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/libsyntax directory under the main Rust source tree. (The middle and back end are in src/librustc.)
Built-in syntax extensions are located in src/libsyntax/ext, including the macro-defining construct macro, located in simplext.rs. It also contains expand.rs, which is responsible for traversing and expanding the tree in the first place, and base.rs, which contains helper functions for extensions and registers the extensions for the expander.
Rust's parser, located in src/libsyntax/parse/parser.rs is a hand-rolled LL-style parser with a small, finite lookahead. Rust's lexer is in src/libsyntax/parse/lexer.rs.
If you need to add a new keyword to Rust (a rare occurrence, with hope!) edit one or the other of the functions reserved_keyword_table or strict_keyword_table in src/libsyntax/parse/token.rs. Together, these functions construct the table that parser functions consult when determining which identifiers are keywords.