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`)][parser]) 
depend on only these.

After the [`AST`][parser] is constructed and other early analysis is done, the
compiler's [query system][query] 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][rustdoc-chapter].


## Tests

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

The test harness is in [`src/tools/compiletest/`][`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][bootstch]. 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:
  run a lot of tests on a lot of platforms.
- And more...