Add a lint to check needless Waker clones

Author: a1phyr

CHANGELOG.md

@@ -5589,6 +5589,7 @@ Released 2018-09-13
 [`verbose_bit_mask`]: https://rust-lang.github.io/rust-clippy/master/index.html#verbose_bit_mask
 [`verbose_file_reads`]: https://rust-lang.github.io/rust-clippy/master/index.html#verbose_file_reads
 [`vtable_address_comparisons`]: https://rust-lang.github.io/rust-clippy/master/index.html#vtable_address_comparisons
+[`waker_clone_and_wake`]: https://rust-lang.github.io/rust-clippy/master/index.html#waker_clone_and_wake
 [`while_immutable_condition`]: https://rust-lang.github.io/rust-clippy/master/index.html#while_immutable_condition
 [`while_let_loop`]: https://rust-lang.github.io/rust-clippy/master/index.html#while_let_loop
 [`while_let_on_iterator`]: https://rust-lang.github.io/rust-clippy/master/index.html#while_let_on_iterator

clippy_lints/src/declared_lints.rs

@@ -441,6 +441,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::methods::USELESS_ASREF_INFO,
     crate::methods::VEC_RESIZE_TO_ZERO_INFO,
     crate::methods::VERBOSE_FILE_READS_INFO,
+    crate::methods::WAKER_CLONE_AND_WAKE_INFO,
     crate::methods::WRONG_SELF_CONVENTION_INFO,
     crate::methods::ZST_OFFSET_INFO,
     crate::min_ident_chars::MIN_IDENT_CHARS_INFO,

clippy_lints/src/methods/mod.rs

@@ -112,6 +112,7 @@ mod useless_asref;
 mod utils;
 mod vec_resize_to_zero;
 mod verbose_file_reads;
+mod waker_clone_and_wake;
 mod wrong_self_convention;
 mod zst_offset;
 
@@ -3632,6 +3633,28 @@ declare_clippy_lint! {
     "`as_str` used to call a method on `str` that is also available on `String`"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for usage of `clone().wake_by_ref()`
+    ///
+    /// ### Why is this bad?
+    /// Cloning the waker is not necessary, `wake_by_ref()` enables the same operation
+    /// without extra cloning/dropping.
+    ///
+    /// ### Example
+    /// ```rust,ignore
+    /// waker.clone().wake();
+    /// ```
+    /// Should be written
+    /// ```rust,ignore
+    /// waker.wake_by_ref();
+    /// ```
+    #[clippy::version = "1.75.0"]
+    pub WAKER_CLONE_AND_WAKE,
+    perf,
+    "cloning a `Waker` only to wake it"
+}
+
 pub struct Methods {
     avoid_breaking_exported_api: bool,
     msrv: Msrv,
@@ -3777,6 +3800,7 @@ impl_lint_pass!(Methods => [
     ITER_OUT_OF_BOUNDS,
     PATH_ENDS_WITH_EXT,
     REDUNDANT_AS_STR,
+    WAKER_CLONE_AND_WAKE,
 ]);
 
 /// Extracts a method call name, args, and `Span` of the method name.
@@ -4365,6 +4389,9 @@ impl Methods {
                     }
                     unnecessary_literal_unwrap::check(cx, expr, recv, name, args);
                 },
+                ("wake", []) => {
+                    waker_clone_and_wake::check(cx, expr, recv);
+                }
                 ("write", []) => {
                     readonly_write_lock::check(cx, expr, recv);
                 }

clippy_lints/src/methods/waker_clone_and_wake.rs

@@ -0,0 +1,34 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::source::snippet_with_applicability;
+use clippy_utils::{match_def_path, paths};
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::LateContext;
+use rustc_span::sym;
+
+use super::WAKER_CLONE_AND_WAKE;
+
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, recv: &'tcx Expr<'_>) {
+    let ty = cx.typeck_results().expr_ty(recv).peel_refs();
+
+    if let Some(did) = ty.ty_adt_def() &&
+        match_def_path(cx, did.did(), &paths::WAKER) &&
+        let ExprKind::MethodCall(func, waker_ref, &[], _) = recv.kind &&
+        func.ident.name == sym::clone
+    {
+        let mut applicability = Applicability::MachineApplicable;
+
+        span_lint_and_sugg(
+            cx,
+            WAKER_CLONE_AND_WAKE,
+            expr.span,
+            "cloning a `Waker` only to wake it",
+            "replace with",
+            format!(
+                "{}.wake_by_ref()",
+                snippet_with_applicability(cx, waker_ref.span, "..", &mut applicability)
+            ),
+            applicability,
+        );
+    }
+}

clippy_utils/src/paths.rs

@@ -112,6 +112,7 @@ pub const VEC_RESIZE: [&str; 4] = ["alloc", "vec", "Vec", "resize"];
 pub const INSTANT_NOW: [&str; 4] = ["std", "time", "Instant", "now"];
 pub const VEC_IS_EMPTY: [&str; 4] = ["alloc", "vec", "Vec", "is_empty"];
 pub const VEC_POP: [&str; 4] = ["alloc", "vec", "Vec", "pop"];
+pub const WAKER: [&str; 4] = ["core", "task", "wake", "Waker"];
 pub const OPTION_UNWRAP: [&str; 4] = ["core", "option", "Option", "unwrap"];
 pub const OPTION_EXPECT: [&str; 4] = ["core", "option", "Option", "expect"];
 #[expect(clippy::invalid_paths)] // not sure why it thinks this, it works so

tests/ui/waker_clone_and_wake.fixed

@@ -0,0 +1,5 @@
+pub fn wake(cx: &mut std::task::Context) {
+    cx.waker().wake_by_ref()
+}
+
+fn main() {}

tests/ui/waker_clone_and_wake.rs

@@ -0,0 +1,5 @@
+pub fn wake(cx: &mut std::task::Context) {
+    cx.waker().clone().wake()
+}
+
+fn main() {}

tests/ui/waker_clone_and_wake.stderr

@@ -0,0 +1,11 @@
+error: cloning a `Waker` only to wake it
+  --> $DIR/waker_clone_and_wake.rs:2:5
+   |
+LL |     cx.waker().clone().wake()
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `cx.waker().wake_by_ref()`
+   |
+   = note: `-D clippy::waker-clone-and-wake` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::waker_clone_and_wake)]`
+
+error: aborting due to previous error
+