Rust has a standardized style, and an implementation of that style in the rustfmt tool. The standardized style helps Rust developers feel comfortable and at home in many different projects, and the tooling support from rustfmt makes it easy to maintain and to incorporate in continuous integration. rustfmt also provides many options to customize the style, but the style guide defines the defaults, and most projects use those defaults.

The standard Rust style resulted from development and discussion within the Rust style team, between 2016 and 2018. After publishing the style guide, the Rust style team concluded its active work, by design.

However, as the Rust language develops, we have a regular need for improvements to the style guide, such as to support new language constructs. This includes minor language changes, as well as highly anticipated new features such as let-chaining (RFC 2497) and let-else (RFC 3137). New constructs like these, by default, get ignored and not formatted by rustfmt, and subsequently need formatting added. Some of this work has fallen to the rustfmt team in recent years, but the rustfmt team would prefer to implement style determinations made by another team rather than making such determinations itself.

In addition, rustfmt maintains backwards compatibility guarantees: code that has been correctly formatted with rustfmt won't get formatted differently with a future version of rustfmt. This avoids churn, and avoids creating CI failures when people use rustfmt to check style in CI. However, this also prevents evolving the Rust style to take community desires into account and improve formatting over time. rustfmt provides various configuration options to change its default formatting, and many of those options represent changes that many people in the community would like enabled by default.

For instance, many people prefer to format their use lines in three blocks: imports from the standard library, imports from external crates, and then imports from modules within the same project. rustfmt supports this via the option group_imports = StdExternalCrate, but cannot make this the default without causing CI failures in existing projects. We need a way to evolve the default Rust style compatibly, similar in spirit to the mechanisms we use for Rust editions: allowing existing style to continue working, and allowing people to opt into new style.

To solve both of these problems, RFC 3309 has revived the Rust style team, with three goals:

• Making determinations about styling for new Rust constructs
• Evolving the existing Rust style
• Defining mechanisms to evolve the Rust style while taking backwards compatibility into account

We don't plan to make any earth-shattering style changes; the look and feel of Rust will remain largely the same. Evolutions to the default Rust style will largely consist of established rustfmt options people already widely enable, or would enable if they were stable.

We expect that the initial work of the style team will focus on clearing a backlog of new language constructs that lack formatting guidance. Afterwards, we will look towards defining and implementing the mechanisms to evolve the default Rust style, and then begin introducing style improvements.