Remove method call receiver special casing in unused_async lint

clippy_lints/src/unused_async.rs

@@ -2,7 +2,7 @@ use clippy_utils::diagnostics::span_lint_hir_and_then;
 use clippy_utils::is_def_id_trait_method;
 use rustc_hir::def::DefKind;
 use rustc_hir::intravisit::{FnKind, Visitor, walk_expr, walk_fn};
-use rustc_hir::{Body, Expr, ExprKind, FnDecl, Node, YieldSource};
+use rustc_hir::{Body, Expr, ExprKind, FnDecl, HirId, Node, YieldSource};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::hir::nested_filter;
 use rustc_session::impl_lint_pass;
@@ -137,17 +137,7 @@ impl<'tcx> LateLintPass<'tcx> for UnusedAsync {
         }
     }
 
-    fn check_path(&mut self, cx: &LateContext<'tcx>, path: &rustc_hir::Path<'tcx>, hir_id: rustc_hir::HirId) {
-        fn is_node_func_call(node: Node<'_>, expected_receiver: Span) -> bool {
-            matches!(
-                node,
-                Node::Expr(Expr {
-                    kind: ExprKind::Call(Expr { span, .. }, _) | ExprKind::MethodCall(_, Expr { span, .. }, ..),
-                    ..
-                }) if *span == expected_receiver
-            )
-        }
-
+    fn check_path(&mut self, cx: &LateContext<'tcx>, path: &rustc_hir::Path<'tcx>, hir_id: HirId) {
         // Find paths to local async functions that aren't immediately called.
         // E.g. `async fn f() {}; let x = f;`
         // Depending on how `x` is used, f's asyncness might be required despite not having any `await`
@@ -156,7 +146,14 @@ impl<'tcx> LateLintPass<'tcx> for UnusedAsync {
             && let Some(local_def_id) = def_id.as_local()
             && cx.tcx.def_kind(def_id) == DefKind::Fn
             && cx.tcx.asyncness(def_id).is_async()
-            && !is_node_func_call(cx.tcx.parent_hir_node(hir_id), path.span)
+            && let parent = cx.tcx.parent_hir_node(hir_id)
+            && !matches!(
+                parent,
+                Node::Expr(Expr {
+                    kind: ExprKind::Call(Expr { span, .. }, _),
+                    ..
+                }) if *span == path.span
+            )
         {
             self.async_fns_as_value.insert(local_def_id);
         }

tests/ui/unused_async.rs

@@ -55,6 +55,22 @@ mod issue9695 {
     }
 }
 
+mod issue13466 {
+    use std::future::Future;
+
+    struct Wrap<F>(F);
+    impl<F> From<F> for Wrap<F> {
+        fn from(f: F) -> Self {
+            Self(f)
+        }
+    }
+    fn takes_fut<F: Fn() -> Fut, Fut: Future>(_: Wrap<F>) {}
+    async fn unused_async() {}
+    fn fp() {
+        takes_fut(unused_async.into());
+    }
+}
+
 async fn foo() -> i32 {
     //~^ ERROR: unused `async` for function with no await statements
     4

tests/ui/unused_async.stderr

@@ -27,7 +27,7 @@ LL |     async fn f3() {}
    = help: consider removing the `async` from this function
 
 error: unused `async` for function with no await statements
-  --> tests/ui/unused_async.rs:58:1
+  --> tests/ui/unused_async.rs:74:1
    |
 LL | / async fn foo() -> i32 {
 LL | |
@@ -38,7 +38,7 @@ LL | | }
    = help: consider removing the `async` from this function
 
 error: unused `async` for function with no await statements
-  --> tests/ui/unused_async.rs:70:5
+  --> tests/ui/unused_async.rs:86:5
    |
 LL | /     async fn unused(&self) -> i32 {
 LL | |