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<main>
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<h1 id="two-phase-borrows"><a class="header" href="#two-phase-borrows">Two-phase borrows</a></h1>
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<p>Two-phase borrows are a more permissive version of mutable borrows that allow
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nested method calls such as <code>vec.push(vec.len())</code>. Such borrows first act as
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shared borrows in a "reservation" phase and can later be "activated" into a
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full mutable borrow.</p>
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<p>Only certain implicit mutable borrows can be two-phase, any <code>&mut</code> or <code>ref mut</code>
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in the source code is never a two-phase borrow. The cases where we generate a
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two-phase borrow are:</p>
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<ol>
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<li>The autoref borrow when calling a method with a mutable reference receiver.</li>
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<li>A mutable reborrow in function arguments.</li>
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<li>The implicit mutable borrow in an overloaded compound assignment operator.</li>
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</ol>
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<p>To give some examples:</p>
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<pre><pre class="playground"><code class="language-rust2018"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>// In the source code
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// Case 1:
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let mut v = Vec::new();
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v.push(v.len());
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let r = &mut Vec::new();
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r.push(r.len());
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// Case 2:
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std::mem::replace(r, vec![1, r.len()]);
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// Case 3:
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let mut x = std::num::Wrapping(2);
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x += x;
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<span class="boring">}</span></code></pre></pre>
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<p>Expanding these enough to show the two-phase borrows:</p>
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<pre><code class="language-rust ignore">// Case 1:
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let mut v = Vec::new();
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let temp1 = &two_phase v;
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let temp2 = v.len();
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Vec::push(temp1, temp2);
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let r = &mut Vec::new();
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let temp3 = &two_phase *r;
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let temp4 = r.len();
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Vec::push(temp3, temp4);
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// Case 2:
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let temp5 = &two_phase *r;
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let temp6 = vec![1, r.len()];
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std::mem::replace(temp5, temp6);
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// Case 3:
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let mut x = std::num::Wrapping(2);
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let temp7 = &two_phase x;
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let temp8 = x;
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std::ops::AddAssign::add_assign(temp7, temp8);</code></pre>
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<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>
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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
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construction.</p>
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<p>Each two-phase borrow is assigned to a temporary that is only used once. As
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such we can define:</p>
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<ul>
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<li>The point where the temporary is assigned to is called the <em>reservation</em>
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point of the two-phase borrow.</li>
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<li>The point where the temporary is used, which is effectively always a
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function call, is called the <em>activation</em> point.</li>
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</ul>
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<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
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<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
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borrow.</p>
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<h2 id="checking-two-phase-borrows"><a class="header" href="#checking-two-phase-borrows">Checking two-phase borrows</a></h2>
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<p>Two-phase borrows are treated as if they were mutable borrows with the
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following exceptions:</p>
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<ol>
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<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
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activated at this location. If a live two phase borrow is activated at a
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location, then we check that there are no borrows that conflict with the
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two-phase borrow.</li>
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<li>At the reservation point we error if there are conflicting live <em>mutable</em>
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borrows. And lint if there are any conflicting shared borrows.</li>
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<li>Between the reservation and the activation point, the two-phase borrow acts
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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
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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>
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<li>After the activation point, the two-phase borrow acts as a mutable borrow.</li>
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</ol>
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