rust/src/librustc/README.md

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:

* http://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.