Lexing and parsing

The very first thing the compiler does is take the program (in UTF-8 Unicode text) and turn it into a data format the compiler can work with more conveniently than strings. This happens in two stages: Lexing and Parsing.

1. Lexing takes strings and turns them into streams of tokens. For example, foo.bar + buz would be turned into the tokens foo, ., bar, +, and buz. This is implemented in rustc_lexer.

2. Parsing 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) .

The AST

The 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).

Every node in the AST has its own NodeId, including top-level items such as structs, but also individual statements and expressions. A NodeId is an identifier number that uniquely identifies an AST node within a crate.

However, because they are absolute within a crate, adding or removing a single node in the AST causes all the subsequent NodeIds to change. This renders NodeIds pretty much useless for incremental compilation, where you want as few things as possible to change.

NodeIds are used in all the rustc bits that operate directly on the AST, like macro expansion and name resolution (more on these over the next couple chapters).

Parsing

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 rustc_parse. 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 minimize the amount of copying that is done, both Lexer 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 Macro Expansion). Expansion itself may require parsing the output of a 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.

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