Coding Conventions for rustc

# Coding conventions This file offers some tips on the coding conventions for rustc. This chapter covers [formatting](#formatting), [coding for correctness](#cc), [using crates from crates.io](#cio), and some tips on [structuring your PR for easy review](#er). <a id="formatting"></a> ## Formatting and the tidy script rustc is moving towards the [Rust standard coding style][fmt]. However, for now we don't use stable `rustfmt`; we use a pinned version with a special config, so this may result in different style from normal [`rustfmt`]. Therefore, formatting this repository using `cargo fmt` is not recommended. Instead, formatting should be done using `./x fmt`. It's a good habit to run `./x fmt` before every commit, as this reduces conflicts later. Formatting is checked by the `tidy` script. It runs automatically when you do `./x test` and can be run in isolation with `./x fmt --check`. If you want to use format-on-save in your editor, the pinned version of `rustfmt` is built under `build/<target>/stage0/bin/rustfmt`. ### Formatting C++ code The compiler contains some C++ code for interfacing with parts of LLVM that don't have a stable C API. When modifying that code, use this command to format it: ```console ./x test tidy --extra-checks cpp:fmt --bless ``` This uses a pinned version of `clang-format`, to avoid relying on the local environment. ### Formatting and linting Python code The Rust repository contains quite a lot of Python code. We try to keep it both linted and formatted by the [ruff] tool. When modifying Python code, use this command to format it: ```console ./x test tidy --extra-checks py:fmt --bless ``` And, the following command to run lints: ```console ./x test tidy --extra-checks py:lint ``` These use a pinned version of `ruff`, to avoid relying on the local environment. <a id="copyright"></a>   ### Copyright notice  In the past, files began with a copyright and license notice. Please **omit** this notice for new files licensed under the standard terms (dual MIT/Apache-2.0). All of the copyright notices should be gone by now, but if you come across one in the rust-lang/rust repo, feel free to open a PR to remove it. ### Line length Lines should be at most 100 characters. It's even better if you can keep things to 80. Sometimes, and particularly for tests, it can be necessary to exempt yourself from this limit. In that case, you can add a comment towards the top of the file like so: ```rust // ignore-tidy-linelength ``` ### Tabs vs spaces Prefer 4-space indents. <a id="cc"></a> ## Coding for correctness Beyond formatting, there are a few other tips that are worth following. ### Prefer exhaustive matches Using `_` in a match is convenient, but it means that when new variants are added to the enum, they may not get handled correctly.

This section outlines the coding conventions for the rustc project, including formatting using `./x fmt` and the `tidy` script, formatting C++ and Python code with pinned versions of `clang-format` and `ruff` respectively, omitting copyright notices for new files, adhering to line length limits (with exceptions via `// ignore-tidy-linelength`), using 4-space indents, and prioritizing exhaustive matches for correctness.