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

(none)

Features that were previously 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

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 ?