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| <h1 id="ast-lowering"><a class="header" href="#ast-lowering">AST lowering</a></h1> |
| <p>The AST lowering step converts AST to <a href="../hir.html">HIR</a>. |
| This means many structures are removed if they are irrelevant |
| for type analysis or similar syntax agnostic analyses. Examples |
| of such structures include but are not limited to</p> |
| <ul> |
| <li>Parenthesis |
| <ul> |
| <li>Removed without replacement, the tree structure makes order explicit</li> |
| </ul> |
| </li> |
| <li><code>for</code> loops |
| <ul> |
| <li>Converted to <code>match</code> + <code>loop</code> + <code>match</code></li> |
| </ul> |
| </li> |
| <li>Universal <code>impl Trait</code> |
| <ul> |
| <li>Converted to generic arguments |
| (but with some flags, to know that the user didn't write them)</li> |
| </ul> |
| </li> |
| <li>Existential <code>impl Trait</code> |
| <ul> |
| <li>Converted to a virtual <code>existential type</code> declaration</li> |
| </ul> |
| </li> |
| </ul> |
| <p>Lowering needs to uphold several invariants in order to not trigger the |
| sanity checks in <code>compiler/rustc_passes/src/hir_id_validator.rs</code>:</p> |
| <ol> |
| <li>A <code>HirId</code> must be used if created. So if you use the <code>lower_node_id</code>, |
| you <em>must</em> use the resulting <code>NodeId</code> or <code>HirId</code> (either is fine, since |
| any <code>NodeId</code>s in the <code>HIR</code> are checked for existing <code>HirId</code>s)</li> |
| <li>Lowering a <code>HirId</code> must be done in the scope of the <em>owning</em> item. |
| This means you need to use <code>with_hir_id_owner</code> if you are creating parts |
| of an item other than the one being currently lowered. This happens for |
| example during the lowering of existential <code>impl Trait</code></li> |
| <li>A <code>NodeId</code> that will be placed into a HIR structure must be lowered, |
| even if its <code>HirId</code> is unused. Calling |
| <code>let _ = self.lower_node_id(node_id);</code> is perfectly legitimate.</li> |
| <li>If you are creating new nodes that didn't exist in the <code>AST</code>, you <em>must</em> |
| create new ids for them. This is done by calling the <code>next_id</code> method, |
| which produces both a new <code>NodeId</code> as well as automatically lowering it |
| for you so you also get the <code>HirId</code>.</li> |
| </ol> |
| <p>If you are creating new <code>DefId</code>s, since each <code>DefId</code> needs to have a |
| corresponding <code>NodeId</code>, it is advisable to add these <code>NodeId</code>s to the |
| <code>AST</code> so you don't have to generate new ones during lowering. This has |
| the advantage of creating a way to find the <code>DefId</code> of something via its |
| <code>NodeId</code>. If lowering needs this <code>DefId</code> in multiple places, you can't |
| generate a new <code>NodeId</code> in all those places because you'd also get a new |
| <code>DefId</code> then. With a <code>NodeId</code> from the <code>AST</code> this is not an issue.</p> |
| <p>Having the <code>NodeId</code> also allows the <code>DefCollector</code> to generate the <code>DefId</code>s |
| instead of lowering having to do it on the fly. Centralizing the <code>DefId</code> |
| generation in one place makes it easier to refactor and reason about.</p> |
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