Tracking issue for future-incompatibility lint `soft_unstable`

[petrochenkov]

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.

Features that are currently emitting this lint

The #[bench] attribute

Tracking issue: #50297

• Support "soft" feature-gating using a lint #64066
• de-stabilize bench attribute #134273

RustcEncodable & RustcDecodable

Tracking issue: #134301

• Soft-destabilize RustcEncodable & RustcDecodable, remove from prelude in next edition #116016
• Remove rustc_encodable_decodable feature #134272

Features that were previously emitting this lint

Inner #![test] and #![rustfmt::skip]

Tracking issue: #54726

• rustc_expand: Mark inner #![test] attributes as soft-unstable #79003
• expand: Resolve and expand inner attributes on out-of-line modules #82399
• fully de-stabilize all custom inner attributes #134276

Before adding new features here, read this

We should really support a separate tracking issue for each case of a soft-unstable feature; having them all point here doesn't make a ton of sense.

[lqd]

#[deny(soft_unstable)] is on by default, but the error message (playground) still has a lint-like note warning mentioning it will become a hard error in the future:

= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #64266 <https://github.com/rust-lang/rust/issues/64266>

do we need an issue for this ?