chore: use multipart_suggestions for manual_async_fn (#13788)

This addresses #13099 for the manual_async_fn test.

changelog: [manual_async_fn]: Updated manual_async_fn to use
multipart_suggestions where appropriate

clippy_lints/src/manual_async_fn.rs

@@ -74,27 +74,22 @@ impl<'tcx> LateLintPass<'tcx> for ManualAsyncFn {
                     if let Some(vis_snip) = vis_span.get_source_text(cx)
                         && let Some(header_snip) = header_span.get_source_text(cx)
                         && let Some(ret_pos) = position_before_rarrow(&header_snip)
-                        && let Some((ret_sugg, ret_snip)) = suggested_ret(cx, output)
+                        && let Some((_, ret_snip)) = suggested_ret(cx, output)
                     {
                         let header_snip = if vis_snip.is_empty() {
                             format!("async {}", &header_snip[..ret_pos])
                         } else {
                             format!("{} async {}", vis_snip, &header_snip[vis_snip.len() + 1..ret_pos])
                         };
 
-                        let help = format!("make the function `async` and {ret_sugg}");
-                        diag.span_suggestion(
-                            header_span,
-                            help,
-                            format!("{header_snip}{ret_snip}"),
-                            Applicability::MachineApplicable,
-                        );
+                        let body_snip = snippet_block(cx, closure_body.value.span, "..", Some(block.span)).to_string();
 
-                        let body_snip = snippet_block(cx, closure_body.value.span, "..", Some(block.span));
-                        diag.span_suggestion(
-                            block.span,
-                            "move the body of the async block to the enclosing function",
-                            body_snip,
+                        diag.multipart_suggestion(
+                            "make the function `async` and return the output of the future directly",
+                            vec![
+                                (header_span, format!("{header_snip}{ret_snip}")),
+                                (block.span, body_snip),
+                            ],
                             Applicability::MachineApplicable,
                         );
                     }

tests/ui/manual_async_fn.fixed

@@ -0,0 +1,116 @@
+#![warn(clippy::manual_async_fn)]
+#![allow(clippy::needless_pub_self, unused)]
+
+use std::future::Future;
+
+async fn fut() -> i32 { 42 }
+
+#[rustfmt::skip]
+async fn fut2() -> i32 { 42 }
+
+#[rustfmt::skip]
+async fn fut3() -> i32 { 42 }
+
+async fn empty_fut() {}
+
+#[rustfmt::skip]
+async fn empty_fut2() {}
+
+#[rustfmt::skip]
+async fn empty_fut3() {}
+
+async fn core_fut() -> i32 { 42 }
+
+// should be ignored
+fn has_other_stmts() -> impl core::future::Future<Output = i32> {
+    let _ = 42;
+    async move { 42 }
+}
+
+// should be ignored
+fn not_fut() -> i32 {
+    42
+}
+
+// should be ignored
+async fn already_async() -> impl Future<Output = i32> {
+    async { 42 }
+}
+
+struct S;
+impl S {
+    async fn inh_fut() -> i32 {
+        // NOTE: this code is here just to check that the indentation is correct in the suggested fix
+        let a = 42;
+        let b = 21;
+        if a < b {
+            let c = 21;
+            let d = 42;
+            if c < d {
+                let _ = 42;
+            }
+        }
+        42
+    }
+
+    // should be ignored
+    fn not_fut(&self) -> i32 {
+        42
+    }
+
+    // should be ignored
+    fn has_other_stmts() -> impl core::future::Future<Output = i32> {
+        let _ = 42;
+        async move { 42 }
+    }
+
+    // should be ignored
+    async fn already_async(&self) -> impl Future<Output = i32> {
+        async { 42 }
+    }
+}
+
+// Tests related to lifetime capture
+
+async fn elided(_: &i32) -> i32 { 42 }
+
+// should be ignored
+fn elided_not_bound(_: &i32) -> impl Future<Output = i32> {
+    async { 42 }
+}
+
+#[allow(clippy::needless_lifetimes)]
+async fn explicit<'a, 'b>(_: &'a i32, _: &'b i32) -> i32 { 42 }
+
+// should be ignored
+#[allow(clippy::needless_lifetimes)]
+fn explicit_not_bound<'a, 'b>(_: &'a i32, _: &'b i32) -> impl Future<Output = i32> {
+    async { 42 }
+}
+
+// should be ignored
+mod issue_5765 {
+    use std::future::Future;
+
+    struct A;
+    impl A {
+        fn f(&self) -> impl Future<Output = ()> {
+            async {}
+        }
+    }
+
+    fn test() {
+        let _future = {
+            let a = A;
+            a.f()
+        };
+    }
+}
+
+pub async fn issue_10450() -> i32 { 42 }
+
+pub(crate) async fn issue_10450_2() -> i32 { 42 }
+
+pub(self) async fn issue_10450_3() -> i32 { 42 }
+
+fn main() {}

tests/ui/manual_async_fn.rs

@@ -1,8 +1,6 @@
 #![warn(clippy::manual_async_fn)]
 #![allow(clippy::needless_pub_self, unused)]
 
-//@no-rustfix: need to change the suggestion to a multipart suggestion
-
 use std::future::Future;
 
 fn fut() -> impl Future<Output = i32> {
@@ -99,6 +97,7 @@ fn elided_not_bound(_: &i32) -> impl Future<Output = i32> {
     async { 42 }
 }
 
+#[allow(clippy::needless_lifetimes)]
 fn explicit<'a, 'b>(_: &'a i32, _: &'b i32) -> impl Future<Output = i32> + 'a + 'b {
     async { 42 }
 }