An informal guide to reading and working on the rustc compiler.
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The crates of rustc
Rustc consists of a number of crates, including
(the names and divisions are not set in stone and may change;
in general, a finer-grained division of crates is preferable):
libsyntaxcontains 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
.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_backcontains some very low-level details that are specific to different LLVM targets and so forth.
librustc_transcontains 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:
librustc_driver | +-----------------+-------------------+ | | libsyntax -> librustc -> librustc_trans
The compiler process:
The Rust compiler is comprised of six main compilation phases.
- Parsing input
- Configuration & expanding (cfg rules & syntax extension expansion)
- Running analysis passes
- Translation to LLVM
- LLVM passes
Phase one is responsible for parsing & lexing the input to the compiler. The
output of this phase is an abstract syntax tree (AST). The AST at this point
includes all macro uses & attributes. This means code which will be later
expanded and/or removed due to
cfg attributes is still present in this
version of the AST. Parsing abstracts away details about individual files which
have been read into the AST.
Phase two handles configuration and macro expansion. You can think of this
phase as a function acting on the AST from the previous phase. The input for
this phase is the unexpanded AST from phase one, and the output is an expanded
version of the same AST. This phase will expand all macros & syntax
extensions and will evaluate all
cfg attributes, potentially removing some
code. The resulting AST will not contain any macros or
The code for these first two phases is in
After this phase, the compiler allocates IDs to each node in the AST (technically not every node, but most of them). If we are writing out dependencies, that happens now.
The third phase is analysis. This is the most complex phase in the compiler,
and makes up much of the code. This phase included name resolution, type
checking, borrow checking, type & lifetime inference, trait selection, method
selection, linting and so on. Most of the error detection in the compiler comes
from this phase (with the exception of parse errors which arise during
parsing). The "output" of this phase is a set of side tables containing
semantic information about the source program. The analysis code is in
librustc and some other crates with the
The fourth phase is translation. This phase translates the AST (and the side
tables from the previous phase) into LLVM IR (intermediate representation).
This is achieved by calling into the LLVM libraries. The code for this is in
Phase five runs the LLVM backend. This runs LLVM's optimization passes on the
generated IR and generates machine code resulting in object files. This phase
is not really part of the Rust compiler, as LLVM carries out all the work.
The interface between LLVM and Rust is in
The final phase, phase six, links the object files into an executable. This is
again outsourced to other tools and not performed by the Rust compiler
directly. The interface is in
librustc_back (which also contains some
things used primarily during translation).
A module called the driver coordinates all these phases. It handles all the highest level coordination of compilation from parsing command line arguments all the way to invoking the linker to produce an executable.
Modules in the librustc crate
The librustc 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
The 3 central data structures:
./../libsyntax/ast.rsdefines 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
nodeis the data itself, the member
spanis 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
middle/ty.rsdefines 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::tytag have a corresponding variant in the
./../librustc_llvm/lib.rsdefines the exported types
BasicBlockRef, and several others. Each of these is an opaque pointer to an LLVM type, manipulated through the