High-level overview of the compiler source

• Workspace structure
• Compiler
  • Big picture
• rustdoc
• Tests
• Build System
• Standard library
• Other

Now that we have seen what the compiler does, let's take a look at the structure of the rust-lang/rust repository, where the rustc source code lives.

You may find it helpful to read the "Overview of the compiler" chapter, which introduces how the compiler works, before this one.

Workspace structure

The rust-lang/rust repository consists of a single large cargo workspace containing the compiler, the standard libraries (core, alloc, std, proc_macro, etc), and rustdoc, along with the build system and a bunch of tools and submodules for building a full Rust distribution.

The repository consists of three main directories:

• compiler/ contains the source code for rustc. It consists of many crates that together make up the compiler.

• library/ contains the standard libraries (core, alloc, std, proc_macro, test), as well as the Rust runtime (backtrace, rtstartup, lang_start).

• tests/ contains the compiler tests.

• src/ contains the source code for rustdoc, clippy, cargo, the build system, language docs, etc.

Compiler

The compiler is implemented in the various compiler/ crates. The compiler/ crates all have names starting with rustc_*. These are a collection of around 50 interdependent crates ranging in size from tiny to huge. There is also the rustc crate which is the actual binary (i.e. the main function); it doesn't actually do anything besides calling the rustc_driver crate, which drives the various parts of compilation in other crates.

The dependency structure of these crates is complex, but roughly it is something like this:

• rustc (the binary) calls rustc_driver::main.
  • rustc_driver depends on a lot of other crates, but the main one is rustc_interface.
    • rustc_interface depends on most of the other compiler crates. It is a fairly generic interface for driving the whole compilation.
      • Most of the other rustc_* crates depend on rustc_middle, which defines a lot of central data structures in the compiler.
        • rustc_middle and most of the other crates depend on a handful of crates representing the early parts of the compiler (e.g. the parser), fundamental data structures (e.g. Span), or error reporting: rustc_data_structures, rustc_span, rustc_errors, etc.

You can see the exact dependencies by reading the Cargo.toml for the various crates, just like a normal Rust crate.

One final thing: src/llvm-project is a submodule for our fork of LLVM. During bootstrapping, LLVM is built and the compiler/rustc_llvm crate contains Rust wrappers around LLVM (which is written in C++), so that the compiler can interface with it.

Most of this book is about the compiler, so we won't have any further explanation of these crates here.

Big picture

The dependency structure of the compiler is influenced by two main factors:

1. Organization. The compiler is a huge codebase; it would be an impossibly large crate. In part, the dependency structure reflects the code structure of the compiler.
2. Compile-time. By breaking the compiler into multiple crates, we can take better advantage of incremental/parallel compilation using cargo. In particular, we try to have as few dependencies between crates as possible so that we don't have to rebuild as many crates if you change one.

At the very bottom of the dependency tree are a handful of crates that are used by the whole compiler (e.g. rustc_span). The very early parts of the compilation process (e.g. parsing and the Abstract Syntax Tree (AST)) depend on only these.

After the AST is constructed and other early analysis is done, the compiler's query system gets set up. The query system is set up in a clever way using function pointers. This allows us to break dependencies between crates, allowing more parallel compilation. The query system is defined in rustc_middle, so nearly all subsequent parts of the compiler depend on this crate. It is a really large crate, leading to long compile times. Some efforts have been made to move stuff out of it with varying success. Another side-effect is that sometimes related functionality gets scattered across different crates. For example, linting functionality is found across earlier parts of the crate, rustc_lint, rustc_middle, and other places.

Ideally there would be fewer, more cohesive crates, with incremental and parallel compilation making sure compile times stay reasonable. However, incremental and parallel compilation haven't gotten good enough for that yet, so breaking things into separate crates has been our solution so far.

At the top of the dependency tree is rustc_driver and rustc_interface which is an unstable wrapper around the query system helping drive various stages of compilation. Other consumers of the compiler may use this interface in different ways (e.g. rustdoc or maybe eventually rust-analyzer). The rustc_driver crate first parses command line arguments and then uses rustc_interface to drive the compilation to completion.

rustdoc

The bulk of rustdoc is in librustdoc. However, the rustdoc binary itself is src/tools/rustdoc, which does nothing except call rustdoc::main.

There is also JavaScript and CSS for the docs in src/tools/rustdoc-js and src/tools/rustdoc-themes.

You can read more about rustdoc in this chapter.

Tests

The test suite for all of the above is in tests/. You can read more about the test suite in this chapter.

The test harness is in src/tools/compiletest/.

Build System

There are a number of tools in the repository just for building the compiler, standard library, rustdoc, etc, along with testing, building a full Rust distribution, etc.

One of the primary tools is src/bootstrap/. You can read more about bootstrapping in this chapter. The process may also use other tools from src/tools/, such as tidy/ or compiletest/.

Standard library

This code is fairly similar to most other Rust crates except that it must be built in a special way because it can use unstable (nightly) features. The standard library is sometimes referred to as libstd or the "standard facade".

Other

There are a lot of other things in the rust-lang/rust repo that are related to building a full Rust distribution. Most of the time you don't need to worry about them.

These include:

• src/ci: The CI configuration. This actually quite extensive because we run a lot of tests on a lot of platforms.
• src/doc: Various documentation, including submodules for a few books.
• src/etc: Miscellaneous utilities.
• And more...