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 four crates altogether: `libsyntax`, `librustc`,
`librustc_back`, and `librustc_trans` (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.
Roughly speaking the "order" of the three crates is as follows:
libsyntax -> librustc -> librustc_trans
| |
+-----------------+-------------------+
|
librustc_trans/driver
Here the role of `librustc_trans/driver` is to invoke the compiler
from libsyntax, then the analysis phases from librustc, and finally
the lowering and codegen passes from librustc_trans.
Modules in the rustc crate
==========================
The rustc crate itself consists of the following subdirectories
(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
util - ubiquitous types and helper functions
lib - bindings to LLVM
The entry-point for the compiler is main() in the librustc_trans
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: lib/llvm.rs (in librustc_trans) 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.
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