Auto merge of #13471 - y21:issue13466-fp, r=llogiq

Remove method call receiver special casing in `unused_async` lint

Fixes the false positive mentioned in https://github.com/rust-lang/rust-clippy/issues/13466#issuecomment-2380025514.

The false negative in the OP would be nice to fix too, but I'd rather do that in a separate PR because it's much more involved

Before this change, the `unused_async` lint would check if the async fn is also used anywhere and avoid linting if so. The exception is if the async function is immediately called, because the returned future handling can be easily removed (and also if we don't have some exceptions then the lint wouldn't trigger anywhere) *or* if it's a method call receiver.

I'm not exactly sure why I implemented that special casing for method call receivers in #11200, but it doesn't make much sense in hindsight imo. Especially given that method calls are essentially equivalent to function calls with the receiver as the first argument, which was the primary motivation for not linting in the first place (async fn passed to another function, like `axum::get(handler)` where handler has to be an async fn).

changelog: none

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 | |