Ferrous Systems Donates Ferrocene Language Specification to Rust Project

Rust 1.0 was published in May 2015. As the language approaches its 10th anniversary, Rust has become one of the fastest growing and most-loved languages [1] among developers, thanks to a combination of speed, safety and a vibrant community. 

Like any growing open source language, Rust has a considerable amount of documentation and tutorials provided by the Rust Project [2]. Included in the online library are the official Rust book [3] Rust Reference [4], courses and tutorials [5], and more. However, there is an important piece of documentation missing that many other languages provide: an official language specification.

In December 2022, an RFC [6] was submitted to encourage the Rust Project to begin working on a specification. After much discussion, the RFC was approved in July 2023, and work began. Initially, the Rust Project specification team (t-spec [7]) were interested in creating the document from scratch using the Rust Reference as a guiding marker. However, the team knew there was already an external Rust specification that was being used successfully for compiler qualification purposes – the FLS.

The FLS is a description of the Rust programming language, developed by Ferrous Systems [8] in July 2022 as part of Ferrocene [9], a Rust compiler and toolchain designed for safety-critical and regulated industries. The FLS provides a structured and detailed reference for Rust’s syntax, semantics, and behavior, serving as a foundation for verification, compliance, and standardization efforts. Since Rust did not have an official language specification back then, nor a plan to write one, the FLS represented a major step toward describing Rust in a way that aligns with industry requirements, particularly in high-assurance domains.

The t-spec team wanted to avoid potential confusion from having two highly visible Rust specifications in the industry and so decided it would be worthwhile to try to integrate the FLS with the Rust Reference to create the official Rust Project specification. They approached Ferrous Systems, which agreed to contribute its FLS to the Rust Project and allow the Rust Project to take over its development and management.

The Rust Foundation is excited to announce that Ferrous Systems has agreed to contribute its Ferrocene Language Specification (FLS) to the Rust Project. This generous donation will provide a clearer path to delivering an official Rust specification. It will also empower the Rust Project to oversee its ongoing evolution, providing confidence to companies and individuals already relying on the FLS, and marking a major milestone for the Rust ecosystem.

“I really appreciate Ferrous taking this step to provide their specification to the Rust Project,” said Joel Marcey, Director of Technology at the Rust Foundation and member of the t-spec team. “They have already done a massive amount of legwork. Having the FLS integrated officially into the Rust Project will allow the t-spec team to supercharge our progress in the delivery of an official Rust specification that can be utilized by developers, safety-critical tool vendors, and others that look to a language specification for their work.”

“We originally created the Ferrocene Language Specification to provide a structured and reliable description of Rust for the certification of the Ferrocene compiler,” said Felix Gilcher, Co-founder of Ferrous Systems. “As an open source-first company, contributing the FLS to the Rust Project is a logical step toward fostering the development of a unified, community-driven specification that benefits all Rust users. We are glad to support this effort and look forward to the long-term impact this will have on Rust’s adoption in regulated and high-assurance domains.”

There will be a transition period as the FLS is integrated into the Rust Project. The first phase of work will involve incorporating FLS into the Project’s tooling and processes, in alignment with existing Rust Project goals [10].

Once the integration is complete, Ferrous Systems will be able to rely on the Project’s version of the specification and discontinue their own. More broadly, this effort will provide others who require a Rust specification with an official, authoritative reference for their work with the Rust programming language.

Moving forward, both the FLS and the Rust Reference will be utilized to form the official Rust Specification. This is an exciting outcome. A heartfelt thank you to the Ferrous Systems team for their invaluable contribution!

View the latest blog from the Rust Project on this topic: https://blog.rust-lang.org/2025/03/26/adopting-the-fls.html