third_party_rust_bindgen/CONTRIBUTING.md

Contributing to bindgen

Hi! We'd love to have your contributions! If you want help or mentorship, reach out to us in a GitHub issue, or stop by #servo on irc.mozilla.org and introduce yourself.

• Code of Conduct
• Filing an Issue
• Building
• Testing
  • Overview
  • Running All Tests
  • Authoring New Tests
• Automatic code formatting
• Generating Graphviz Dot Files
• Debug Logging
• Using creduce to Minimize Test Cases
  • Isolating Your Test Case
  • Writing a Predicate Script

Code of Conduct

We abide by the Rust Code of Conduct and ask that you do as well.

Filing an Issue

Think you've found a bug? File an issue! To help us understand and reproduce the issue, provide us with:

• A (preferrably reduced) C/C++ header file that reproduces the issue
• The bindgen flags used to reproduce the issue with the header file
• The expected bindgen output
• The actual bindgen output
• The debugging logs generated when running bindgen on this testcase

Building

To build the bindgen library and the bindgen executable:

$ cargo build

If you installed multiple versions of llvm, it may not be able to locate the latest version of libclang. In that case, you may want to either uninstall other versions of llvm, or specify the path of the desired libclang explicitly:

$ export LIBCLANG_PATH=path/to/clang-3.9/lib

On Linux and macOS, you may also need to add a path to libclang.so (usually the same path as above) to library search path. This can be done as below:

$ export LD_LIBRARY_PATH=path/to/clang-3.9/lib # for Linux
$ export DYLD_LIBRARY_PATH=path/to/clang-3.9/lib # for macOS

Additionally, you may want to build and test with the testing_only_docs feature to ensure that you aren't forgetting to document types and functions. CI will catch it if you forget, but the turn around will be a lot slower ;)

$ cargo build --features testing_only_docs

Testing

Code for binding generation and testing thereof is in the bindgen crate. The following sections assume you are working in that subdirectory.

Overview

Input C/C++ test headers reside in the tests/headers directory. Expected output Rust bindings live in tests/expectations/tests.

For example, tests/headers/my_header.h's expected generated Rust bindings would be tests/expectations/tests/my_header.rs.

Run cargo test to compare generated Rust bindings to the expectations.

Running All Tests

$ cargo test [--all-features]

Running a Single Test

To generate bindings for a single test header, compile the bindings, and run the layout assertion tests for those bindings, use the tests/test-one.sh script. It supports fuzzy searching for test headers. For example, to test tests/headers/what_is_going_on.hpp, execute this command:

$ ./tests/test-one.sh going

Authoring New Tests

To add a new test header to the suite, simply put it in the tests/headers directory. Next, run bindgen to generate the initial expected output Rust bindings. Put those in tests/expectations/tests.

If your new test requires certain flags to be passed to bindgen, you can specify them at the top of the test header, with a comment like this:

// bindgen-flags: --enable-cxx-namespaces -- -std=c++14

Then verify the new Rust bindings compile and pass some basic tests:

$ cargo test -p tests_expectations

Automatic code formatting

We use rustfmt to enforce a consistent code style across the whole bindgen code base. This is enforced in CI, and your pull requests will get automatically rejected if you don't re-format with the latest rustfmt before pushing.

You can install the latest version of rustfmt with this command:

$ cargo install -f rustfmt

Ensure that ~/.cargo/bin is on your path.

Once that is taken care of, you can (re)format all code by running this command:

$ cargo fmt

The code style is described in the rustfmt.toml file in top level of the repo.

Generating Graphviz Dot Files

We can generate Graphviz dot files from our internal representation of a C/C++ input header, and then you can create a PNG or PDF from it with Graphviz's dot program. This is very useful when debugging bindgen!

First, make sure you have Graphviz and dot installed:

$ brew install graphviz         # OS X
$ sudo dnf install graphviz     # Fedora
$ # Etc...

Then, use the --emit-ir-graphviz flag to generate a dot file from our IR:

$ cargo run -- example.hpp --emit-ir-graphviz output.dot

Finally, convert the dot file to an image:

$ dot -Tpng output.dot -o output.png

The final result will look something like this:

Debug Logging

To help debug what bindgen is doing, you can define the environment variable RUST_LOG=bindgen to get a bunch of debugging log spew.

$ RUST_LOG=bindgen ./target/debug/bindgen [flags...] ~/path/to/some/header.h

This logging can also be used when debugging failing tests:

$ RUST_LOG=bindgen cargo test

Using creduce to Minimize Test Cases

If you are hacking on bindgen and find a test case that causes an unexpected panic, results in bad Rust bindings, or some other incorrectness in bindgen, then using creduce can help reduce the test case to a minimal one.

Follow these instructions for building and/or installing creduce.

Running creduce requires two things:

1. Your isolated test case, and

2. A script to act as a predicate script describing whether the behavior you're trying to isolate occurred.

With those two things in hand, running creduce looks like this:

$ creduce ./predicate.sh ./isolated_test_case.h

Isolating Your Test Case

Use the -save-temps flag to make Clang spit out its intermediate representations when compiling the test case into an object file.

$ clang[++ -x c++ --std=c++14] -save-temps -c my_test_case.h

There should now be a my_test_case.ii file, which is the results after the C pre-processor has processed all the #includes, #defines, and #ifdefs. This is generally what we're looking for.

Writing a Predicate Script

Writing a predicate.sh script for a bindgen test case is fairly straightforward. One potential gotcha is that creduce can and will attempt to reduce test cases into invalid C/C++ code. That might be useful for C/C++ compilers, but we generally only care about valid C/C++ input headers.

Here is a skeleton predicate script:

#!/usr/bin/env bash

# Exit the script with a nonzero exit code if:
# * any individual command finishes with a nonzero exit code, or
# * we access any undefined variable.
set -eu

# Print out Rust backtraces on panic. Useful for minimizing a particular panic.
export RUST_BACKTRACE=1

# If the `libclang.so` you're using for `bindgen` isn't the system
# `libclang.so`, let the linker find it.
export LD_LIBRARY_PATH=~/path/to/your/directory/containing/libclang

# Make sure that the reduced test case is valid C/C++ by compiling it. If it
# isn't valid C/C++, this command will exit with a nonzero exit code and cause
# the whole script to do the same.
clang[++ --std=c++14] -c ./pre_processed_header.hpp

# Run `bindgen` and `grep` for the thing your hunting down! Make sure to include
# `2>&1` to get at stderr if you're hunting down a panic.
~/src/rust-bindgen/target/debug/bindgen \
    ./pre_processed_header.hpp \
    [ <extra flags> ] \
    2>&1 \
    | grep "<pattern in generated bindings or a panic string or ...>"

When hunting down a panic, I greped like this:

... | grep "thread main panicked at '<panic error message here>'"

When hunting down bad codegen for a base member, I greped like this:

... | grep "pub _base: MyInvalidBaseTypeThatShouldntBeHere"

That's pretty much it! I want to impress upon you that creduce is really helpful and has enabled me to reduce 30k lines of test case into 5 lines. And it works pretty quickly too. Super valuable tool to have in your belt when hacking on bindgen!

Happy bug hunting and test case reducing!

More information on using creduce.