The `rustc_ast` crate contains those things concerned purely with syntax – that is, the AST ("abstract syntax tree"), parser, pretty-printer, lexer, macro expander, and utilities for traversing ASTs. For more information about how these things work in rustc, see the rustc dev guide: - [Parsing](https://rustc-dev-guide.rust-lang.org/the-parser.html) - [Macro Expansion](https://rustc-dev-guide.rust-lang.org/macro-expansion.html)