Document AST lowering implementation

Author: reddevilmidzy

src/hir/lowering.md

@@ -15,6 +15,24 @@ 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`][hir_id_validator]:
 
@@ -33,6 +51,13 @@ sanity checks in [`compiler/rustc_passes/src/hir_id_validator.rs`][hir_id_valida
   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