Ferrocene Language Specification

Read the specification »

The Ferrocene Language Specification (FLS) is a document describing the Rust language. It was created as one of the prerequisites for qualifying Ferrocene, a Rust toolchain qualified for safety-critical environments. The FLS is compiled of existing Rust documentation, but presented with a rigorous structure in order to meet the requirements of qualification.

The FLS is not intended to be used as the normative specification of the Rust language, nor is it meant to replace the decision-making processes of the Rust project. Any difference between the FLS and the behavior of the Rust compiler is considered an error on our part and the FLS will be updated accordingly.

The Ferrocene Language Specification text is licensed under either the MIT or Apache-2.0 licenses, at your option. Individual files might have different licensing. Licensing metadata is present in each file, and the full licenses text is present in the LICENSES/ directory.

Building the specification

FLS uses Sphinx to build a rendered version of the specification, and uv to install and manage Python dependencies (including Sphinx itself). To simplify building the rendered version, we created a script called make.py that takes care of invoking Sphinx with the right flags.

You can build the rendered version by running:

./make.py

By default, Sphinx uses incremental rebuilds to generate the content that changed since the last invocation. If you notice a problem with incremental rebuilds, you can pass the -c flag to clear the existing artifacts before building:

./make.py -c

The rendered version will be available in build/html/.

You can also start a local server on port 8000 with automatic rebuild and reload whenever you change a file by passing the -s flag:

./make.py -s

Checking links consistency

It's possible to run Rust's linkchecker tool on the rendered documentation, to see if there are broken links. To do so, pass the --check-links flag:

./make.py --check-links

This will clone the source code of the tool, build it, and execute it on the rendered documentation.

Updating build dependencies

The FLS uses uv to manage the Python dependencies used for builds. If you change the list of dependencies in pyproject.toml they will automatically be installed the next time you run make.py. If you want to update the packages in the lockfile, run:

uv lock --upgrade