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Document AST lowering implementation #2800

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Add reference link for HIR ID validator
reddevilmidzy Mar 12, 2026
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Document AST lowering implementation
reddevilmidzy Mar 19, 2026
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29 changes: 28 additions & 1 deletion src/hir/lowering.md
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Expand Up @@ -15,8 +15,26 @@ of such structures include but are not limited to
* Existential `impl Trait`
* Converted to a virtual `existential type` declaration

The implementation of AST lowering is in the [`rustc_ast_lowering`] crate.
The entry point is [`lower_to_hir`], which retrieves the post-expansion AST
and resolver data from [`TyCtxt`] and builds the [`hir::Crate`] for the whole crate.

Lowering is organized around HIR owners. [`lower_to_hir`] first indexes the
crate and then [`ItemLowerer::lower_node`] lowers each crate, item, associated
item, and foreign item.

Most of the lowering logic lives on [`LoweringContext`]. The implementation is
split across multiple files in the [`rustc_ast_lowering`] crate such as `item.rs`,
`expr.rs`, `pat.rs`, `path.rs`, and others, but they all share the same [`LoweringContext`]
state and ID‑lowering machinery.

Each owner is lowered in its own [`with_hir_id_owner`] scope. This is why the
`HirId` invariants below matter: `lower_node_id` maps AST `NodeId`s into the
current owner, while `next_id` creates fresh HIR-only nodes introduced during
desugaring.

Lowering needs to uphold several invariants in order to not trigger the
sanity checks in `compiler/rustc_passes/src/hir_id_validator.rs`:
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
Expand All @@ -33,6 +51,15 @@ sanity checks in `compiler/rustc_passes/src/hir_id_validator.rs`:
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
[`TyCtxt`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html
[`hir::Crate`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/hir/struct.Crate.html
[`ItemLowerer::lower_node`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast_lowering/item/struct.ItemLowerer.html
[`LoweringContext`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast_lowering/struct.LoweringContext.html
[`with_hir_id_owner`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ast_lowering/struct.LoweringContext.html#method.with_hir_id_owner
[hir_id_validator]: https://github.com/rust-lang/rust/blob/HEAD/compiler/rustc_passes/src/hir_id_validator.rs

If you are creating new `DefId`s, since each `DefId` needs to have a
corresponding `NodeId`, it is advisable to add these `NodeId`s to the
`AST` so you don't have to generate new ones during lowering. This has
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