-inference"><a class="header" href="#closure-capture-inference">Closure Capture Inference</a></h1> |
| <p>This section describes how rustc handles closures. Closures in Rust are |
| effectively “desugared” into structs that contain the values they use (or |
| references to the values they use) from their creator’s stack frame. rustc has |
| the job of figuring out which values a closure uses and how, so it can decide |
| whether to capture a given variable by shared reference, mutable reference, or |
| by move. rustc also has to figure out which of the closure traits (<a href="https://doc.rust-lang.org/std/ops/trait.Fn.html"><code>Fn</code></a>, |
| <a href="https://doc.rust-lang.org/std/ops/trait.FnMut.html"><code>FnMut</code></a>, or <a href="https://doc.rust-lang.org/std/ops/trait.FnOnce.html"><code>FnOnce</code></a>) a closure is capable of |
| implementing.</p> |
| <p>Let’s start with a few examples:</p> |
| <h3 id="example-1"><a class="header" href="#example-1">Example 1</a></h3> |
| <p>To start, let’s take a look at how the closure in the following example is desugared:</p> |
| <pre class="playground"><code class="language-rust">fn closure(f: impl Fn()) { |
| f(); |
| } |
| |
| fn main() { |
| let x: i32 = 10; |
| closure(|| println!("Hi {}", x)); // The closure just reads x. |
| println!("Value of x after return {}", x); |
| }</code></pre> |
| <p>Let’s say the above is the content of a file called <code>immut.rs</code>. If we compile |
| <code>immut.rs</code> using the following command. The <a href="./mir/passes.html"><code>-Z dump-mir=all</code></a> flag will cause |
| <code>rustc</code> to generate and dump the <a href="./mir/index.html">MIR</a> to a directory called <code>mir_dump</code>.</p> |
| <pre><code class="language-console">> rustc +stage1 immut.rs -Z dump-mir=all |
| </code></pre> |
| <p>After we run this command, we will see a newly generated directory in our |
| current working directory called <code>mir_dump</code>, which will contain several files. |
| If we look at file <code>rustc.main.-------.mir_map.0.mir</code>, we will find, among |
| other things, it also contains this line:</p> |
| <pre><code class="language-rust ignore">_4 = &_1; |
| _3 = [closure@immut.rs:7:13: 7:36] { x: move _4 };</code></pre> |
| <p>Note that in the MIR examples in this chapter, <code>_1</code> is <code>x</code>.</p> |
| <p>Here in first line <code>_4 = &_1;</code>, the <code>mir_dump</code> tells us that <code>x</code> was borrowed |
| as an immutable reference. This is what we would hope as our closure just |
| reads <code>x</code>.</p> |
| <h3 id="example-2"><a class="header" href="#example-2">Example 2</a></h3> |
| <p>Here is another example:</p> |
| <pre class="playground"><code class="language-rust">fn closure(mut f: impl FnMut()) { |
| f(); |
| } |
| |
| fn main() { |
| let mut x: i32 = 10; |
| closure(|| { |
| x += 10; // The closure mutates the value of x |
| println!("Hi {}", x) |
| }); |
| println!("Value of x after return {}", x); |
| }</code></pre> |
| <pre><code class="language-rust ignore">_4 = &mut _1; |
| _3 = [closure@mut.rs:7:13: 10:6] { x: move _4 };</code></pre> |
| <p>This time along, in the line <code>_4 = &mut _1;</code>, we see that the borrow is changed to mutable borrow. |
| Fair enough! The closure increments <code>x</code> by 10.</p> |
| <h3 id="example-3"><a class="header" href="#example-3">Example 3</a></h3> |
| <p>One more example:</p> |
| <pre class="playground"><code class="language-rust">fn closure(f: impl FnOnce()) { |
| f(); |
| } |
| |
| fn main() { |
| let x = vec![21]; |
| closure(|| { |
| drop(x); // Makes x unusable after the fact. |
| }); |
| // println!("Value of x after return {:?}", x); |
| }</code></pre> |
| <pre><code class="language-rust ignore">_6 = [closure@move.rs:7:13: 9:6] { x: move _1 }; // bb16[3]: scope 1 at move.rs:7:13: 9:6</code></pre> |
| <p>Here, <code>x</code> is directly moved into the closure and the access to it will not be permitted after the |
| closure.</p> |
| <h2 id="inferences-in-the-compiler"><a class="header" href="#inferences-in-the-compiler">Inferences in the compiler</a></h2> |
| <p>Now let’s dive into rustc code and see how all these inferences are done by the compiler.</p> |
| <p>Let’s start with defining a term that we will be using quite a bit in the rest of the discussion - |
| <em>upvar</em>. An <strong>upvar</strong> is a variable that is local to the function where the closure is defined. So, |
| in the above examples, <strong>x</strong> will be an upvar to the closure. They are also sometimes referred to as |
| the <em>free variables</em> meaning they are not bound to the context of the closure. |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_passes/upvars/index.html"><code>compiler/rustc_passes/src/upvars.rs</code></a> defines a query called <em>upvars_mentioned</em> |
| for this purpose.</p> |
| <p>Other than lazy invocation, one other thing that distinguishes a closure from a |
| normal function is that it can use the upvars. It borrows these upvars from its surrounding |
| context; therefore the compiler has to determine the upvar’s borrow type. The compiler starts with |
| assigning an immutable borrow type and lowers the restriction (that is, changes it from |
| <strong>immutable</strong> to <strong>mutable</strong> to <strong>move</strong>) as needed, based on the usage. In the Example 1 above, the |
| closure only uses the variable for printing but does not modify it in any way and therefore, in the |
| <code>mir_dump</code>, we find the borrow type for the upvar <code>x</code> to be immutable. In example 2, however, the |
| closure modifies <code>x</code> and increments it by some value. Because of this mutation, the compiler, which |
| started off assigning <code>x</code> as an immutable reference type, has to adjust it as a mutable reference. |
| Likewise in the third example, the closure drops the vector and therefore this requires the variable |
| <code>x</code> to be moved into the closure. Depending on the borrow kind, the closure has to implement the |
| appropriate trait: <code>Fn</code> trait for immutable borrow, <code>FnMut</code> for mutable borrow, |
| and <code>FnOnce</code> for move semantics.</p> |
| <p>Most of the code related to the closure is in the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_typeck/upvar/index.html"><code>compiler/rustc_hir_typeck/src/upvar.rs</code></a> file and the data structures are |
| declared in the file <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/index.html"><code>compiler/rustc_middle/src/ty/mod.rs</code></a>.</p> |
| <p>Before we go any further, let’s discuss how we can examine the flow of control through the rustc |
| codebase. For closures specifically, set the <code>RUSTC_LOG</code> env variable as below and collect the |
| output in a file:</p> |
| <pre><code class="language-console">> RUSTC_LOG=rustc_hir_typeck::upvar rustc +stage1 -Z dump-mir=all \ |
| <.rs file to compile> 2> <file where the output will be dumped> |
| </code></pre> |
| <p>This uses the stage1 compiler and enables <code>debug!</code> logging for the |
| <code>rustc_hir_typeck::upvar</code> module.</p> |
| <p>The other option is to step through the code using lldb or gdb.</p> |
| <ol> |
| <li><code>rust-lldb build/host/stage1/bin/rustc test.rs</code></li> |
| <li>In lldb: |
| <ol> |
| <li><code>b upvar.rs:134</code> // Setting the breakpoint on a certain line in the upvar.rs file</li> |
| <li><code>r</code> // Run the program until it hits the breakpoint</li> |
| </ol> |
| </li> |
| </ol> |
| <p>Let’s start with <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_typeck/upvar/index.html"><code>upvar.rs</code></a>. This file has something called |
| the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_typeck/expr_use_visitor/struct.ExprUseVisitor.html"><code>euv::ExprUseVisitor</code></a> which walks the source of the closure and |
| invokes a callback for each upvar that is borrowed, mutated, or moved.</p> |
| <pre class="playground"><code class="language-rust">fn main() { |
| let mut x = vec![21]; |
| let _cl = || { |
| let y = x[0]; // 1. |
| x[0] += 1; // 2. |
| }; |
| }</code></pre> |
| <p>In the above example, our visitor will be called twice, for the lines marked 1 and 2, once for a |
| shared borrow and another one for a mutable borrow. It will also tell us what was borrowed.</p> |
| <p>The callbacks are defined by implementing the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_typeck/expr_use_visitor/trait.Delegate.html"><code>Delegate</code></a> trait. The |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_typeck/upvar/struct.InferBorrowKind.html"><code>InferBorrowKind</code></a> type implements <code>Delegate</code> and keeps a map that |
| records for each upvar which mode of capture was required. The modes of capture |
| can be <code>ByValue</code> (moved) or <code>ByRef</code> (borrowed). For <code>ByRef</code> borrows, the possible |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.BorrowKind.html"><code>BorrowKind</code></a>s are <code>ImmBorrow</code>, <code>UniqueImmBorrow</code>, <code>MutBorrow</code> as defined in the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/index.html"><code>compiler/rustc_middle/src/ty/mod.rs</code></a>.</p> |
| <p><code>Delegate</code> defines a few different methods (the different callbacks): |
| <strong>consume</strong> for <em>move</em> of a variable, <strong>borrow</strong> for a <em>borrow</em> of some kind |
| (shared or mutable), and <strong>mutate</strong> when we see an <em>assignment</em> of something.</p> |
| <p>All of these callbacks have a common argument <em>cmt</em> which stands for Category, |
| Mutability and Type and is defined in |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_typeck/expr_use_visitor/index.html"><code>compiler/rustc_hir_typeck/src/expr_use_visitor.rs</code></a>. Borrowing from the code |
| comments, “<code>cmt</code> is a complete categorization of a value indicating where it |
| originated and how it is located, as well as the mutability of the memory in |
| which the value is stored”. Based on the callback (consume, borrow etc.), we |
| will call the relevant <code>adjust_upvar_borrow_kind_for_<something></code> and pass the |
| <code>cmt</code> along. Once the borrow type is adjusted, we store it in the table, which |
| basically says what borrows were made for each closure.</p> |
| <pre><code class="language-rust ignore">self.tables |
| .borrow_mut() |
| .upvar_capture_map |
| .extend(delegate.adjust_upvar_captures);</code></pre> |
| |
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