fix: incorrect suggestion for async closure Future trait errors

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -838,10 +838,22 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         if matches!(obligation.cause.code(), ObligationCauseCode::FunctionArg { .. })
             && obligation.cause.span.can_be_used_for_suggestions()
         {
-            let (span, sugg) = if let Some(snippet) =
-                self.tcx.sess.source_map().span_to_snippet(obligation.cause.span).ok()
-                && snippet.starts_with("|")
+            // For inline closures (including async closures), suggest wrapping the entire
+            // closure expression. We check for "async " and "async|" to only match inline
+            // async closures, avoiding variables like `async_closure` or other constructs.
+            let (span, sugg) = if let Ok(snippet) =
+                self.tcx.sess.source_map().span_to_snippet(obligation.cause.span)
+                && (snippet.starts_with("|")
+                    || snippet.starts_with("async ")
+                    || snippet.starts_with("async|"))
             {
+                // Check if this closure already has a call at the end (e.g., `|| {}()` or
+                // `async || {}()`). If so, suppress this suggestion as it would be redundant
+                // with the E0618 error.
+                if snippet.ends_with("()") {
+                    return false;
+                }
+
                 (obligation.cause.span, format!("({snippet})({args})"))
             } else {
                 (obligation.cause.span.shrink_to_hi(), format!("({args})"))

tests/ui/suggestions/async-closure-inline-call-suggestion.rs

@@ -0,0 +1,19 @@
+//@ edition:2021
+use std::future::Future;
+
+async fn f(c: impl Future<Output = ()>) {
+    c.await
+}
+
+fn main() {
+    f(async || {});
+    //~^ ERROR: not a future
+    //~| HELP: the trait `Future` is not implemented for
+    //~| HELP: use parentheses to call this closure
+
+    f(async || {}());
+    //~^ ERROR: expected function, found `()`
+    //~| HELP: if you meant to create this closure and immediately call it, surround the closure with parentheses
+    //~| ERROR: not a future
+    //~| HELP: the trait `Future` is not implemented for
+}

tests/ui/suggestions/async-closure-inline-call-suggestion.stderr

@@ -0,0 +1,52 @@
+error[E0277]: `{async closure@$DIR/async-closure-inline-call-suggestion.rs:9:7: 9:15}` is not a future
+  --> $DIR/async-closure-inline-call-suggestion.rs:9:7
+   |
+LL |     f(async || {});
+   |     - ^^^^^^^^^^^ `{async closure@$DIR/async-closure-inline-call-suggestion.rs:9:7: 9:15}` is not a future
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Future` is not implemented for `{async closure@$DIR/async-closure-inline-call-suggestion.rs:9:7: 9:15}`
+note: required by a bound in `f`
+  --> $DIR/async-closure-inline-call-suggestion.rs:4:20
+   |
+LL | async fn f(c: impl Future<Output = ()>) {
+   |                    ^^^^^^^^^^^^^^^^^^^ required by this bound in `f`
+help: use parentheses to call this closure
+   |
+LL -     f(async || {});
+LL +     f((async || {})());
+   |
+
+error[E0618]: expected function, found `()`
+  --> $DIR/async-closure-inline-call-suggestion.rs:14:16
+   |
+LL |     f(async || {}());
+   |                ^^--
+   |                |
+   |                call expression requires function
+   |
+help: if you meant to create this closure and immediately call it, surround the closure with parentheses
+   |
+LL |     f((async || {})());
+   |       +           +
+
+error[E0277]: `{async closure@$DIR/async-closure-inline-call-suggestion.rs:14:7: 14:15}` is not a future
+  --> $DIR/async-closure-inline-call-suggestion.rs:14:7
+   |
+LL |     f(async || {}());
+   |     - ^^^^^^^^^^^^^ `{async closure@$DIR/async-closure-inline-call-suggestion.rs:14:7: 14:15}` is not a future
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Future` is not implemented for `{async closure@$DIR/async-closure-inline-call-suggestion.rs:14:7: 14:15}`
+note: required by a bound in `f`
+  --> $DIR/async-closure-inline-call-suggestion.rs:4:20
+   |
+LL | async fn f(c: impl Future<Output = ()>) {
+   |                    ^^^^^^^^^^^^^^^^^^^ required by this bound in `f`
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0277, E0618.
+For more information about an error, try `rustc --explain E0277`.