rustc-dev-guide/src/the-parser.md

Lexing and Parsing

The parser and lexer are currently undergoing a lot of refactoring, so parts of this chapter may be out of date.

The very first thing the compiler does is take the program (in Unicode characters) and turn it into something the compiler can work with more conveniently than strings. This happens in two stages: Lexing and Parsing.

Lexing takes strings and turns them into streams of tokens. For example, a.b + c would be turned into the tokens a, ., b, +, and c. The lexer lives in rustc_lexer.

Parsing then takes streams of tokens and turns them into a structured form which is easier for the compiler to work with, usually called an Abstract Syntax Tree (AST). An AST mirrors the structure of a Rust program in memory, using a Span to link a particular AST node back to its source text.

The AST is defined in rustc_ast, 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).

The parser is defined in rustc_parse, along with a high-level interface to the lexer and some validation routines that run after macro expansion. In particular, the rustc_parse::parser contains the parser implementation.

The main entrypoint to the parser is via the various parse_* functions and others in the parser crate. They let you do things like turn a SourceFile (e.g. the source in a single file) into a token stream, create a parser from the token stream, and then execute the parser to get a Crate (the root AST node).

To minimise the amount of copying that is done, both the StringReader and Parser have lifetimes which bind them to the parent ParseSess. This contains all the information needed while parsing, as well as the SourceMap itself.

Note that while parsing, we may encounter macro definitions or invocations. We set these aside to be expanded (see this chapter). Expansion may itself require parsing the output of the macro, which may reveal more macros to be expanded, and so on.

More on Lexical Analysis

Code for lexical analysis is split between two crates:

• rustc_lexer crate is responsible for breaking a &str into chunks constituting tokens. Although it is popular to implement lexers as generated finite state machines, the lexer in rustc_lexer is hand-written.

• StringReader from rustc_ast integrates rustc_lexer with rustc specific data structures. Specifically, it adds Span information to tokens returned by rustc_lexer and interns identifiers.