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.


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.

  • 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 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.

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 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.