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improve hir/lowering.md

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Tshepang Mbambo
2026-04-22 14:23:38 +02:00
parent 14dd4b2086
commit dc675c72d3
1 changed files with 14 additions and 14 deletions
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src/doc/rustc-dev-guide/src/hir/lowering.md
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@@ -2,7 +2,7 @@
The AST lowering step converts AST to [HIR](../hir.md).
This means many structures are removed if they are irrelevant
for type analysis or similar syntax agnostic analyses.
for type analysis or similar syntax-agnostic analyses.
Examples of such structures include but are not limited to
* Parenthesis
@@ -40,21 +40,21 @@ Lowering needs to uphold several invariants in order to not trigger the
sanity checks in [`compiler/rustc_passes/src/hir_id_validator.rs`][hir_id_validator]:
1. A `HirId` must be used if created.
So if you use the `lower_node_id`,
you *must* use the resulting `NodeId` or `HirId` (either is fine, since
any `NodeId`s in the `HIR` are checked for existing `HirId`s)
So, if you use the `lower_node_id`,
you *must* use the resulting `NodeId` or `HirId` (either is fine, since
any `NodeId`s in the `HIR` are checked for existing `HirId`s).
2. Lowering a `HirId` must be done in the scope of the *owning* item.
This means you need to use `with_hir_id_owner` if you are creating parts
of an item other than the one being currently lowered.
This happens for example during the lowering of existential `impl Trait`
This means you need to use `with_hir_id_owner` if you are creating parts
of an item other than the one being currently lowered.
This happens, for example, during the lowering of existential `impl Trait`.
3. A `NodeId` that will be placed into a HIR structure must be lowered,
even if its `HirId` is unused.
Calling `let _ = self.lower_node_id(node_id);` is perfectly legitimate.
even if its `HirId` is unused.
Calling `let _ = self.lower_node_id(node_id);` is perfectly legitimate.
4. If you are creating new nodes that didn't exist in the `AST`, you *must*
create new ids for them.
This is done by calling the `next_id` method,
which produces both a new `NodeId` as well as automatically lowering it
for you so you also get the `HirId`.
create new ids for them.
This is done by calling the `next_id` method,
which produces both a new `NodeId` as well as automatically lowering it
for you so you also get the `HirId`.
[`rustc_ast_lowering`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast_lowering/index.html
[`lower_to_hir`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast_lowering/fn.lower_to_hir.html
@@ -74,7 +74,7 @@ the advantage of creating a way to find the `DefId` of something via its
If lowering needs this `DefId` in multiple places, you can't
generate a new `NodeId` in all those places because you'd also get a new
`DefId` then.
With a `NodeId` from the `AST` this is not an issue.
With a `NodeId` from the `AST`, this is not an issue.
Having the `NodeId` also allows the `DefCollector` to generate the `DefId`s
instead of lowering having to do it on the fly.