An informal guide to reading and working on the rustc compiler.
==================================================================

If you wish to expand on this document, or have a more experienced
Rust contributor add anything else to it, please get in touch:

* https://internals.rust-lang.org/
* https://chat.mibbit.com/?server=irc.mozilla.org&channel=%23rust

or file a bug:

https://github.com/rust-lang/rust/issues

Your concerns are probably the same as someone else's.

You may also be interested in the
[Rust Forge](https://forge.rust-lang.org/), which includes a number of
interesting bits of information.

Finally, at the end of this file is a GLOSSARY defining a number of
common (and not necessarily obvious!) names that are used in the Rust
compiler code. If you see some funky name and you'd like to know what
it stands for, check there!

The crates of rustc
===================

Rustc consists of a number of crates, including `syntax`,
`rustc`, `rustc_back`, `rustc_trans`, `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.

(NB. 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 improves that may change.)

The dependency structure of these crates is roughly a diamond:

```
                  rustc_driver
                /      |       \
              /        |         \
            /          |           \
          /            v             \
rustc_trans    rustc_borrowck   ...  rustc_metadata
          \            |            /
            \          |          /
              \        |        /
                \      v      /
                    rustc
                       |
                       v
                    syntax
                    /    \
                  /       \
           syntax_pos  syntax_ext
```                    

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](ty/maps/README.md), however, the
"flow" of compilation is becoming less centrally defined.)

At the other extreme, the `rustc` 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`, 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` lie various crates that make up the parser and error
reporting mechanism. For historical reasons, these crates do not have
the `rustc_` prefix, but they are really just as much 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).

Each crate has a `README.md` file that describes, at a high-level,
what it contains, and tries to give some kind of explanation (some
better than others).

The compiler process
====================

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 `src/librustc/ty/maps/README.md`.)

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 `syntax/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 IR". The HIR is defined in `src/librustc/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.
3. **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.
4. **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 the borrowck but also
      certain high-level optimizations. 
5. **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").
6. **Linking**
    - Finally, those `.o` files are linked together.

Glossary
========

The compiler uses a number of...idiosyncratic abbreviations and
things. This glossary attempts to list them and give you a few
pointers for understanding them better.

- AST -- the **abstract syntax tree** produced by the `syntax` crate; reflects user syntax
  very closely. 
- codegen unit -- when we produce LLVM IR, we group the Rust code into a number of codegen
  units. Each of these units is processed by LLVM independently from one another,
  enabling parallelism. They are also the unit of incremental re-use. 
- cx -- we tend to use "cx" as an abbrevation for context. See also tcx, infcx, etc.
- `DefId` -- an index identifying a **definition** (see `librustc/hir/def_id.rs`). Uniquely
  identifies a `DefPath`.
- HIR -- the **High-level IR**, created by lowering and desugaring the AST. See `librustc/hir`.
- `HirId` -- identifies a particular node in the HIR by combining a
  def-id with an "intra-definition offset".
- `'gcx` -- the lifetime of the global arena (see `librustc/ty`).
- generics -- the set of generic type parameters defined on a type or item
- ICE -- internal compiler error. When the compiler crashes.
- ICH -- incremental compilation hash.
- infcx -- the inference context (see `librustc/infer`)
- MIR -- the **Mid-level IR** that is created after type-checking for use by borrowck and trans.
  Defined in the `src/librustc/mir/` module, but much of the code that manipulates it is
  found in `src/librustc_mir`.
- obligation -- something that must be proven by the trait system; see `librustc/traits`.
- local crate -- the crate currently being compiled.
- node-id or `NodeId` -- an index identifying a particular node in the
  AST or HIR; gradually being phased out and replaced with `HirId`.
- query -- perhaps some sub-computation during compilation; see `librustc/maps`.
- provider -- the function that executes a query; see `librustc/maps`.
- sess -- the **compiler session**, which stores global data used throughout compilation
- side tables -- because the AST and HIR are immutable once created, we often carry extra
  information about them in the form of hashtables, indexed by the id of a particular node.
- span -- a location in the user's source code, used for error
  reporting primarily.  These are like a file-name/line-number/column
  tuple on steroids: they carry a start/end point, and also track
  macro expansions and compiler desugaring. All while being packed
  into a few bytes (really, it's an index into a table). See the
  `Span` datatype for more.
- substs -- the **substitutions** for a given generic type or item
  (e.g., the `i32, u32` in `HashMap<i32, u32>`)
- tcx -- the "typing context", main data structure of the compiler (see `librustc/ty`).
- trans -- the code to **translate** MIR into LLVM IR.
- trait reference -- a trait and values for its type parameters (see `librustc/ty`).
- ty -- the internal representation of a **type** (see `librustc/ty`).
