
 Hacking on Rust in Tor
========================

 Getting Started
-----------------

Please read or review our documentation on Rust coding standards
(`.../doc/HACKING/CodingStandardsRust.md`) before doing anything.

Please also read
[the Rust Code of Conduct](https://www.rust-lang.org/en-US/conduct.html). We aim
to follow the good example set by the Rust community and be excellent to one
another.  Let's be careful with each other, so we can be memory-safe together!

Next, please contact us before rewriting anything!  Rust in Tor is still an
experiment.  It is an experiment that we very much want to see succeed, so we're
going slowly and carefully.  For the moment, it's also a completely
volunteer-driven effort: while many, if not most, of us are paid to work on Tor,
we are not yet funded to write Rust code for Tor.  Please be patient with the
other people who are working on getting more Rust code into Tor, because they
are graciously donating their free time to contribute to this effort.

 Resources for learning Rust
-----------------------------

**Beginning resources**

The primary resource for learning Rust is
[The Book](https://doc.rust-lang.org/book/).  If you'd like to start writing
Rust immediately, without waiting for anything to install, there is
[an interactive browser-based playground](https://play.rust-lang.org/).

**Advanced resources**

If you're interested in playing with various Rust compilers and viewing a very
nicely displayed output of the generated assembly, there is
[the Godbolt compiler explorer](https://rust.godbolt.org/)

For learning how to write unsafe Rust, read
[The Rustonomicon](https://doc.rust-lang.org/nomicon/).

For learning everything you ever wanted to know about Rust macros, there is
[The Little Book of Rust Macros](https://danielkeep.github.io/tlborm/book/index.html).

For learning more about FFI and Rust, see Jake Goulding's
[Rust FFI Omnibus](http://jakegoulding.com/rust-ffi-omnibus/).

 Compiling Tor with Rust enabled
---------------------------------

You will need to run the `configure` script with the `--enable-rust` flag to
explicitly build with Rust. Additionally, you will need to specify where to
fetch Rust dependencies, as we allow for either fetching dependencies from Cargo
or specifying a local directory.

**Fetch dependencies from Cargo**

    ./configure --enable-rust --enable-cargo-online-mode

**Using a local dependency cache**

You'll need the following Rust dependencies (as of this writing):

    libc==0.2.22

We vendor our Rust dependencies in a separate repo using
[cargo-vendor](https://github.com/alexcrichton/cargo-vendor).  To use them, do:

    git submodule init
    git submodule update

To specify the local directory containing the dependencies, (assuming you are in
the top level of the repository) configure tor with:

    TOR_RUST_DEPENDENCIES='path_to_dependencies_directory' ./configure --enable-rust

(Note that RUST_DEPENDENCIES must be the full path to the directory; it cannot
be relative.)

Assuming you used the above `git submodule` commands and you're in the topmost
directory of the repository, this would be:

    TOR_RUST_DEPENDENCIES=`pwd`/src/ext/rust/crates ./configure --enable-rust


 Identifying which modules to rewrite
======================================

The places in the Tor codebase that are good candidates for porting to Rust are:

1. loosely coupled to other Tor submodules,
2. have high test coverage, and
3. would benefit from being implemented in a memory safe language.

Help in either identifying places such as this, or working to improve existing
areas of the C codebase by adding regression tests and simplifying dependencies,
would be really helpful.

Furthermore, as submodules in C are implemented in Rust, this is a good
opportunity to refactor, add more tests, and split modules into smaller areas of
responsibility.

A good first step is to build a module-level callgraph to understand how
interconnected your target module is.

    git clone https://git.torproject.org/user/nickm/calltool.git
    cd tor
    CFLAGS=0 ./configure
    ../calltool/src/main.py module_callgraph

The output will tell you each module name, along with a set of every module that
the module calls.  Modules which call fewer other modules are better targets.

 Writing your Rust module
==========================

Strive to change the C API as little as possible.

We are currently targetting Rust nightly, *for now*. We expect this to change
moving forward, as we understand more about which nightly features we need. It
is on our TODO list to try to cultivate good standing with various distro
maintainers of `rustc` and `cargo`, in order to ensure that whatever version we
solidify on is readily available.

If parts of your Rust code needs to stay in sync with C code (such as handling
enums across the FFI boundary), annonotate these places in a comment structured
as follows:

  /// C_RUST_COUPLED: <path_to_file> `<name_of_c_object>`

Where <name_of_c_object> can be an enum, struct, constant, etc.  Then, do the
same in the C code, to note that rust will need to be changed when the C
does.

 Adding your Rust module to Tor's build system
-----------------------------------------------

0. Your translation of the C module should live in its own crate(s)
   in the `.../tor/src/rust/` directory.
1. Add your crate to `.../tor/src/rust/Cargo.toml`, in the
   `[workspace.members]` section.
2. Add your crate's files to src/rust/include.am

If your crate should be available to C (rather than just being included as a
dependency of other Rust modules):
0. Declare the crate as a dependency of tor_rust in
   `src/rust/tor_util/Cargo.toml` and include it in
   `src/rust/tor_rust/lib.rs`

 How to test your Rust code
----------------------------

Everything should be tested full stop.  Even non-public functionality.

Be sure to edit `.../tor/src/test/test_rust.sh` to add the name of your crate to
the `crates` variable! This will ensure that `cargo test` is run on your crate.

Configure Tor's build system to build with Rust enabled:

    ./configure --enable-fatal-warnings --enable-rust --enable-cargo-online-mode

Tor's test should be run by doing:

    make check

Tor's integration tests should also pass:

    make test-stem

 Submitting a patch
=====================

Please follow the instructions in `.../doc/HACKING/GettingStarted.md`.
