[unused_async]: don't lint on async trait impls

clippy_lints/src/unused_async.rs

@@ -1,6 +1,6 @@
 use clippy_utils::diagnostics::span_lint_and_then;
 use rustc_hir::intravisit::{walk_body, walk_expr, walk_fn, FnKind, Visitor};
-use rustc_hir::{Body, Expr, ExprKind, FnDecl, YieldSource};
+use rustc_hir::{Body, Expr, ExprKind, FnDecl, ItemKind, Node, YieldSource};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::hir::nested_filter;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
@@ -81,6 +81,20 @@ impl<'a, 'tcx> Visitor<'tcx> for AsyncFnVisitor<'a, 'tcx> {
     }
 }
 
+/// Checks if the `def_id` belongs to a function and that function is part of a trait impl,
+/// in which case we shouldn't lint because `async` is part of the trait definition and therefore
+/// can't be removed.
+fn is_in_trait_impl(cx: &LateContext<'_>, def_id: LocalDefId) -> bool {
+    if let Some(hir_id) = cx.tcx.opt_local_def_id_to_hir_id(def_id)
+        && let Node::Item(item) = cx.tcx.hir().get_parent(hir_id)
+        && let ItemKind::Impl(imp) = item.kind
+    {
+        imp.of_trait.is_some()
+    } else {
+        false
+    }
+}
+
 impl<'tcx> LateLintPass<'tcx> for UnusedAsync {
     fn check_fn(
         &mut self,
@@ -91,7 +105,7 @@ impl<'tcx> LateLintPass<'tcx> for UnusedAsync {
         span: Span,
         def_id: LocalDefId,
     ) {
-        if !span.from_expansion() && fn_kind.asyncness().is_async() {
+        if !span.from_expansion() && fn_kind.asyncness().is_async() && !is_in_trait_impl(cx, def_id) {
             let mut visitor = AsyncFnVisitor {
                 cx,
                 found_await: false,

tests/ui/unused_async.rs

@@ -1,4 +1,6 @@
 #![warn(clippy::unused_async)]
+#![feature(async_fn_in_trait)]
+#![allow(incomplete_features)]
 
 use std::future::Future;
 use std::pin::Pin;
@@ -23,6 +25,18 @@ mod issue10800 {
     }
 }
 
+mod issue10459 {
+    trait HasAsyncMethod {
+        async fn do_something() -> u32;
+    }
+
+    impl HasAsyncMethod for () {
+        async fn do_something() -> u32 {
+            1
+        }
+    }
+}
+
 async fn foo() -> i32 {
     4
 }

tests/ui/unused_async.stderr

@@ -1,5 +1,5 @@
 error: unused `async` for function with no await statements
-  --> $DIR/unused_async.rs:11:5
+  --> $DIR/unused_async.rs:13:5
    |
 LL | /     async fn async_block_await() {
 LL | |         async {
@@ -10,14 +10,14 @@ LL | |     }
    |
    = help: consider removing the `async` from this function
 note: `await` used in an async block, which does not require the enclosing function to be `async`
-  --> $DIR/unused_async.rs:13:23
+  --> $DIR/unused_async.rs:15:23
    |
 LL |             ready(()).await;
    |                       ^^^^^
    = note: `-D clippy::unused-async` implied by `-D warnings`
 
 error: unused `async` for function with no await statements
-  --> $DIR/unused_async.rs:26:1
+  --> $DIR/unused_async.rs:40:1
    |
 LL | / async fn foo() -> i32 {
 LL | |     4
@@ -27,7 +27,7 @@ LL | | }
    = help: consider removing the `async` from this function
 
 error: unused `async` for function with no await statements
-  --> $DIR/unused_async.rs:37:5
+  --> $DIR/unused_async.rs:51:5
    |
 LL | /     async fn unused(&self) -> i32 {
 LL | |         1