Bug 1647987 - Create Rust testing docs. r=froydnj.

Rust crates that you want to link into libxul should be listed in the

``dependencies`` section of

`toolkit/library/rust/shared/Cargo.toml <https://dxr.mozilla.org/mozilla-central/source/toolkit/library/rust/shared/Cargo.toml>`_.

`toolkit/library/rust/shared/Cargo.toml <https://searchfox.org/mozilla-central/source/toolkit/library/rust/shared/Cargo.toml>`_.

After adding your crate, execute ``cargo update -p gkrust-shared``

to update the ``Cargo.lock`` file. You'll also need to add an ``extern crate``

reference to

`toolkit/library/rust/shared/lib.rs <https://dxr.mozilla.org/mozilla-central/source/toolkit/library/rust/shared/lib.rs>`_.

`toolkit/library/rust/shared/lib.rs <https://searchfox.org/mozilla-central/source/toolkit/library/rust/shared/lib.rs>`_.

This ensures that the Rust code will be linked properly into libxul as well

as the copy of libxul used for gtests.

By default, all Cargo packages in the mozilla-central repository are part of

the same

`workspace <https://dxr.mozilla.org/mozilla-central/source/toolkit/library/rust/shared/lib.rs>`_

`workspace <https://searchfox.org/mozilla-central/source/toolkit/library/rust/shared/lib.rs>`_

and will share the ``Cargo.lock`` file and ``target`` directory in the root of

the repository.  You can change this behavior by adding a path to the

``exclude`` list in the top-level ``Cargo.toml`` file.  You may want to do

    RustLibrary('crate_name')

where ``crate_name`` matches the name from the ``[package]`` section of your

``Cargo.toml``. You can refer to `the moz.build file <https://dxr.mozilla.org/mozilla-central/rev/3f4c3a3cabaf94958834d3a8935adfb4a887942d/toolkit/library/rust/moz.build#7>`_ and `the Cargo.toml file <https://dxr.mozilla.org/mozilla-central/rev/3f4c3a3cabaf94958834d3a8935adfb4a887942d/toolkit/library/rust/Cargo.toml>`_ that are used for libxul.

``Cargo.toml``. You can refer to `the moz.build file <https://searchfox.org/mozilla-central/rev/3f4c3a3cabaf94958834d3a8935adfb4a887942d/toolkit/library/rust/moz.build#7>`_ and `the Cargo.toml file <https://searchfox.org/mozilla-central/rev/3f4c3a3cabaf94958834d3a8935adfb4a887942d/toolkit/library/rust/Cargo.toml>`_ that are used for libxul.

You can then add ``USE_LIBS += ['crate_name']`` to the ``moz.build`` file

that defines the binary as you would with any other library in the tree.

- ``mach vendor rust`` will check that the licenses of all crates are suitable.

- You should review the crate code to some degree to check that it looks

  reasonable (especially for unsafe code) and that it has reasonable tests.

- Third-party crate tests aren't run, which means that large test fixtures will

  bloat mozilla-central. Consider working with upstream to mark those test

  fixtures with ``[package] exclude = ...`` as described

  `here <https://doc.rust-lang.org/cargo/reference/manifest.html#the-exclude-and-include-fields>`_.

- Other than that, there is no formal sign-off procedure, but one may be added

  in the future.

Finally, make sure you don't accidentally land the changes to the crate or the

``Cargo.lock`` file.

For an example of a more complex workflow involving a third-party crate, see

`mp4parse-rust/README.md <https://searchfox.org/mozilla-central/source/media/mp4parse-rust/README.md>`_.

It describes the workflow for a crate that is hosted on GitHub, and for which

changes are made via GitHub pull requests, but all pull requests must also be

tested within mozilla-central before being merged.

        - testing/perfdocs

        - testing/perftest

        - tools/code-coverage

        - testing-rust-code

    l10n_doc:

        - intl

        - l10n

# Testing & Debugging Rust Code

## Testing Mozilla crates

Rust code will naturally be tested as part of system tests such as Mochitests.

This section describes the two methods for unit testing of individual Rust

crates. Which method should be used depends on the circumstances.

### Rust tests

If a Mozilla crate has "normal" Rust tests (i.e. `#[test]` functions that run

with `cargo test`), you can add the crate's name to `RUST_TESTS` in

[toolkit/library/rust/moz.build](https://searchfox.org/mozilla-central/source/toolkit/library/rust/moz.build).

(Cargo features can be activated for Rust tests by adding them to

`RUST_TEST_FEATURES` in the same file.)

Rust tests are run with `./mach rusttests`. They run on automation in a couple

of `rusttests` jobs, but not on all platforms.

Rust tests have one major restriction: they cannot link against Gecko symbols.

Therefore, Rust tests cannot be used for crates that use Gecko crates like

`nsstring` and `xpcom`.

It's also possible to use `RUST_TESTS` in a different `moz.build` file. See

`testing/geckodriver/moz.build` and the [geckodriver testing docs] for an

example.

[geckodriver testing docs]: ../testing/geckodriver/Testing.html

### GTests

Another way to unit test a Mozilla crate is by writing a GTest that uses FFI to

call into Rust code. This requires the following steps.

- Create a new test crate whose name is the same as the name of crate being

  tested, with a `-gtest` suffix.

- Add to the test crate a Rust file, a C++ file containing GTest `TEST()`

  functions that use FFI to call into the Rust file, a `Cargo.toml` file that

  references the Rust file, and a `moz.build` file that references the C++

  file.

- Add an entry to the `[dependencies]` section in

  [toolkit/library/gtest/rust/Cargo.toml](https://searchfox.org/mozilla-central/source/toolkit/library/gtest/rust/Cargo.toml).

- Add an `extern crate` entry to

  [toolkit/library/gtest/rust/lib.rs](https://searchfox.org/mozilla-central/source/toolkit/library/gtest/rust/lib.rs).

See

[xpcom/rust/gtest/nsstring/](https://searchfox.org/mozilla-central/source/xpcom/rust/gtest/nsstring)

for a simple example. (Note that the `moz.build` file is in the parent

directory for that crate.)

A Rust GTest can be run like any other GTest via `./mach gtest`, using the C++

`TEST()` functions as the starting point.

Unlike Rust tests, GTests can be used when linking against Gecko symbols is required.

## Testing third-party crates

In general we don't run tests for third-party crates. The assumption is that

these crates are sufficiently well-tested elsewhere.

## Debugging Rust code

In theory, Rust code is debuggable much like C++ code, using standard tools

like `gdb`, `rr`, and the Microsoft Visual Studio Debugger. In practice, the

experience can be worse, because shortcomings such as the following can occur.

- Inability to print local variables, even in non-optimized builds.

- Inability to call generic functions.

- Missing line numbers and stack frames.

- Printing of basic types such as `Option` and `Vec` is sometimes sub-optimal.

  If you see a warning "Missing auto-load script at offset 0 in section

  `.debug_gdb_scripts`" when starting `gdb`, the `rust-gdb` wrapper may give

  better results.

## Logging from Rust code

### Rust logging

The `RUST_LOG` environment variable (from the `env_logger` crate) can be used

to enable logging to stderr from Rust code in Firefox. The logging macros from

the `log` crate can be used. In order of importance, they are: `error!`,

`warn!`, `info!`, `debug!`, `trace!`.

For example, to show all log messages of `info` level or higher, run:

```

RUST_LOG=info firefox

```

Module-level logging can also be specified, see the [documentation] for the

`env_logger` crate for details.

To restrict logging to child processes, use `RUST_LOG_CHILD` instead of

`RUST_LOG`.

[documentation]: https://docs.rs/env_logger/

### Gecko logging

Rust logging can also be forwarded to the [Gecko logger] for capture via

`MOZ_LOG` and `MOZ_LOG_FILE`.

[Gecko logger]: https://developer.mozilla.org/en-US/docs/Mozilla/Developer_guide/Gecko_Logging

- When parsing modules from `MOZ_LOG`, modules containing `::` are considered

  to be Rust modules. To log everything in a top-level module like

  `neqo_transport`, specify it as `neqo_transport::*`. For example:

```

MOZ_LOG=timestamp,sync,nsHostResolver:5,neqo_transport::*:5,proxy:5 firefox

```

- When logging from a submodule the `::*` is allowed but isn't necessary.

  So these two lines are equivalent:

```

MOZ_LOG=timestamp,sync,neqo_transport::recovery:5 firefox

MOZ_LOG=timestamp,sync,neqo_transport::recovery::*:5 firefox

```

- `debug!` and `trace!` logs will not appear in non-debug builds. This is due

  to our use of the `release_max_level_info` feature in the `log` crate.

- When using both `MOZ_LOG` and `RUST_LOG`, modules that are specified in

  `MOZ_LOG` will not appear in `RUST_LOG`.

SPHINX_TREES['code-quality'] = 'docs/code-quality'

SPHINX_TREES['testing-rust-code'] = 'docs/testing-rust-code'

SPHINX_TREES['bug-mgmt'] = 'docs/bug-mgmt'

SPHINX_TREES['setup'] = 'docs/setup'