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<h1 id="type-checking"><a class="header" href="#type-checking">Type checking</a></h1>
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<p>The <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_analysis/index.html"><code>hir_analysis</code></a> crate contains the source for "type collection" as well
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as a bunch of related functionality.
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Checking the bodies of functions is implemented in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_typeck/index.html"><code>hir_typeck</code></a> crate.
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These crates draw heavily on the <a href="./type-inference.html">type inference</a> and <a href="./traits/resolution.html">trait solving</a>.</p>
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<h2 id="type-collection"><a class="header" href="#type-collection">Type collection</a></h2>
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<p>Type "collection" is the process of converting the types found in the HIR
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(<code>hir::Ty</code>), which represent the syntactic things that the user wrote, into the
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<strong>internal representation</strong> used by the compiler (<code>Ty<'tcx></code>) – we also do
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similar conversions for where-clauses and other bits of the function signature.</p>
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<p>To try and get a sense of the difference, consider this function:</p>
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<pre><code class="language-rust ignore">struct Foo { }
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fn foo(x: Foo, y: self::Foo) { ... }
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// ^^^ ^^^^^^^^^</code></pre>
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<p>Those two parameters <code>x</code> and <code>y</code> each have the same type: but they will have
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distinct <code>hir::Ty</code> nodes. Those nodes will have different spans, and of course
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they encode the path somewhat differently. But once they are "collected" into
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<code>Ty<'tcx></code> nodes, they will be represented by the exact same internal type.</p>
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<p>Collection is defined as a bundle of <a href="./query.html">queries</a> for computing information about
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the various functions, traits, and other items in the crate being compiled.
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Note that each of these queries is concerned with <em>interprocedural</em> things –
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for example, for a function definition, collection will figure out the type and
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signature of the function, but it will not visit the <em>body</em> of the function in
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any way, nor examine type annotations on local variables (that's the job of
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type <em>checking</em>).</p>
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<p>For more details, see the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_analysis/collect/index.html"><code>collect</code></a> module.</p>
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<p><strong>TODO</strong>: actually talk about type checking... <a href="https://github.com/rust-lang/rustc-dev-guide/issues/1161">#1161</a></p>
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