Note rustc hacking guide front end

The compiler front-end includes the lexer, parser and syntax extensions (macros), among other things. The front-end is in a separate crate, called syntax, whose source files live in the src/librustsyntax directory under the main Rust source tree. (The middle and back end are in src/rustc.)

Adding a new keyword

If you need to add a new keyword to Rust (a rare occurrence, with hope!) edit one or the other of the functions contextual_keyword_table or restricted_keyword_table in src/librustsyntax/parse/token.rs. Together, these functions construct the table that parser functions consult when determining which identifiers are keywords.