Testing & Debugging Rust Code
This page explains how to test and debug Rust code in Firefox.
The build documentation explains how to add new Rust code to Firefox. The code documentation explains how to write and work with Rust code in Firefox.
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.
(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.
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, aCargo.toml
file that references the Rust file, and amoz.build
file that references the C++ file.Add an entry to the
[dependencies]
section in toolkit/library/gtest/rust/Cargo.toml.Add an
extern crate
entry to toolkit/library/gtest/rust/lib.rs.
See
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
andVec
is sometimes sub-optimal. If you see a warning “Missing auto-load script at offset 0 in section.debug_gdb_scripts
” when startinggdb
, therust-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
.
Gecko logging
Rust logging can also be forwarded to the Gecko logger for capture via
MOZ_LOG
and MOZ_LOG_FILE
.
When parsing modules from
MOZ_LOG
, modules containing::
are considered to be Rust modules. To log everything in a top-level module likeneqo_transport
, specify it asneqo_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!
andtrace!
logs will not appear in non-debug builds. This is due to our use of therelease_max_level_info
feature in thelog
crate.When using both
MOZ_LOG
andRUST_LOG
, modules that are specified inMOZ_LOG
will not appear inRUST_LOG
.