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| <h1 id="the-hir"><a class="header" href="#the-hir">The HIR</a></h1> |
| <p>The HIR – “High-Level Intermediate Representation” – is the primary IR used |
| in most of rustc. It is a compiler-friendly representation of the abstract |
| syntax tree (AST) that is generated after parsing, macro expansion, and name |
| resolution (see <a href="./hir/lowering.html">Lowering</a> for how the HIR is created). |
| Many parts of HIR resemble Rust surface syntax quite closely, with |
| the exception that some of Rust’s expression forms have been desugared away. |
| For example, <code>for</code> loops are converted into a <code>loop</code> and do not appear in |
| the HIR. This makes HIR more amenable to analysis than a normal AST.</p> |
| <p>This chapter covers the main concepts of the HIR.</p> |
| <p>You can view the HIR representation of your code by passing the |
| <code>-Z unpretty=hir-tree</code> flag to rustc:</p> |
| <pre><code class="language-bash">cargo rustc -- -Z unpretty=hir-tree |
| </code></pre> |
| <p>You can also use the <code>-Z unpretty=hir</code> option to generate a HIR |
| that is closer to the original source code expression:</p> |
| <pre><code class="language-bash">cargo rustc -- -Z unpretty=hir |
| </code></pre> |
| <h2 id="out-of-band-storage-and-the-crate-type"><a class="header" href="#out-of-band-storage-and-the-crate-type">Out-of-band storage and the <code>Crate</code> type</a></h2> |
| <p>The top-level data-structure in the HIR is the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/struct.Crate.html"><code>Crate</code></a>, which stores |
| the contents of the crate currently being compiled (we only ever |
| construct HIR for the current crate). Whereas in the AST the crate |
| data structure basically just contains the root module, the HIR |
| <code>Crate</code> structure contains a number of maps and other things that |
| serve to organize the content of the crate for easier access.</p> |
| <p>For example, the contents of individual items (e.g. modules, |
| functions, traits, impls, etc) in the HIR are not immediately |
| accessible in the parents. So, for example, if there is a module item |
| <code>foo</code> containing a function <code>bar()</code>:</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>mod foo { |
| fn bar() { } |
| } |
| <span class="boring">}</span></code></pre> |
| <p>then in the HIR the representation of module <code>foo</code> (the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/struct.Mod.html"><code>Mod</code></a> |
| struct) would only have the <strong><code>ItemId</code></strong> <code>I</code> of <code>bar()</code>. To get the |
| details of the function <code>bar()</code>, we would lookup <code>I</code> in the |
| <code>items</code> map.</p> |
| <p>One nice result from this representation is that one can iterate |
| over all items in the crate by iterating over the key-value pairs |
| in these maps (without the need to trawl through the whole HIR). |
| There are similar maps for things like trait items and impl items, |
| as well as “bodies” (explained below).</p> |
| <p>The other reason to set up the representation this way is for better |
| integration with incremental compilation. This way, if you gain access |
| to an <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/struct.Item.html"><code>&rustc_hir::Item</code></a> (e.g. for the mod <code>foo</code>), you do not immediately |
| gain access to the contents of the function <code>bar()</code>. Instead, you only |
| gain access to the <strong>id</strong> for <code>bar()</code>, and you must invoke some |
| function to lookup the contents of <code>bar()</code> given its id; this gives |
| the compiler a chance to observe that you accessed the data for |
| <code>bar()</code>, and then record the dependency.</p> |
| <p><a id="hir-id"></a></p> |
| <h2 id="identifiers-in-the-hir"><a class="header" href="#identifiers-in-the-hir">Identifiers in the HIR</a></h2> |
| <p>The HIR uses a bunch of different identifiers that coexist and serve different purposes.</p> |
| <ul> |
| <li> |
| <p>A <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.DefId.html"><code>DefId</code></a>, as the name suggests, identifies a particular definition, or top-level |
| item, in a given crate. It is composed of two parts: a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.CrateNum.html"><code>CrateNum</code></a> which identifies |
| the crate the definition comes from, and a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.DefIndex.html"><code>DefIndex</code></a> which identifies the definition |
| within the crate. Unlike <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/struct.HirId.html"><code>HirId</code></a>s, there isn’t a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.DefId.html"><code>DefId</code></a> for every expression, which |
| makes them more stable across compilations.</p> |
| </li> |
| <li> |
| <p>A <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.LocalDefId.html"><code>LocalDefId</code></a> is basically a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.DefId.html"><code>DefId</code></a> that is known to come from the current crate. |
| This allows us to drop the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.CrateNum.html"><code>CrateNum</code></a> part, and use the type system to ensure that |
| only local definitions are passed to functions that expect a local definition.</p> |
| </li> |
| <li> |
| <p>A <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/struct.HirId.html"><code>HirId</code></a> uniquely identifies a node in the HIR of the current crate. It is composed |
| of two parts: an <code>owner</code> and a <code>local_id</code> that is unique within the <code>owner</code>. This |
| combination makes for more stable values which are helpful for incremental compilation. |
| Unlike <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.DefId.html"><code>DefId</code></a>s, a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/struct.HirId.html"><code>HirId</code></a> can refer to <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/enum.Node.html">fine-grained entities</a> like expressions, |
| but stays local to the current crate.</p> |
| </li> |
| <li> |
| <p>A <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/struct.BodyId.html"><code>BodyId</code></a> identifies a HIR <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/struct.Body.html"><code>Body</code></a> in the current crate. It is currently only |
| a wrapper around a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/struct.HirId.html"><code>HirId</code></a>. For more info about HIR bodies, please refer to the |
| <a href="./hir.html#hir-bodies">HIR chapter</a>.</p> |
| </li> |
| </ul> |
| <p>These identifiers can be converted into one another through the <code>TyCtxt</code>.</p> |
| <h2 id="hir-operations"><a class="header" href="#hir-operations">HIR Operations</a></h2> |
| <p>Most of the time when you are working with the HIR, you will do so via |
| <code>TyCtxt</code>. It contains a number of methods, defined in the <code>hir::map</code> module and |
| mostly prefixed with <code>hir_</code>, to convert between IDs of various kinds and to |
| lookup data associated with a HIR node.</p> |
| <p>For example, if you have a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.LocalDefId.html"><code>LocalDefId</code></a>, and you would like to convert it |
| to a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/struct.HirId.html"><code>HirId</code></a>, you can use <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html#method.local_def_id_to_hir_id"><code>tcx.local_def_id_to_hir_id(def_id)</code></a>. |
| You need a <code>LocalDefId</code>, rather than a <code>DefId</code>, since only local items have HIR nodes.</p> |
| <p>Similarly, you can use <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html#method.hir_node"><code>tcx.hir_node(n)</code></a> to lookup the node for a |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/struct.HirId.html"><code>HirId</code></a>. This returns a <code>Option<Node<'hir>></code>, where <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/enum.Node.html"><code>Node</code></a> is an enum |
| defined in the map. By matching on this, you can find out what sort of |
| node the <code>HirId</code> referred to and also get a pointer to the data |
| itself. Often, you know what sort of node <code>n</code> is – e.g. if you know |
| that <code>n</code> must be some HIR expression, you can do |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html#method.expect_expr"><code>tcx.hir_expect_expr(n)</code></a>, which will extract and return the |
| <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/struct.Expr.html"><code>&hir::Expr</code></a>, panicking if <code>n</code> is not in fact an expression.</p> |
| <p>Finally, you can find the parents of nodes, via |
| calls like <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html#method.parent_hir_node"><code>tcx.parent_hir_node(n)</code></a>.</p> |
| <h2 id="hir-bodies"><a class="header" href="#hir-bodies">HIR Bodies</a></h2> |
| <p>A <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/struct.Body.html"><code>rustc_hir::Body</code></a> represents some kind of executable code, such as the body |
| of a function/closure or the definition of a constant. Bodies are |
| associated with an <strong>owner</strong>, which is typically some kind of item |
| (e.g. an <code>fn()</code> or <code>const</code>), but could also be a closure expression |
| (e.g. <code>|x, y| x + y</code>). You can use the <code>TyCtxt</code> to find the body |
| associated with a given def-id (<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html#method.hir_maybe_body_owned_by"><code>hir_maybe_body_owned_by</code></a>) or to find |
| the owner of a body (<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html#method.hir_body_owner_def_id"><code>hir_body_owner_def_id</code></a>).</p> |
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