Suggest async block instead of async closure when possible

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

@@ -798,6 +798,44 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             return false;
         }
 
+        // If this is a zero-argument async closure directly passed as an argument
+        // and the expected type is `Future`, suggest using `async {}` block instead
+        // of `async || {}`
+        if let ty::CoroutineClosure(def_id, args) = *self_ty.kind()
+            && let sig = args.as_coroutine_closure().coroutine_closure_sig().skip_binder()
+            && let ty::Tuple(inputs) = *sig.tupled_inputs_ty.kind()
+            && inputs.is_empty()
+            && self.tcx.is_lang_item(trait_pred.def_id(), LangItem::Future)
+            && let Some(hir::Node::Expr(hir::Expr {
+                kind:
+                    hir::ExprKind::Closure(hir::Closure {
+                        kind: hir::ClosureKind::CoroutineClosure(CoroutineDesugaring::Async),
+                        fn_arg_span: Some(arg_span),
+                        ..
+                    }),
+                ..
+            })) = self.tcx.hir_get_if_local(def_id)
+            && obligation.cause.span.contains(*arg_span)
+        {
+            let sm = self.tcx.sess.source_map();
+            let removal_span = if let Ok(snippet) =
+                sm.span_to_snippet(arg_span.with_hi(arg_span.hi() + rustc_span::BytePos(1)))
+                && snippet.ends_with(' ')
+            {
+                // There's a space after `||`, include it in the removal
+                arg_span.with_hi(arg_span.hi() + rustc_span::BytePos(1))
+            } else {
+                *arg_span
+            };
+            err.span_suggestion_verbose(
+                removal_span,
+                "use `async {}` instead of `async || {}` to introduce an async block",
+                "",
+                Applicability::MachineApplicable,
+            );
+            return true;
+        }
+
         // Get the name of the callable and the arguments to be used in the suggestion.
         let msg = match def_id_or_name {
             DefIdOrName::DefId(def_id) => match self.tcx.def_kind(def_id) {

tests/ui/async-await/async-closures/suggest-async-block-issue-140265.rs

@@ -0,0 +1,26 @@
+//@ edition:2024
+// Test that we suggest using `async {}` block instead of `async || {}` closure if possible
+
+use std::future::Future;
+
+fn takes_future(_fut: impl Future<Output = ()>) {}
+
+fn main() {
+    // Basic case: suggest using async block
+    takes_future(async || {
+        //~^ ERROR is not a future
+        println!("hi!");
+    });
+
+    // Without space between `||` and `{`: should also suggest using async block
+    takes_future(async||{
+        //~^ ERROR is not a future
+        println!("no space!");
+    });
+
+    // With arguments: should suggest calling the closure, not using async block
+    takes_future(async |x: i32| {
+        //~^ ERROR is not a future
+        println!("{x}");
+    });
+}

tests/ui/async-await/async-closures/suggest-async-block-issue-140265.stderr

@@ -0,0 +1,74 @@
+error[E0277]: `{async closure@$DIR/suggest-async-block-issue-140265.rs:10:18: 10:26}` is not a future
+  --> $DIR/suggest-async-block-issue-140265.rs:10:18
+   |
+LL |       takes_future(async || {
+   |  _____------------_^
+   | |     |
+   | |     required by a bound introduced by this call
+LL | |
+LL | |         println!("hi!");
+LL | |     });
+   | |_____^ `{async closure@$DIR/suggest-async-block-issue-140265.rs:10:18: 10:26}` is not a future
+   |
+   = help: the trait `Future` is not implemented for `{async closure@$DIR/suggest-async-block-issue-140265.rs:10:18: 10:26}`
+note: required by a bound in `takes_future`
+  --> $DIR/suggest-async-block-issue-140265.rs:6:28
+   |
+LL | fn takes_future(_fut: impl Future<Output = ()>) {}
+   |                            ^^^^^^^^^^^^^^^^^^^ required by this bound in `takes_future`
+help: use `async {}` instead of `async || {}` to introduce an async block
+   |
+LL -     takes_future(async || {
+LL +     takes_future(async {
+   |
+
+error[E0277]: `{async closure@$DIR/suggest-async-block-issue-140265.rs:16:18: 16:25}` is not a future
+  --> $DIR/suggest-async-block-issue-140265.rs:16:18
+   |
+LL |       takes_future(async||{
+   |  _____------------_^
+   | |     |
+   | |     required by a bound introduced by this call
+LL | |
+LL | |         println!("no space!");
+LL | |     });
+   | |_____^ `{async closure@$DIR/suggest-async-block-issue-140265.rs:16:18: 16:25}` is not a future
+   |
+   = help: the trait `Future` is not implemented for `{async closure@$DIR/suggest-async-block-issue-140265.rs:16:18: 16:25}`
+note: required by a bound in `takes_future`
+  --> $DIR/suggest-async-block-issue-140265.rs:6:28
+   |
+LL | fn takes_future(_fut: impl Future<Output = ()>) {}
+   |                            ^^^^^^^^^^^^^^^^^^^ required by this bound in `takes_future`
+help: use `async {}` instead of `async || {}` to introduce an async block
+   |
+LL -     takes_future(async||{
+LL +     takes_future(async{
+   |
+
+error[E0277]: `{async closure@$DIR/suggest-async-block-issue-140265.rs:22:18: 22:32}` is not a future
+  --> $DIR/suggest-async-block-issue-140265.rs:22:18
+   |
+LL |       takes_future(async |x: i32| {
+   |  _____------------_^
+   | |     |
+   | |     required by a bound introduced by this call
+LL | |
+LL | |         println!("{x}");
+LL | |     });
+   | |_____^ `{async closure@$DIR/suggest-async-block-issue-140265.rs:22:18: 22:32}` is not a future
+   |
+   = help: the trait `Future` is not implemented for `{async closure@$DIR/suggest-async-block-issue-140265.rs:22:18: 22:32}`
+note: required by a bound in `takes_future`
+  --> $DIR/suggest-async-block-issue-140265.rs:6:28
+   |
+LL | fn takes_future(_fut: impl Future<Output = ()>) {}
+   |                            ^^^^^^^^^^^^^^^^^^^ required by this bound in `takes_future`
+help: use parentheses to call this closure
+   |
+LL |     }(/* i32 */));
+   |      +++++++++++
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/suggestions/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.fixed

@@ -0,0 +1,14 @@
+//@ edition:2018
+//@ run-rustfix
+#![allow(unused_variables)]
+use std::future::Future;
+
+async fn foo() {}
+
+fn bar(f: impl Future<Output=()>) {}
+
+fn main() {
+    bar(foo()); //~ERROR E0277
+    let async_closure = async || ();
+    bar(async_closure()); //~ERROR E0277
+}

tests/ui/suggestions/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs

@@ -1,4 +1,6 @@
 //@ edition:2018
+//@ run-rustfix
+#![allow(unused_variables)]
 use std::future::Future;
 
 async fn foo() {}

tests/ui/suggestions/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.stderr

@@ -1,5 +1,5 @@
 error[E0277]: `fn() -> impl Future<Output = ()> {foo}` is not a future
-  --> $DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:9:9
+  --> $DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:11:9
    |
 LL |     bar(foo);
    |     --- ^^^ `fn() -> impl Future<Output = ()> {foo}` is not a future
@@ -8,7 +8,7 @@ LL |     bar(foo);
    |
    = help: the trait `Future` is not implemented for fn item `fn() -> impl Future<Output = ()> {foo}`
 note: required by a bound in `bar`
-  --> $DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:6:16
+  --> $DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:8:16
    |
 LL | fn bar(f: impl Future<Output=()>) {}
    |                ^^^^^^^^^^^^^^^^^ required by this bound in `bar`
@@ -17,17 +17,17 @@ help: use parentheses to call this function
 LL |     bar(foo());
    |            ++
 
-error[E0277]: `{async closure@$DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:10:25: 10:33}` is not a future
-  --> $DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:11:9
+error[E0277]: `{async closure@$DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:12:25: 12:33}` is not a future
+  --> $DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:13:9
    |
 LL |     bar(async_closure);
-   |     --- ^^^^^^^^^^^^^ `{async closure@$DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:10:25: 10:33}` is not a future
+   |     --- ^^^^^^^^^^^^^ `{async closure@$DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:12:25: 12:33}` is not a future
    |     |
    |     required by a bound introduced by this call
    |
-   = help: the trait `Future` is not implemented for `{async closure@$DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:10:25: 10:33}`
+   = help: the trait `Future` is not implemented for `{async closure@$DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:12:25: 12:33}`
 note: required by a bound in `bar`
-  --> $DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:6:16
+  --> $DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:8:16
    |
 LL | fn bar(f: impl Future<Output=()>) {}
    |                ^^^^^^^^^^^^^^^^^ required by this bound in `bar`