Detect unknown features inside `#[cfg(feature = "…")]` blocks

[emilk]

What it does

Look for #[cfg(feature = "NAME")], cfg!(feature = "NAME"), #[cfg_attr(feature = "NAME", …)] etc and check that NAME is a real feature found in Cargo.toml

Advantage

• Catches typos (#[cfg(feature = "sedre")])
• Catches refactors that removes or renames a feature

Example

#[cfg_attr(feature = "sedre", derive(serde::Deserialize, serde::Serialize))]
struct Foo {
}

Suggested output:

Clippy: "No feature called sedre. Available features are: default, serde

Related

• https://rust-lang.github.io/rfcs/3013-conditional-compilation-checking.html
• https://doc.rust-lang.org/cargo/reference/unstable.html#check-cfg

May sounds similar, but are different:

• Detect invalid cfg expressions, such as the cfg(feature_s_ = ...) typo #10224
• typos inside cfg macros go undetected #5051

[emilk]

Tagging @GuillaumeGomez, as they suggested I do so

[GuillaumeGomez]

I'll check what can be done about it. https://rust-lang.github.io/rfcs/3013-conditional-compilation-checking.html seems to be covering this case as well. The big blocker here is in case a project is not using Cargo. But I guess the lint can be skipped in this case.

[Centri3]

I'd say this is ok. I don't think you can easily run clippy without cargo anyway, or is even supported.

Couple things of note:

• This data must be available in cargo metadata (which I believe it is).
• This will be allow-by-default as it would be in the cargo category (I don't think it should be disabled by default, though)
• This must be a pre-expansion lint as these attributes would be stripped during macro expansion since it will always evaluate to false (since the feature is missing). This would probably be in attrs.rs despite being based around cargo metadata due to this, but I'm not sure.

[GuillaumeGomez]

There is a work in progress for this to be done in rust directly: rust-lang/rust#82450

[epage]

FYI the cargo/rustc feature has been proposed for merge. Cargo has approved its side of it and its waiting on rustc.

[Alexendoo]

Closing in favour of rust-lang/rust#82450