High-level overview of the compiler source

Crate structure

The main Rust repository consists of a src directory, under which there live many crates. These crates contain the sources for the standard library and the compiler. This document, of course, focuses on the latter.

Rustc consists of a number of crates, including rustc_ast, rustc, rustc_target, rustc_codegen, rustc_driver, and many more. The source for each crate can be found in a directory like src/libXXX, where XXX is the crate name.

(N.B. The names and divisions of these crates are not set in stone and may change over time. For the time being, we tend towards a finer-grained division to help with compilation time, though as incremental compilation improves, that may change.)

The dependency structure of these crates is roughly a diamond:

                /      |       \
              /        |         \
            /          |           \
          /            v             \
rustc_codegen  rustc_borrowck   ...  rustc_metadata
          \            |            /
            \          |          /
              \        |        /
                \      v      /
                    /    \
                  /       \
           rustc_span  rustc_builtin_macros

The rustc_driver crate, at the top of this lattice, is effectively the "main" function for the rust compiler. It doesn't have much "real code", but instead ties together all of the code defined in the other crates and defines the overall flow of execution. (As we transition more and more to the query model, however, the "flow" of compilation is becoming less centrally defined.)

At the other extreme, the rustc_middle crate defines the common and pervasive data structures that all the rest of the compiler uses (e.g. how to represent types, traits, and the program itself). It also contains some amount of the compiler itself, although that is relatively limited.

Finally, all the crates in the bulge in the middle define the bulk of the compiler – they all depend on rustc_middle, so that they can make use of the various types defined there, and they export public routines that rustc_driver will invoke as needed (more and more, what these crates export are "query definitions", but those are covered later on).

Below rustc_middle lie various crates that make up the parser and error reporting mechanism. They are also an internal part of the compiler and not intended to be stable (though they do wind up getting used by some crates in the wild; a practice we hope to gradually phase out).

The main stages of compilation

The Rust compiler is in a bit of transition right now. It used to be a purely "pass-based" compiler, where we ran a number of passes over the entire program, and each did a particular check of transformation. We are gradually replacing this pass-based code with an alternative setup based on on-demand queries. In the query-model, we work backwards, executing a query that expresses our ultimate goal (e.g. "compile this crate"). This query in turn may make other queries (e.g. "get me a list of all modules in the crate"). Those queries make other queries that ultimately bottom out in the base operations, like parsing the input, running the type-checker, and so forth. This on-demand model permits us to do exciting things like only do the minimal amount of work needed to type-check a single function. It also helps with incremental compilation. (For details on defining queries, check out the query model.)

Regardless of the general setup, the basic operations that the compiler must perform are the same. The only thing that changes is whether these operations are invoked front-to-back, or on demand. In order to compile a Rust crate, these are the general steps that we take:

  1. Parsing input
    • this processes the .rs files and produces the AST ("abstract syntax tree")
    • the AST is defined in src/librustc_ast/ast.rs. It is intended to match the lexical syntax of the Rust language quite closely.
  2. Name resolution, macro expansion, and configuration
    • once parsing is complete, we process the AST recursively, resolving paths and expanding macros. This same process also processes #[cfg] nodes, and hence may strip things out of the AST as well.
  3. Lowering to HIR
    • Once name resolution completes, we convert the AST into the HIR, or "high-level intermediate representation". The HIR is defined in src/librustc_middle/hir/; that module also includes the lowering code.
    • The HIR is a lightly desugared variant of the AST. It is more processed than the AST and more suitable for the analyses that follow. It is not required to match the syntax of the Rust language.
    • As a simple example, in the AST, we preserve the parentheses that the user wrote, so ((1 + 2) + 3) and 1 + 2 + 3 parse into distinct trees, even though they are equivalent. In the HIR, however, parentheses nodes are removed, and those two expressions are represented in the same way.
  4. Type-checking and subsequent analyses
    • An important step in processing the HIR is to perform type checking. This process assigns types to every HIR expression, for example, and also is responsible for resolving some "type-dependent" paths, such as field accesses (x.f – we can't know what field f is being accessed until we know the type of x) and associated type references (T::Item – we can't know what type Item is until we know what T is).
    • Type checking creates "side-tables" (TypeckTables) that include the types of expressions, the way to resolve methods, and so forth.
    • After type-checking, we can do other analyses, such as privacy checking.
  5. Lowering to MIR and post-processing
    • Once type-checking is done, we can lower the HIR into MIR ("middle IR"), which is a very desugared version of Rust, well suited to borrowck but also to certain high-level optimizations.
  6. Translation to LLVM and LLVM optimizations
    • From MIR, we can produce LLVM IR.
    • LLVM then runs its various optimizations, which produces a number of .o files (one for each "codegen unit").
  7. Linking
    • Finally, those .o files are linked together.