Incremental Parsing
Simple explanation
When source changes slightly, most of the old syntax tree is still right. Incremental parsing finds the still-right pieces and only re-parses the neighborhood of the edit.
Technical explanation
Soundness rests on LR determinism: the same parser state plus the same upcoming bytes always produces the same subtree. The parser therefore records every node's start state (reuse_state), translates old positions through the InputEdit, and collects candidates fully outside the edited span. During parsing, a candidate whose translated start, symbol, and start state match the live state is cloned into the new pool and skipped — whole subtrees at a time, preferring the largest span. reused_node_count reports the take-up. Old trees are never mutated; reuse copies.