A note on conditional compilation

Hey,

I personally found this to be a bit of a surprise, so I'm sharing it in the hopes that it'll help someone out.

It's important to note that I'm not saying this is necessarily an issue, but something that I think might not immediately be intuitive for everyone.

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When running cargo (doc|clippy|miri|test...), it's important to note that you're only running on the currently enabled feature set and target. If using conditional compilation for features or targets in your project, to cover 100% of your code (or the cases you wish to cover), you need to run on all features and combinations + all target platforms (when linting or testing for example).

To do this, there are two helpful methods:

For features: You can use cargo-hack or cargo-all-features to run on all feature combinations.

For targets: You can run different jobs in the CI for supported platforms. Some tools also support --target flags.

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I've opened a PR to add this more prominently to the Conditional Compilation entry in the Rust reference.