Copyright, Line Length, Indentation and Correctness

### 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. Ask yourself: if a new variant were added to this enum, what's the chance that it would want to use the `_` code, versus having some other treatment? Unless the answer is "low", then prefer an exhaustive match. The same advice applies to `if let` and `while let`, which are effectively tests for a single variant. ### Use "TODO" comments for things you don't want to forget As a useful tool to yourself, you can insert a `// TODO` comment for something that you want to get back to before you land your PR: ```rust,ignore fn do_something() { if something_else { unimplemented!(); // TODO write this } } ``` The tidy script will report an error for a `// TODO` comment, so this code would not be able to land until the TODO is fixed (or removed). This can also be useful in a PR as a way to signal from one commit that you are leaving a bug that a later commit will fix: ```rust,ignore if foo { return true; // TODO wrong, but will be fixed in a later commit

This section discusses coding conventions related to copyright notices (which should be omitted from new files), line length (aim for under 100 characters, preferably 80, with exceptions using `// ignore-tidy-linelength`), indentation (prefer 4 spaces), and coding for correctness. It emphasizes using exhaustive matches instead of `_` in match statements and `if let`/`while let` constructs. It also recommends using `// TODO` comments to flag unfinished code that needs attention before landing the PR, as the tidy script will check for these.