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| <h1 id="generic-parameter-definitions"><a class="header" href="#generic-parameter-definitions">Generic parameter definitions</a></h1> |
| <p>This chapter will discuss how rustc tracks what generic parameters are introduced. For example given some <code>struct Foo<T></code> how does rustc track that <code>Foo</code> defines some type parameter <code>T</code> (and no other generic parameters).</p> |
| <p>This will <em>not</em> cover how we track generic parameters introduced via <code>for<'a></code> syntax (e.g. in where clauses or <code>fn</code> types), which is covered elsewhere in the <a href="./ty_module/binders.html">chapter on <code>Binder</code>s </a>.</p> |
| <h1 id="tygenerics"><a class="header" href="#tygenerics"><code>ty::Generics</code></a></h1> |
| <p>The generic parameters introduced by an item are tracked by the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Generics.html"><code>ty::Generics</code></a> struct. Sometimes items allow usage of generics defined on parent items, this is accomplished via the <code>ty::Generics</code> struct having an optional field to specify a parent item to inherit generic parameters of. For example given the following code:</p> |
| <pre><code class="language-rust ignore">trait Trait<T> { |
| fn foo<U>(&self); |
| }</code></pre> |
| <p>The <code>ty::Generics</code> used for <code>foo</code> would contain <code>[U]</code> and a parent of <code>Some(Trait)</code>. <code>Trait</code> would have a <code>ty::Generics</code> containing <code>[Self, T]</code> with a parent of <code>None</code>.</p> |
| <p>The <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/generics/struct.GenericParamDef.html"><code>GenericParamDef</code></a> struct is used to represent each individual generic parameter in a <code>ty::Generics</code> listing. The <code>GenericParamDef</code> struct contains information about the generic parameter, for example its name, defid, what kind of parameter it is (i.e. type, const, lifetime).</p> |
| <p><code>GenericParamDef</code> also contains a <code>u32</code> index representing what position the parameter is (starting from the outermost parent), this is the value used to represent usages of generic parameters (more on this in the <a href="./ty.html">chapter on representing types</a>).</p> |
| <p>Interestingly, <code>ty::Generics</code> does not currently contain <em>every</em> generic parameter defined on an item. In the case of functions it only contains the <em>early bound</em> parameters.</p> |
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