Explanation

Why this crate exists

Desugaring, lowering and rewrite passes all want the same thing: walk an arena bottom-up, remap changed nodes, hand back a new root. Extracting that shape once (ADR-0094) turned the legacy desugar_pass into a reusable engine and made passes registerable data.

Where it sits

This crate belongs to the engine-* family. Plugins, kits and hosts reach it through the kit's operation slot, so the engine can be swapped without touching a language definition.

See the reference for the API and the how-to guides for tasks.