[feat] report unused feature

What it does

When dev, may enable some features.

// lib.rs
#![allow(incomplete_features)]
#![feature(generic_const_exprs)]

// hello.rs
code
code using generic_const_exprs
code

Then later, the related code using feature are removed.

// hello.rs
code
code

But maybe forget to remove these header.

// lib.rs
#![allow(incomplete_features)]
#![feature(generic_const_exprs)]

Advantage

• clean lib.rs

• per delete line, per SLOW build

Drawbacks

There seems many features, not sure if clippy can get info from rustc about unused feature.

Example

For scenario, related code are remove.

// hello.rs
code
code

Rerport all unused #![feature(XXX)]

// lib.rs
#![allow(incomplete_features)]
#![feature(generic_const_exprs)]

And maybe also report not need to enable features, feature need to enable before prev rust version but no need within new rust.

Start with the examples in lib.rs and hello.rs, then investigate whether rustc exposes enough information for Clippy to identify unused or no-longer-needed #![feature(...)] declarations. Done would be a defined, supportable report for the requested unused-feature scenario, with its scope and limitations documented.