Add allow_attribute lint

CHANGELOG.md

@@ -4382,6 +4382,7 @@ Released 2018-09-13
 <!-- begin autogenerated links to lint list -->
 [`absurd_extreme_comparisons`]: https://rust-lang.github.io/rust-clippy/master/index.html#absurd_extreme_comparisons
 [`alloc_instead_of_core`]: https://rust-lang.github.io/rust-clippy/master/index.html#alloc_instead_of_core
+[`allow_attributes`]: https://rust-lang.github.io/rust-clippy/master/index.html#allow_attributes
 [`allow_attributes_without_reason`]: https://rust-lang.github.io/rust-clippy/master/index.html#allow_attributes_without_reason
 [`almost_complete_letter_range`]: https://rust-lang.github.io/rust-clippy/master/index.html#almost_complete_letter_range
 [`almost_complete_range`]: https://rust-lang.github.io/rust-clippy/master/index.html#almost_complete_range

clippy_lints/src/allow_attributes.rs

@@ -0,0 +1,71 @@
+use ast::AttrStyle;
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use rustc_ast as ast;
+use rustc_errors::Applicability;
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// Detects uses of the `#[allow]` attribute and suggests replacing it with
+    /// the `#[expect]` (See [RFC 2383](https://rust-lang.github.io/rfcs/2383-lint-reasons.html))
+    ///
+    /// The expect attribute is still unstable and requires the `lint_reasons`
+    /// on nightly. It can be enabled by adding `#![feature(lint_reasons)]` to
+    /// the crate root.
+    ///
+    /// This lint only warns outer attributes (`#[allow]`), as inner attributes
+    /// (`#![allow]`) are usually used to enable or disable lints on a global scale.
+    ///
+    /// ### Why is this bad?
+    ///
+    /// `#[expect]` attributes suppress the lint emission, but emit a warning, if
+    /// the expectation is unfulfilled. This can be useful to be notified when the
+    /// lint is no longer triggered.
+    ///
+    /// ### Example
+    /// ```rust,ignore
+    /// #[allow(unused_mut)]
+    /// fn foo() -> usize {
+    ///    let mut a = Vec::new();
+    ///    a.len()
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```rust,ignore
+    /// #![feature(lint_reasons)]
+    /// #[expect(unused_mut)]
+    /// fn foo() -> usize {
+    ///     let mut a = Vec::new();
+    ///     a.len()
+    /// }
+    /// ```
+    #[clippy::version = "1.69.0"]
+    pub ALLOW_ATTRIBUTES,
+    restriction,
+    "`#[allow]` will not trigger if a warning isn't found. `#[expect]` triggers if there are no warnings."
+}
+
+declare_lint_pass!(AllowAttribute => [ALLOW_ATTRIBUTES]);
+
+impl LateLintPass<'_> for AllowAttribute {
+    // Separate each crate's features.
+    fn check_attribute(&mut self, cx: &LateContext<'_>, attr: &ast::Attribute) {
+        if_chain! {
+            if cx.tcx.features().lint_reasons;
+            if let AttrStyle::Outer = attr.style;
+            if let Some(ident) = attr.ident();
+            if ident.name == rustc_span::symbol::sym::allow;
+            then {
+                span_lint_and_sugg(
+                    cx,
+                    ALLOW_ATTRIBUTES,
+                    ident.span,
+                    "#[allow] attribute found",
+                    "replace it with",
+                    "expect".into(),
+                    Applicability::MachineApplicable,
+                );
+            }
+        }
+    }
+}

clippy_lints/src/declared_lints.rs

@@ -35,6 +35,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::utils::internal_lints::produce_ice::PRODUCE_ICE_INFO,
     #[cfg(feature = "internal")]
     crate::utils::internal_lints::unnecessary_def_path::UNNECESSARY_DEF_PATH_INFO,
+    crate::allow_attributes::ALLOW_ATTRIBUTES_INFO,
     crate::almost_complete_range::ALMOST_COMPLETE_RANGE_INFO,
     crate::approx_const::APPROX_CONSTANT_INFO,
     crate::as_conversions::AS_CONVERSIONS_INFO,

clippy_lints/src/lib.rs

@@ -67,6 +67,7 @@ mod declared_lints;
 mod renamed_lints;
 
 // begin lints modules, do not remove this comment, it’s used in `update_lints`
+mod allow_attributes;
 mod almost_complete_range;
 mod approx_const;
 mod as_conversions;
@@ -933,6 +934,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|_| Box::new(missing_assert_message::MissingAssertMessage));
     store.register_early_pass(|| Box::new(redundant_async_block::RedundantAsyncBlock));
     store.register_late_pass(|_| Box::new(let_with_type_underscore::UnderscoreTyped));
+    store.register_late_pass(|_| Box::new(allow_attributes::AllowAttribute));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

tests/ui/allow_attributes.fixed

@@ -0,0 +1,25 @@
+// run-rustfix
+#![allow(unused)]
+#![warn(clippy::allow_attributes)]
+#![feature(lint_reasons)]
+
+fn main() {}
+
+// Using clippy::needless_borrow just as a placeholder, it isn't relevant.
+
+// Should lint
+#[expect(dead_code)]
+struct T1;
+
+struct T2; // Should not lint
+#[deny(clippy::needless_borrow)] // Should not lint
+struct T3;
+#[warn(clippy::needless_borrow)] // Should not lint
+struct T4;
+// `panic = "unwind"` should always be true
+#[cfg_attr(panic = "unwind", expect(dead_code))]
+struct CfgT;
+
+fn ignore_inner_attr() {
+    #![allow(unused)] // Should not lint
+}

tests/ui/allow_attributes.rs

@@ -0,0 +1,25 @@
+// run-rustfix
+#![allow(unused)]
+#![warn(clippy::allow_attributes)]
+#![feature(lint_reasons)]
+
+fn main() {}
+
+// Using clippy::needless_borrow just as a placeholder, it isn't relevant.
+
+// Should lint
+#[allow(dead_code)]
+struct T1;
+
+struct T2; // Should not lint
+#[deny(clippy::needless_borrow)] // Should not lint
+struct T3;
+#[warn(clippy::needless_borrow)] // Should not lint
+struct T4;
+// `panic = "unwind"` should always be true
+#[cfg_attr(panic = "unwind", allow(dead_code))]
+struct CfgT;
+
+fn ignore_inner_attr() {
+    #![allow(unused)] // Should not lint
+}

tests/ui/allow_attributes.stderr

@@ -0,0 +1,16 @@
+error: #[allow] attribute found
+  --> $DIR/allow_attributes.rs:11:3
+   |
+LL | #[allow(dead_code)]
+   |   ^^^^^ help: replace it with: `expect`
+   |
+   = note: `-D clippy::allow-attributes` implied by `-D warnings`
+
+error: #[allow] attribute found
+  --> $DIR/allow_attributes.rs:20:30
+   |
+LL | #[cfg_attr(panic = "unwind", allow(dead_code))]
+   |                              ^^^^^ help: replace it with: `expect`
+
+error: aborting due to 2 previous errors
+