What is this lint about

When some feature is unstable, but was allowed on stable due to some stability checking hole or oversight we may unstabilize it in a "soft" way to avoid breaking crates depending on the crates using the feature.

This soft unstabilization can be done using this lint.

How to fix this warning/error

Remove or cfg-out uses of the unstable feature in configurations of your code that are supposed to build on stable.

Current status

• Support "soft" feature-gating using a lint #64066 uses soft_unstable for the attribute #[bench] (feature(test)).
• rustc_expand: Mark inner #![test] attributes as soft-unstable #79003 uses soft_unstable for inner #![test] (feature(custom_inner_attributes)).
• expand: Resolve and expand inner attributes on out-of-line modules #82399 uses soft_unstable for inner #![rustfmt::skip] on out-of-line modules (feature(custom_inner_attributes)).