rustyline is a mature Rust library providing GNU Readline-style interactive line editing for terminal applications. It is not a standalone application but a reusable dependency used by CLIs, REPLs, and shells. The core is an Editor generic over helper and history types, with pluggable traits for completion, hints, highlighting, validation, and custom key bindings. It supports Emacs and Vi keymaps, Unicode/grapheme-aware editing, multi-line input, kill ring, undo/redo, and multiple history backends including in-memory, file, and SQLite. The architecture is trait-driven and platform-aware, with separate Unix and Windows TTY implementations behind a common abstraction, plus a derive macro crate to reduce boilerplate for custom helpers.