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<main>
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<h1 id="mir-visitor"><a class="header" href="#mir-visitor">MIR visitor</a></h1>
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<p>The MIR visitor is a convenient tool for traversing the MIR and either
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looking for things or making changes to it. The visitor traits are
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defined in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/visit/index.html">the <code>rustc_middle::mir::visit</code> module</a> – there are two of
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them, generated via a single macro: <code>Visitor</code> (which operates on a
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<code>&Mir</code> and gives back shared references) and <code>MutVisitor</code> (which
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operates on a <code>&mut Mir</code> and gives back mutable references).</p>
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<p>To implement a visitor, you have to create a type that represents
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your visitor. Typically, this type wants to "hang on" to whatever
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state you will need while processing MIR:</p>
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<pre><code class="language-rust ignore">struct MyVisitor<...> {
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tcx: TyCtxt<'tcx>,
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...
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}</code></pre>
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<p>and you then implement the <code>Visitor</code> or <code>MutVisitor</code> trait for that type:</p>
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<pre><code class="language-rust ignore">impl<'tcx> MutVisitor<'tcx> for MyVisitor {
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fn visit_foo(&mut self, ...) {
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...
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self.super_foo(...);
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}
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}</code></pre>
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<p>As shown above, within the impl, you can override any of the
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<code>visit_foo</code> methods (e.g., <code>visit_terminator</code>) in order to write some
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code that will execute whenever a <code>foo</code> is found. If you want to
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recursively walk the contents of the <code>foo</code>, you then invoke the
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<code>super_foo</code> method. (NB. You never want to override <code>super_foo</code>.)</p>
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<p>A very simple example of a visitor can be found in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_transform/prettify/struct.LocalFinder.html"><code>LocalFinder</code></a>.
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By implementing <code>visit_local</code> method, this visitor identifies local variables that
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can be candidates for reordering.</p>
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<h2 id="traversal"><a class="header" href="#traversal">Traversal</a></h2>
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<p>In addition the visitor, <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/traversal/index.html">the <code>rustc_middle::mir::traversal</code> module</a>
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contains useful functions for walking the MIR CFG in
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<a href="https://en.wikipedia.org/wiki/Tree_traversal">different standard orders</a> (e.g. pre-order, reverse
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post-order, and so forth).</p>
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