rustc/compiler/rustc_ast

The rustc_ast crate contains those things concerned purely with syntax – that is, the AST ("abstract syntax tree"), along with some definitions for tokens and token streams, data structures/traits for mutating ASTs, and shared definitions for other AST-related parts of the compiler (like the lexer and macro-expansion).

For more information about how these things work in rustc, see the rustc dev guide:

• Parsing
• Macro Expansion