Have you ever run into a situation where cargo fmt, Rust’s code formatter, produces different output on different machines, even though you’re working on the same project? This can be incredibly frustrating, especially when you’re trying to maintain consistent code style across a team or between your own development environments. I recently encountered this issue, and after a bit of digging, I found a somewhat obscure cause and a straightforward solution that I wanted to share.
The Problem: Formatting Discrepancies
I was working on a Rust project, and I noticed that cargo fmt was formatting the code slightly differently on my macOS machine compared to my Linux server. The differences were subtle, but they were enough to cause unnecessary diffs in my Git repository and make code reviews a bit more challenging.
Naturally, I started by checking the usual suspects:
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Rust Versions: I made sure that both machines were running the same version of the Rust toolchain using rustc –version and cargo –version. They were identical.
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rustfmt.toml: I searched my project directory and all parent directories for a rustfmt.toml file, which is used to customize the formatting rules. I couldn’t find any.
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Environment Variables: I checked for any environment variables that might be influencing rustfmt‘s behavior, but nothing seemed out of the ordinary.
Despite these efforts, the formatting differences persisted. It was a real head-scratcher.
The Unexpected Culprit: rustfmt.toml in stdarch Source Code
After further investigation, I discovered the root cause:
rustfmt was somehow picking up rustfmt.toml files located within the source code of the stdarch library, which is part of the Rust standard library.
These files are located in the .rustup directory, specifically under the toolchain directory (e.g., ~/.rustup/toolchains/<toolchain_name>/lib/rustlib/src/rust/library/stdarch/).
The issue seemed to be happening only on my macOS machine, not on my Linux server, suggesting a platform-specific difference in how rustfmt searches for configuration files.
These rustfmt.toml files are not intended to be user configuration files.
They are used internally to format the stdarch library’s code.
For some reason, rustfmt was incorrectly identifying them as valid configuration files for my project.
The Solution: .rustfmt.ignore to the Rescue
You’d think, well, maybe I can just delete these files, or copy them to my other machine, but the best solution is neither.
The solution is to tell rustfmt to ignore these unintended configuration files using a .rustfmt.ignore file.
Similar to .gitignore, a .rustfmt.ingore file in your project allows you to specify paths that rustfmt should skip when searching for configuration files.
I created a .rustfmt.ignore file in the root of my project (where my Cargo.toml file is) and added the following lines:
./toolchains/*/lib/rustlib/src/rust/library/stdarch/crates/core_arch/rustfmt.toml
./toolchains/*/lib/rustlib/src/rust/library/stdarch/rustfmt.tomlThis tells rustfmt to ignore any rustfmt.toml files found in those specific paths within the .rustup directory, using a glob to match the varying toolchain names.
Why This Works
rustfmt respects the .rustfmt.ignore file and will skip any files or directories listed in it when searching for configuration files. By adding the paths to the stdarch library’s rustfmt.toml files, I prevented rustfmt from accidentally picking them up.
Conclusion
Inconsistent cargo fmt formatting can be a frustrating problem, but it’s often caused by unexpected configuration issues. In this case, the culprit was rustfmt mistakenly picking up configuration files from within the standard library’s source code. By using a .rustfmt.ignore file, I was able to resolve the issue and ensure consistent formatting across all my development environments.
If you encounter similar formatting discrepancies, I hope this post helps you troubleshoot the problem and find a solution!
