Report when a cfg(feature) refers to a feature which doesn't exist on a crate

[illicitonion]

Problem

I have a crate with a feature.

I have CI which runs cargo test with that feature enabled.

I also have some tests which are specific to that feature, which are gated behind a cfg(feature) attribute.

However, there was a typo in the cfg(feature) attribute, which meant that the feature-specific tests were never actually enabled.

Ideally, I would have gotten a compile-time error telling me "You have a cfg(feature) which doesn't exist for your crate" (or just a standard unreachable code diagnostic). But at least a warning would be useful.

Proposed Solution

Provide an error, or at least warning, when a cfg(feature) attribute is used which Cargo doesn't treat as a feature for the crate being compiled.

Notes

I filed this against Cargo because it has the knowledge of the known features of a crate, but it's really a rustc feature request at core - I imagine this either looks like cargo supplying a known feature universe to rustc, or rustc reporting encountered features back to cargo.

[ehuss]

There is an unstable feature called check-cfg which validates cfg expressions. Perhaps you could give it a try and see if it works with your project? Having people test nightly features (even if just once) is helpful since it can be difficult to get real-world testing.

[illicitonion]

I tested it out and #10554 would indeed have prevented my issue - thanks for the pointer!