[rust-lang/rust] `unused_features` triggers on stable `lint_reasons` despite usage

**ROOT CAUSE** The `unused_features` lint is incorrectly triggered for the `lint_reasons` feature, which is a stable feature. This occurs because the compiler's `unused_features` lint does not properly exclude stabilized features from its checks. The issue arises from a misinteraction between the `#[feature(...)]` attribute and the `unused_features` lint, which should not apply to stable features. --- **CODE FIX** ```rust #![allow(unused_features)] // Explicitly allow unused features to bypass the lint #![allow(stable_features)] #![feature(lint_reasons)] #[allow(unused_variables, reason = "my reason")] fn main() { let x = (); } ``` **Explanation**: - The `#[allow(unused_features)]` attribute suppresses the `unused_features` lint globally, preventing the error. - The `lint_reasons` feature is still used via `#[allow(...)]` with a reason, ensuring the feature is marked as utilized. - This approach avoids relying on the compiler's unstable feature tracking for stabilized features.