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 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

The compiler process:

The Rust compiler is comprised of six main compilation phases.

  1. Parsing input
  2. Configuration & expanding (cfg rules & syntax extension expansion)
  3. Running analysis passes
  4. Translation to LLVM
  5. LLVM passes
  6. Linking

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 macro_use statements.

The code for these first two phases is in libsyntax.

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 librustc_ prefix.

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 librustc_trans.

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 librustc_llvm.

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 librustc_driver crate.

The 3 central data structures:

  1. ./../libsyntax/ 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/, 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/ 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/ 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.