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<h1 id="adts-and-generic-arguments"><a class="header" href="#adts-and-generic-arguments">ADTs and Generic Arguments</a></h1>
<p>The term <code>ADT</code> stands for “Algebraic data type”, in rust this refers to a struct, enum, or union.</p>
<h2 id="adts-representation"><a class="header" href="#adts-representation">ADTs Representation</a></h2>
<p>Let’s consider the example of a type like <code>MyStruct&lt;u32&gt;</code>, where <code>MyStruct</code> is defined like so:</p>
<pre><code class="language-rust ignore">struct MyStruct&lt;T&gt; { x: u8, y: T }</code></pre>
<p>The type <code>MyStruct&lt;u32&gt;</code> would be an instance of <code>TyKind::Adt</code>:</p>
<pre><code class="language-rust ignore">Adt(&amp;'tcx AdtDef, GenericArgs&lt;'tcx&gt;)
// ------------ ---------------
// (1) (2)
//
// (1) represents the `MyStruct` part
// (2) represents the `&lt;u32&gt;`, or "substitutions" / generic arguments</code></pre>
<p>There are two parts:</p>
<ul>
<li>The <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.AdtDef.html"><code>AdtDef</code></a> references the struct/enum/union but without the values for its type
parameters. In our example, this is the <code>MyStruct</code> part <em>without</em> the argument <code>u32</code>.
(Note that in the HIR, structs, enums and unions are represented differently, but in <code>ty::Ty</code>,
they are all represented using <code>TyKind::Adt</code>.)</li>
<li>The <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/type.GenericArgs.html"><code>GenericArgs</code></a> is a list of values that are to be substituted
for the generic parameters. In our example of <code>MyStruct&lt;u32&gt;</code>, we would end up with a list like
<code>[u32]</code>. We’ll dig more into generics and substitutions in a little bit.</li>
</ul>
<h3 id="adtdef-and-defid"><a class="header" href="#adtdef-and-defid"><strong><code>AdtDef</code> and <code>DefId</code></strong></a></h3>
<p>For every type defined in the source code, there is a unique <code>DefId</code> (see <a href="../hir.html#identifiers-in-the-hir">this
chapter</a>). This includes ADTs and generics. In the <code>MyStruct&lt;T&gt;</code>
definition we gave above, there are two <code>DefId</code>s: one for <code>MyStruct</code> and one for <code>T</code>. Notice that
the code above does not generate a new <code>DefId</code> for <code>u32</code> because it is not defined in that code (it
is only referenced).</p>
<p><code>AdtDef</code> is more or less a wrapper around <code>DefId</code> with lots of useful helper methods. There is
essentially a one-to-one relationship between <code>AdtDef</code> and <code>DefId</code>. You can get the <code>AdtDef</code> for a
<code>DefId</code> with the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html#method.adt_def"><code>tcx.adt_def(def_id)</code> query</a>. <code>AdtDef</code>s are all interned, as shown
by the <code>'tcx</code> lifetime.</p>
<h2 id="question-why-not-substitute-inside-the-adtdef"><a class="header" href="#question-why-not-substitute-inside-the-adtdef">Question: Why not substitute “inside” the <code>AdtDef</code>?</a></h2>
<p>Recall that we represent a generic struct with <code>(AdtDef, args)</code>. So why bother with this scheme?</p>
<p>Well, the alternate way we could have chosen to represent types would be to always create a new,
fully-substituted form of the <code>AdtDef</code> where all the types are already substituted. This seems like
less of a hassle. However, the <code>(AdtDef, args)</code> scheme has some advantages over this.</p>
<p>First, <code>(AdtDef, args)</code> scheme has an efficiency win:</p>
<pre><code class="language-rust ignore">struct MyStruct&lt;T&gt; {
... 100s of fields ...
}
// Want to do: MyStruct&lt;A&gt; ==&gt; MyStruct&lt;B&gt;</code></pre>
<p>in an example like this, we can instantiate <code>MyStruct&lt;A&gt;</code> as <code>MyStruct&lt;B&gt;</code> (and so on) very cheaply,
by just replacing the one reference to <code>A</code> with <code>B</code>. But if we eagerly instantiated all the fields,
that could be a lot more work because we might have to go through all of the fields in the <code>AdtDef</code>
and update all of their types.</p>
<p>A bit more deeply, this corresponds to structs in Rust being <a href="https://en.wikipedia.org/wiki/Nominal_type_system"><em>nominal</em> types</a> — which
means that they are defined by their <em>name</em> (and that their contents are then indexed from the
definition of that name, and not carried along “within” the type itself).</p>
<h2 id="the-genericargs-type"><a class="header" href="#the-genericargs-type">The <code>GenericArgs</code> type</a></h2>
<p>Given a generic type <code>MyType&lt;A, B, …&gt;</code>, we have to store the list of generic arguments for <code>MyType</code>.</p>
<p>In rustc this is done using <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/type.GenericArgs.html"><code>GenericArgs</code></a>. <code>GenericArgs</code> is a thin pointer to a slice of <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.GenericArg.html"><code>GenericArg</code></a> representing a list of generic arguments for a generic item. For example, given a <code>struct HashMap&lt;K, V&gt;</code> with two type parameters, <code>K</code> and <code>V</code>, the <code>GenericArgs</code> used to represent the type <code>HashMap&lt;i32, u32&gt;</code> would be represented by <code>&amp;'tcx [tcx.types.i32, tcx.types.u32]</code>.</p>
<p><code>GenericArg</code> is conceptually an <code>enum</code> with three variants, one for type arguments, one for const arguments and one for lifetime arguments.
In practice that is actually represented by <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/type.GenericArgKind.html"><code>GenericArgKind</code></a> and <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.GenericArg.html"><code>GenericArg</code></a> is a more space efficient version that has a method to
turn it into a <code>GenericArgKind</code>.</p>
<p>The actual <code>GenericArg</code> struct stores the type, lifetime or const as an interned pointer with the discriminant stored in the lower 2 bits.
Unless you are working with the <code>GenericArgs</code> implementation specifically, you should generally not have to deal with <code>GenericArg</code> and instead
make use of the safe <a href="#genericargkind"><code>GenericArgKind</code></a> abstraction obtainable via the <code>GenericArg::unpack()</code> method.</p>
<p>In some cases you may have to construct a <code>GenericArg</code>, this can be done via <code>Ty/Const/Region::into()</code> or <code>GenericArgKind::pack</code>.</p>
<pre><code class="language-rust ignore">// An example of unpacking and packing a generic argument.
fn deal_with_generic_arg&lt;'tcx&gt;(generic_arg: GenericArg&lt;'tcx&gt;) -&gt; GenericArg&lt;'tcx&gt; {
// Unpack a raw `GenericArg` to deal with it safely.
let new_generic_arg: GenericArgKind&lt;'tcx&gt; = match generic_arg.unpack() {
GenericArgKind::Type(ty) =&gt; { /* ... */ }
GenericArgKind::Lifetime(lt) =&gt; { /* ... */ }
GenericArgKind::Const(ct) =&gt; { /* ... */ }
};
// Pack the `GenericArgKind` to store it in a generic args list.
new_generic_arg.pack()
}</code></pre>
<p>So pulling it all together:</p>
<pre><code class="language-rust ignore">struct MyStruct&lt;T&gt;(T);
type Foo = MyStruct&lt;u32&gt;</code></pre>
<p>For the <code>MyStruct&lt;U&gt;</code> written in the <code>Foo</code> type alias, we would represent it in the following way:</p>
<ul>
<li>There would be an <code>AdtDef</code> (and corresponding <code>DefId</code>) for <code>MyStruct</code>.</li>
<li>There would be a <code>GenericArgs</code> containing the list <code>[GenericArgKind::Type(Ty(u32))]</code></li>
<li>And finally a <code>TyKind::Adt</code> with the <code>AdtDef</code> and <code>GenericArgs</code> listed above.</li>
</ul>
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