`expect_unstable` lint for feature instability assertions

[onkoe]

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;

[Jarcho]

Just since I was confused when I read this the first time, this is intended to label code which is supposed to be changed once a feature becomes stable?

Nominated as this is related to a potential change to how we handle MSRVs.

[onkoe]

@Jarcho Yes, you have it right! It's intended to alert users when unstable features are stabilized.

[Jarcho]

Discussed this briefly at the last meeting. There were no objections to doing this. This won't need a config option for MSRV handling since #13416 would handle that in a more general way.