rust/src/librustc

An informal guide to reading and working on the rustc compiler.

If you wish to expand on this document, or have a more experienced Rust contributor add anything else to it, please get in touch:

• https://internals.rust-lang.org/
• https://chat.mibbit.com/?server=irc.mozilla.org&channel=%23rust

or file a bug:

https://github.com/rust-lang/rust/issues

Your concerns are probably the same as someone else's.

The crates of rustc

Rustc consists of a number of crates, including libsyntax, librustc, librustc_back, librustc_trans, and librustc_driver (the names and divisions are not set in stone and may change; in general, a finer-grained division of crates is preferable):

• libsyntax contains those things concerned purely with syntax – that is, the AST, parser, pretty-printer, lexer, macro expander, and utilities for traversing ASTs – are in a separate crate called "syntax", whose files are in ./../libsyntax, where . is the current directory (that is, the parent directory of front/, middle/, back/, and so on).

• librustc (the current directory) contains the high-level analysis passes, such as the type checker, borrow checker, and so forth. It is the heart of the compiler.

• librustc_back contains some very low-level details that are specific to different LLVM targets and so forth.

• librustc_trans contains the code to convert from Rust IR into LLVM IR, and then from LLVM IR into machine code, as well as the main driver that orchestrates all the other passes and various other bits of miscellany. In general it contains code that runs towards the end of the compilation process.

• librustc_driver invokes the compiler from libsyntax, then the analysis phases from librustc, and finally the lowering and codegen passes from librustc_trans.

Roughly speaking the "order" of the three crates is as follows:

libsyntax -> librustc -> librustc_trans
|                                     |
+-----------------+-------------------+
                  |
          librustc_driver

Modules in the rustc crate

The rustc crate itself consists of the following submodules (mostly, but not entirely, in their own directories):

• session: options and data that pertain to the compilation session as a whole
• middle: middle-end: name resolution, typechecking, LLVM code generation
• metadata: encoder and decoder for data required by separate compilation
• plugin: infrastructure for compiler plugins
• lint: infrastructure for compiler warnings
• util: ubiquitous types and helper functions
• lib: bindings to LLVM

The entry-point for the compiler is main() in the librustc_driver crate.

The 3 central data structures:

1. ./../libsyntax/ast.rs defines the AST. The AST is treated as immutable after parsing, but it depends on mutable context data structures (mainly hash maps) to give it meaning.

   • Many – though not all – nodes within this data structure are wrapped in the type spanned<T>, meaning that the front-end has marked the input coordinates of that node. The member node is the data itself, the member span is the input location (file, line, column; both low and high).

   • Many other nodes within this data structure carry a def_id. These nodes represent the 'target' of some name reference elsewhere in the tree. When the AST is resolved, by middle/resolve.rs, all names wind up acquiring a def that they point to. So anything that can be pointed-to by a name winds up with a def_id.

2. middle/ty.rs defines the datatype sty. This is the type that represents types after they have been resolved and normalized by the middle-end. The typeck phase converts every ast type to a ty::sty, and the latter is used to drive later phases of compilation. Most variants in the ast::ty tag have a corresponding variant in the ty::sty tag.

3. ./../librustc_llvm/lib.rs defines the exported types ValueRef, TypeRef, BasicBlockRef, and several others. Each of these is an opaque pointer to an LLVM type, manipulated through the lib::llvm interface.

Control and information flow within the compiler:

• main() in lib.rs assumes control on startup. Options are parsed, platform is detected, etc.

• ./../libsyntax/parse/parser.rs parses the input files and produces an AST that represents the input crate.

• Multiple middle-end passes (middle/resolve.rs, middle/typeck.rs) analyze the semantics of the resulting AST. Each pass generates new information about the AST and stores it in various environment data structures. The driver passes environments to each compiler pass that needs to refer to them.

• Finally, the trans module in librustc_trans translates the Rust AST to LLVM bitcode in a type-directed way. When it's finished synthesizing LLVM values, rustc asks LLVM to write them out in some form (.bc, .o) and possibly run the system linker.