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Contributing to rust-bitcoin

👍🎉 First off, thanks for taking the time to contribute! 🎉👍

The following is a set of guidelines for contributing to Rust Bitcoin implementation and other Rust Bitcoin-related projects, which are hosted in the Rust Bitcoin Community on GitHub. These are mostly guidelines, not rules. Use your best judgment, and feel free to propose changes to this document in a pull request.

Table Of Contents

General

The Rust Bitcoin project operates an open contributor model where anyone is welcome to contribute towards development in the form of peer review, documentation, testing and patches.

Anyone is invited to contribute without regard to technical experience, "expertise", OSS experience, age, or other concern. However, the development of standards & reference implementations demands a high-level of rigor, adversarial thinking, thorough testing and risk-minimization. Any bug may cost users real money. That being said, we deeply welcome people contributing for the first time to an open source project or pick up Rust while contributing. Don't be shy, you'll learn.

Communication channels

Communication about Rust Bitcoin happens primarily in #bitcoin-rust IRC chat on Libera with the logs available at https://gnusha.org/bitcoin-rust/ (starting from Jun 2021 and now on) and https://gnusha.org/rust-bitcoin/ (historical archive before Jun 2021).

Discussion about code base improvements happens in GitHub issues and on pull requests.

Major projects are tracked here. Major milestones are tracked here.

Asking questions

Note: Please don't file an issue to ask a question. You'll get faster results by using the resources below.

We have a dedicated developer channel on IRC, #[email protected] where you may get helpful advice if you have questions.

Contribution workflow

The codebase is maintained using the "contributor workflow" where everyone without exception contributes patch proposals using "pull requests". This facilitates social contribution, easy testing and peer review.

To contribute a patch, the workflow is a as follows:

  1. Fork Repository
  2. Create topic branch
  3. Commit patches

Please keep commits atomic and diffs easy to read. For this reason do not mix any formatting fixes or code moves with actual code changes. Further, each commit, individually, should compile and pass tests, in order to ensure git bisect and other automated tools function properly.

Please cover every new feature with unit tests.

When refactoring, structure your PR to make it easy to review and don't hesitate to split it into multiple small, focused PRs.

Commits should cover both the issue fixed and the solution's rationale. Please keep these guidelines in mind.

Preparing PRs

The main library development happens in the master branch. This branch must always compile without errors (using GitHub CI). All external contributions are made within PRs into this branch.

Prerequisites that a PR must satisfy for merging into the master branch:

  • each commit within a PR must compile and pass unit tests with no errors, with every feature combination (including compiling the fuzztests) on some reasonably recent compiler (this is partially automated with CI, so the rule is that we will not accept commits which do not pass GitHub CI);
  • the tip of any PR branch must also compile and pass tests with no errors on MSRV (check [README.md] on current MSRV requirements) and pass fuzz tests on nightly rust;
  • contain all necessary tests for the introduced functional (either as a part of commits, or, more preferably, as separate commits, so that it's easy to reorder them during review and check that the new tests fail without the new code);
  • contain all inline docs for newly introduced API and pass doc tests including running just lint without any errors or warnings;
  • be based on the recent master tip from the original repository at https://github.com/rust-bitcoin/rust-bitcoin.

NB: reviewers may run more complex test/CI scripts, thus, satisfying all the requirements above is just a preliminary, but not necessary sufficient step for getting the PR accepted as a valid candidate PR for the master branch.

High quality commits help us review and merge you contributions. We attempt to adhere to the ideas presented in the following two blog posts:

Deprecation and Versioning

Whenever any part of your code wants to mention the version number the code will be released in, primarily in deprecation notices, you should use the string TBD (verbatim), so that the release preparation script can detect the change and the correct version number can be filled in in preparation of the release.

    #[deprecated(since = "TBD", note = "use `alternative_method()` instead")]

Peer review

Anyone may participate in peer review which is expressed by comments in the pull request. Typically, reviewers will review the code for obvious errors, as well as test out the patch set and opine on the technical merits of the patch. Please, first review PR on the conceptual level before focusing on code style or grammar fixes.

API changes

The API of the following crates is almost stable. Changing it is supposed to be non-trivial. To assist in this effort ll PRs that change the public API of any these crates must include a patch to the api/ text files. This should be a separate final patch to the PR that is the diff created by running just check-api.

  • hashes
  • io
  • primitives
  • units

Repository maintainers

Pull request merge requirements:

  • all CI test should pass,
  • at least one "accepts"/ACKs from the repository maintainers
  • no reasonable "rejects"/NACKs from anybody who reviewed the code.

Current list of the project maintainers:

Backporting

We maintain release branches (e.g. 0.32.x for the v0.32 releases).

In order to backport changes to these branches the process we use is as follows:

  • PR change into master.
  • Mark the PR with the appropriate labels if backporting is needed (e.g. port-0.32.x).
  • Once PR merges create another PR that targets the appropriate branch.
  • If, and only if, the backport PR is identical to the original PR (i.e. created using git cherry-pick) then the PR may be one-ACK merged.

Any other changes to the release branches should follow the normal 2-ACK merge policy.

Coding conventions

Library reflects Bitcoin Core approach whenever possible.

Naming conventions

Naming of data structures/enums and their fields/variants must follow names used in Bitcoin Core, with the following exceptions:

  • The case should follow Rust standards (i.e. PascalCase for types and snake_case for fields and variants).
  • Omit C-prefixes.
  • If function foo needs a private helper function, use foo_internal.

Upgrading dependencies

If your change requires a dependency to be upgraded you must do the following:

  1. Modify Cargo.toml
  2. Copy Cargo-minimal.lock to Cargo.lock
  3. Trigger cargo to update the required entries in the lock file - use --precise using the minimum version number that works
  4. Test your change
  5. Copy Cargo.lock to Cargo-minimal.lock
  6. Update Cargo-recent.lock if it is also behind
  7. Commit both lock files together with Cargo.toml and your code changes

Unsafe code

Use of unsafe code is prohibited unless there is a unanimous decision among library maintainers on the exclusion from this rule. In such cases there is a requirement to test unsafe code with sanitizers including Miri.

API changes

All PRs that change the public API of rust-bitcoin will be checked on CI for semversioning compliance. This means that if the PR changes the public API in a way that is not backwards compatible, the PR will be flagged as a breaking change. Please check the semver-checks workflow. Under the hood we use cargo-semver-checks.

Policy

We have various rust-bitcoin specific coding styles and conventions that are grouped here loosely under the term 'policy'. These are things we try to adhere to but that you should not need to worry too much about if you are a new contributor. Think of this as a place to collect group knowledge that exists in the various PRs over the last few years.

Import statements

We use the following style for import statements, see (rust-bitcoin#2088) for the discussion that led to this.

// Modules first, as they are part of the project's structure.
pub mod aa_this;
mod bb_private;
pub mod cc_that;

// Private imports, rustfmt will sort and merge them correctly.
use crate::aa_this::{This, That};
use crate::bb_that;

// Public re-exports.
#[rustfmt::skip] // Keeps public re-exports separate, because of this we have to sort manually.
pub use {
    crate::aa_aa_this,
    crate::bb_bb::That,
}

// Avoid wildcard imports, except for 3 rules:

// Rule 1 - test modules.
#[cfg(test)]
mod tests {
    use super::*; // OK
}

// Rule 2 - enum variants.
use LockTime::*; // OK

// Rule 3 - opcodes.
use opcodes::all::*; // OK

// Finally here is an example where we don't allow wildcard imports:
use crate::prelude::*; // *NOT* OK
use crate::prelude::{DisplayHex, String, Vec} // OK

Return Self

Use Self as the return type instead of naming the type. When constructing the return value use Self or the type name, whichever you prefer.

/// A counter that is always smaller than 100.
pub struct Counter(u32);

impl Counter {
    /// Constructs a new `Counter`.
    pub fn new() -> Self { Self(0) }

    /// Returns a counter if it is possible to create one from x.
    pub fn maybe(x: u32) -> Option<Self> {
        match x {
            x if x >= 100 => None,
            c => Some(Counter(c)),
        }
    }
}

impl TryFrom<u32> for Counter {
    type Error = TooBigError;

    fn try_from(x: u32) -> Result<Self, Self::Error> {
        if x >= 100 {
            return Err(TooBigError);
        }
        Ok(Counter(x))
    }
}

When constructing the return value for error enums use Self.

impl From<foo::Error> for LongDescriptiveError {
    fn from(e: foo::Error) -> Self { Self::Foo(e) }
}

Errors

Return as much context as possible with errors e.g., if an error was encountered parsing a string include the string in the returned error type. If a function consumes costly-to-compute input (allocations are also considered costly) it should return the input back in the error type.

More specifically an error should

  • be non_exhaustive unless we really never want to change it.
  • have private fields unless we are very confident they won't change.
  • derive Debug, Clone, PartialEq, Eq (and Copy iff not non_exhaustive).
  • implement Display using write_err!() macro if a variant contains an inner error source.
  • have Error suffix on error types (structs and enums).
  • not have Error suffix on enum variants.
  • call internals::impl_from_infallible!.
  • implement std::error::Error if they are public (feature gated on "std").
  • have messages in lower case, except for proper nouns and variable names.
/// Documentation for the `Error` type.
#[derive(Debug, Clone, PartialEq, Eq)]
#[non_exhaustive]  // Add liberally; if the error type may ever have new variants added.
pub enum Error {
    /// Documentation for variant A.
    A,
    /// Documentation for variant B.
    B,
}

internals::impl_from_infallible!(Error);

All errors that live in an error module (eg, foo/error.rs) and appear in a public function in foo module should be available from foo i.e., should be re-exported from foo/mod.rs.

expect messages

With respect to expect messages, they should follow the Rust standard library guidelines. More specifically, expect messages should be used to to describe the reason you expect the operation to succeed. For example, this expect message clearly states why the operation should succeed:

/// Serializes the public key to bytes.
pub fn to_bytes(self) -> Vec<u8> {
    let mut buf = Vec::new();
    self.write_into(&mut buf).expect("vecs don't error");
    buf
}

Also note that expect messages, as with all error messages, should be lower case, except for proper nouns and variable names.

The details on why we chose this style

According to the Rust standard library, there are two common styles for how to write expect messages:

  • using the message to present information to users encountering a panic ("expect as error message"); and
  • using the message to present information to developers debugging the panic ("expect as precondition").

We opted to use the "expect as precondition" since it clearly states why the operation should succeed. This may be better for communicating with developers, since they are the target audience for the error message and rust-bitcoin.

If you want to know more about the decision error messages and expect messages, please check:

Rustdocs

Be liberal with references to BIPs or other documentation; the aim is that devs can learn about Bitcoin by hacking on this codebase as opposed to having to learn about Bitcoin first and then start hacking on this codebase. Consider the following format, not all sections will be required for all types.

/// The Bitcoin foobar.
///
/// Contains all the data used when passing a foobar around the Bitcoin network.
///
/// <details>
/// <summary>FooBar Original Design</summary>
///
/// The foobar was introduced in Bitcoin x.y.z to increase the amount of foo in bar.
///
/// </details>
///
/// ### Relevant BIPs
///
/// * [BIP X - FooBar in Bitcoin](https://github.com/bitcoin/bips/blob/master/bip-0000.mediawiki)
pub struct FooBar {
    /// The version in use.
    pub version: Version
}

Do use rustdoc subheadings. Do put an empty newline below each heading e.g.,

impl FooBar {
    /// Constructs a `FooBar` from a [`Baz`].
    ///
    /// # Errors
    ///
    /// Returns an error if `Baz` is not ...
    ///
    /// # Panics
    ///
    /// If the `Baz`, converted to a `usize`, is out of bounds.
    pub fn from_baz(baz: Baz) -> Result<Self, Error> {
        ...
    }
}

Add Panics section if any input to the function can trigger a panic.

Generally we prefer to have non-panicking APIs but it is impractical in some cases. If you're not sure, feel free to ask. If we determine panicking is more practical it must be documented. Internal panics that could theoretically occur because of bugs in our code must not be documented.

Example code within the rustdocs should compile and lint with just lint without any errors or warnings.

Derives

We try to use standard set of derives if it makes sense:

#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
enum Foo {
    Bar,
    Baz,
}

For types that do should not form a total or partial order, or that technically do but it does not make sense to compare them, we use the Ordered trait from the ordered crate. See absolute::LockTime for an example.

For error types you likely want to use #[derive(Debug, Clone, PartialEq, Eq)].

See Errors section.

Attributes

Licensing

We use SPDX license tags, all files should start with

// SPDX-License-Identifier: CC0-1.0

Security

Security is the primary focus for this library; disclosure of security vulnerabilities helps prevent user loss of funds. If you believe a vulnerability may affect other implementations, please disclose this information according to the security guidelines, work on which is currently in progress. Before it is completed, feel free to send disclosure to Andrew Poelstra, [email protected], encrypted with his public key from https://www.wpsoftware.net/andrew/andrew.gpg.

Testing

Related to the security aspect, rust bitcoin developers take testing very seriously. Due to the modular nature of the project, writing new test cases is easy and good test coverage of the codebase is an important goal. Refactoring the project to enable fine-grained unit testing is also an ongoing effort.

Various methods of testing are in use (e.g. fuzzing, mutation), please see the readme for more information.

Going further

You may be interested in the guide by Jon Atack on How to review Bitcoin Core PRs and How to make Bitcoin Core PRs. While there are differences between the projects in terms of context and maturity, many of the suggestions offered apply to this project.

Overall, have fun :)