`expect_unstable` lint for feature instability assertions

What it does

This lint notifies a user when a given feature is stable. It lets users mark blocks, items, and statements with an expectation: #[expect_unstable("some_unstable_feature")]

Configuration

• follow-msrv: Whether the crate's MSRV should be considered when checking for this lint. If the feature is stable, but the MSRV is set to something lower than the stabilized version, the lint will not apply. (default: true)

Advantage

• Avoids confusing (and difficult-to-track) TODOs and FIXMEs where unstable features would improve source code.
• Allows users to get rid of manual checks in CI.
• Refines project quality as Rust contributors refine the language.

Drawbacks

No response

Example

// FIXME: make sure to use `core::error::Error` when stable!
// i sure hope someone notices this after 1.81..!
use std::error::Error;

Could be written as:

#[expect_unstable("error_in_core")]
use std::error::Error;

No implementation file or test is named. Start by locating Clippy's existing lint and configuration patterns, then review how Rust feature stability and MSRV checks are represented; done means the proposed expect_unstable syntax, follow-msrv behavior, and diagnostics are covered by tests.