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<main>
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<h1 id="mir-borrow-check"><a class="header" href="#mir-borrow-check">MIR borrow check</a></h1>
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<p>The borrow check is Rust's "secret sauce" – it is tasked with
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enforcing a number of properties:</p>
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<ul>
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<li>That all variables are initialized before they are used.</li>
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<li>That you can't move the same value twice.</li>
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<li>That you can't move a value while it is borrowed.</li>
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<li>That you can't access a place while it is mutably borrowed (except through
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the reference).</li>
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<li>That you can't mutate a place while it is immutably borrowed.</li>
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<li>etc</li>
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</ul>
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<p>The borrow checker operates on the MIR. An older implementation operated on the
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HIR. Doing borrow checking on MIR has several advantages:</p>
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<ul>
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<li>The MIR is <em>far</em> less complex than the HIR; the radical desugaring
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helps prevent bugs in the borrow checker. (If you're curious, you
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can see
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<a href="https://github.com/rust-lang/rust/issues/47366">a list of bugs that the MIR-based borrow checker fixes here</a>.)</li>
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<li>Even more importantly, using the MIR enables <a href="https://rust-lang.github.io/rfcs/2094-nll.html">"non-lexical lifetimes"</a>,
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which are regions derived from the control-flow graph.</li>
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</ul>
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<h3 id="major-phases-of-the-borrow-checker"><a class="header" href="#major-phases-of-the-borrow-checker">Major phases of the borrow checker</a></h3>
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<p>The borrow checker source is found in
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<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/index.html">the <code>rustc_borrowck</code> crate</a>. The main entry point is
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the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/fn.mir_borrowck.html"><code>mir_borrowck</code></a> query.</p>
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<ul>
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<li>We first create a <strong>local copy</strong> of the MIR. In the coming steps,
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we will modify this copy in place to modify the types and things to
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include references to the new regions that we are computing.</li>
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<li>We then invoke <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/nll/fn.replace_regions_in_mir.html"><code>replace_regions_in_mir</code></a> to modify our local MIR.
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Among other things, this function will replace all of the <a href="./appendix/glossary.html#region">regions</a>
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in the MIR with fresh <a href="./appendix/glossary.html#inf-var">inference variables</a>.</li>
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<li>Next, we perform a number of
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<a href="./appendix/background.html#dataflow">dataflow analyses</a> that
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compute what data is moved and when.</li>
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<li>We then do a <a href="borrow_check/type_check.html">second type check</a> across the MIR:
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the purpose of this type check is to determine all of the constraints between
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different regions.</li>
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<li>Next, we do <a href="borrow_check/region_inference.html">region inference</a>, which computes
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the values of each region — basically, the points in the control-flow graph where
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each lifetime must be valid according to the constraints we collected.</li>
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<li>At this point, we can compute the "borrows in scope" at each point.</li>
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<li>Finally, we do a second walk over the MIR, looking at the actions it
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does and reporting errors. For example, if we see a statement like
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<code>*a + 1</code>, then we would check that the variable <code>a</code> is initialized
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and that it is not mutably borrowed, as either of those would
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require an error to be reported. Doing this check requires the results of all
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the previous analyses.</li>
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