How to Use the Command 'rustfmt' (with examples)

Rustfmt is a formatting tool for Rust source code, designed to help maintain a consistent coding style and format across projects. It automates the process of organizing your Rust code by adhering to community-driven style guidelines, which fosters readability and maintainability. This tool is part of the Rust toolchain and simplifies code collaboration by ensuring that everyone contributes code that looks the same. Below, we delve into various practical uses of rustfmt, each illustrated with explicit examples.

Format a file, overwriting the original file in-place:

Code:

rustfmt path/to/source.rs

Motivation:

When working on a large Rust codebase, individual contributions can have differing stylistic choices. Over time, these differences can clutter the code and make it harder to read and maintain. By using rustfmt, a developer ensures that their code adheres to the conventionally accepted Rust formatting guidelines. This not only improves readability for themselves but also for anyone else who may read or contribute to that code in the future.

Explanation:

• rustfmt: This invokes the rustfmt tool.
• path/to/source.rs: This is the path indicating the specific source file you wish to format. The file name should have a .rs extension, indicating it is a Rust source file.

Example Output:

Upon running this command, you may not see any output in the terminal if everything processed without issues. Instead, the tool will modify your source file in-place to match the style conventions defined by the Rust community guidelines. If your file was not already formatted according to those guidelines, you will notice changes in the alignment, indentation, and possibly spacing within your code.

Check a file for formatting and display any changes on the console:

Code:

rustfmt --check path/to/source.rs

Motivation:

Before committing changes to a shared repository, it’s advantageous to verify if your code is properly formatted without altering the original file. This allows a developer to see what rustfmt would change, ensuring that no unexpected alterations are made automatically. In continuous integration pipelines, --check can be used to enforce style guidelines, failing builds when the code doesn’t conform.

Explanation:

• rustfmt: This is the command to run the formatting tool.
• --check: This flag tells rustfmt not to overwrite the file but instead to print the changes that would be made, allowing for review.
• path/to/source.rs: This indicates the specific Rust file being checked for formatting issues.

Example Output:

The output in the terminal will show a diff format of what rustfmt would change in the file. It will highlight parts of your code that do not align with Rust’s formatting standards. If the code is correctly formatted, the command will return a prompt without additional output.

Backup any modified files before formatting:

Code:

rustfmt --backup path/to/source.rs

Motivation:

Sometimes it is important to preserve the original version of a file before any modifications are applied. This could be due to a need to review changes line-by-line, to save a snapshot of the pre-format state, or to ensure that any unintended consequences of formatting can be easily reverted. The --backup option is particularly useful in collaborative environments or when working on delicate code sections where formatting changes could inadvertently introduce bugs.

Explanation:

• rustfmt: Initiates the tool for formatting.
• --backup: This additional option instructs rustfmt to save the current state of the file by renaming it with a .bk extension before applying any formatting changes.
• path/to/source.rs: The path to the specific file you want to format and back up.

Example Output:

After executing this command, you will notice two files in the directory: the newly formatted source.rs file and the source.rs.bk backup. You will not see detailed output in the terminal unless an error occurs during the formatting process.

Conclusion:

Rustfmt is an indispensable tool for maintaining consistent code style in Rust projects. By automating the formatting process, it helps developers focus more on problem-solving and less on stylistic choices. Whether formatting in place, checking for desired changes, or ensuring a safe backup, rustfmt offers the flexibility needed to fit various development workflows.