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| <h1 id="two-phase-borrows"><a class="header" href="#two-phase-borrows">Two-phase borrows</a></h1> |
| <p>Two-phase borrows are a more permissive version of mutable borrows that allow |
| nested method calls such as <code>vec.push(vec.len())</code>. Such borrows first act as |
| shared borrows in a "reservation" phase and can later be "activated" into a |
| full mutable borrow.</p> |
| <p>Only certain implicit mutable borrows can be two-phase, any <code>&mut</code> or <code>ref mut</code> |
| in the source code is never a two-phase borrow. The cases where we generate a |
| two-phase borrow are:</p> |
| <ol> |
| <li>The autoref borrow when calling a method with a mutable reference receiver.</li> |
| <li>A mutable reborrow in function arguments.</li> |
| <li>The implicit mutable borrow in an overloaded compound assignment operator.</li> |
| </ol> |
| <p>To give some examples:</p> |
| <pre><pre class="playground"><code class="language-rust edition2018"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>// In the source code |
| |
| // Case 1: |
| let mut v = Vec::new(); |
| v.push(v.len()); |
| let r = &mut Vec::new(); |
| r.push(r.len()); |
| |
| // Case 2: |
| std::mem::replace(r, vec![1, r.len()]); |
| |
| // Case 3: |
| let mut x = std::num::Wrapping(2); |
| x += x; |
| <span class="boring">}</span></code></pre></pre> |
| <p>Expanding these enough to show the two-phase borrows:</p> |
| <pre><code class="language-rust ignore">// Case 1: |
| let mut v = Vec::new(); |
| let temp1 = &two_phase v; |
| let temp2 = v.len(); |
| Vec::push(temp1, temp2); |
| let r = &mut Vec::new(); |
| let temp3 = &two_phase *r; |
| let temp4 = r.len(); |
| Vec::push(temp3, temp4); |
| |
| // Case 2: |
| let temp5 = &two_phase *r; |
| let temp6 = vec![1, r.len()]; |
| std::mem::replace(temp5, temp6); |
| |
| // Case 3: |
| let mut x = std::num::Wrapping(2); |
| let temp7 = &two_phase x; |
| let temp8 = x; |
| std::ops::AddAssign::add_assign(temp7, temp8);</code></pre> |
| <p>Whether a borrow can be two-phase is tracked by a flag on the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/adjustment/enum.AutoBorrow.html"><code>AutoBorrow</code></a> |
| after type checking, which is then <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_build/thir/cx/expr/trait.ToBorrowKind.html#method.to_borrow_kind">converted</a> to a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/enum.BorrowKind.html"><code>BorrowKind</code></a> during MIR |
| construction.</p> |
| <p>Each two-phase borrow is assigned to a temporary that is only used once. As |
| such we can define:</p> |
| <ul> |
| <li>The point where the temporary is assigned to is called the <em>reservation</em> |
| point of the two-phase borrow.</li> |
| <li>The point where the temporary is used, which is effectively always a |
| function call, is called the <em>activation</em> point.</li> |
| </ul> |
| <p>The activation points are found using the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/borrow_set/struct.GatherBorrows.html"><code>GatherBorrows</code></a> visitor. The |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/borrow_set/struct.BorrowData.html"><code>BorrowData</code></a> then holds both the reservation and activation points for the |
| borrow.</p> |
| <h2 id="checking-two-phase-borrows"><a class="header" href="#checking-two-phase-borrows">Checking two-phase borrows</a></h2> |
| <p>Two-phase borrows are treated as if they were mutable borrows with the |
| following exceptions:</p> |
| <ol> |
| <li>At every location in the MIR we <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/struct.MirBorrowckCtxt.html#method.check_activations">check</a> if any two-phase borrows are |
| activated at this location. If a live two phase borrow is activated at a |
| location, then we check that there are no borrows that conflict with the |
| two-phase borrow.</li> |
| <li>At the reservation point we error if there are conflicting live <em>mutable</em> |
| borrows. And lint if there are any conflicting shared borrows.</li> |
| <li>Between the reservation and the activation point, the two-phase borrow acts |
| as a shared borrow. We determine (in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/path_utils/fn.is_active.html"><code>is_active</code></a>) if we're at such a point |
| by using the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_data_structures/graph/dominators/struct.Dominators.html"><code>Dominators</code></a> for the MIR graph.</li> |
| <li>After the activation point, the two-phase borrow acts as a mutable borrow.</li> |
| </ol> |
| |
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