manual_async_fn: take input lifetimes into account

The anonymous future returned from an `async fn` captures all input
lifetimes. This was not being taken into account.

See https://github.com/rust-lang/rfcs/blob/master/text/2394-async_await.md#lifetime-capture-in-the-anonymous-future

tests/ui/await_holding_lock.rs

@@ -47,6 +47,7 @@ async fn not_good(x: &Mutex<u32>) -> u32 {
     first + second + third
 }
 
+#[allow(clippy::manual_async_fn)]
 fn block_bad(x: &Mutex<u32>) -> impl std::future::Future<Output = u32> + '_ {
     async move {
         let guard = x.lock().unwrap();

tests/ui/await_holding_lock.stderr

@@ -46,13 +46,13 @@ LL | |     };
    | |_____^
 
 error: this MutexGuard is held across an 'await' point. Consider using an async-aware Mutex type or ensuring the MutexGuard is dropped before calling await.
-  --> $DIR/await_holding_lock.rs:52:13
+  --> $DIR/await_holding_lock.rs:53:13
    |
 LL |         let guard = x.lock().unwrap();
    |             ^^^^^
    |
 note: these are all the await points this lock is held through
-  --> $DIR/await_holding_lock.rs:52:9
+  --> $DIR/await_holding_lock.rs:53:9
    |
 LL | /         let guard = x.lock().unwrap();
 LL | |         baz().await

tests/ui/manual_async_fn.fixed

@@ -43,10 +43,6 @@ impl S {
         42
     }
 
-    async fn meth_fut(&self) -> i32 { 42 }
-
-    async fn empty_fut(&self)  {}
-
     // should be ignored
     fn not_fut(&self) -> i32 {
         42
@@ -64,4 +60,40 @@ impl S {
     }
 }
 
+// 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 }
+}
+
+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()
+        };
+    }
+}
+
 fn main() {}

tests/ui/manual_async_fn.rs

@@ -51,14 +51,6 @@ impl S {
         }
     }
 
-    fn meth_fut(&self) -> impl Future<Output = i32> {
-        async { 42 }
-    }
-
-    fn empty_fut(&self) -> impl Future<Output = ()> {
-        async {}
-    }
-
     // should be ignored
     fn not_fut(&self) -> i32 {
         42
@@ -76,4 +68,44 @@ impl S {
     }
 }
 
+// Tests related to lifetime capture
+
+fn elided(_: &i32) -> impl Future<Output = i32> + '_ {
+    async { 42 }
+}
+
+// should be ignored
+fn elided_not_bound(_: &i32) -> impl Future<Output = i32> {
+    async { 42 }
+}
+
+fn explicit<'a, 'b>(_: &'a i32, _: &'b i32) -> impl Future<Output = i32> + 'a + 'b {
+    async { 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()
+        };
+    }
+}
+
 fn main() {}

tests/ui/manual_async_fn.stderr

@@ -65,34 +65,34 @@ LL |             let c = 21;
  ...
 
 error: this function can be simplified using the `async fn` syntax
-  --> $DIR/manual_async_fn.rs:54:5
+  --> $DIR/manual_async_fn.rs:73:1
    |
-LL |     fn meth_fut(&self) -> impl Future<Output = i32> {
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | fn elided(_: &i32) -> impl Future<Output = i32> + '_ {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 help: make the function `async` and return the output of the future directly
    |
-LL |     async fn meth_fut(&self) -> i32 {
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | async fn elided(_: &i32) -> i32 {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 help: move the body of the async block to the enclosing function
    |
-LL |     fn meth_fut(&self) -> impl Future<Output = i32> { 42 }
-   |                                                     ^^^^^^
+LL | fn elided(_: &i32) -> impl Future<Output = i32> + '_ { 42 }
+   |                                                      ^^^^^^
 
 error: this function can be simplified using the `async fn` syntax
-  --> $DIR/manual_async_fn.rs:58:5
+  --> $DIR/manual_async_fn.rs:82:1
    |
-LL |     fn empty_fut(&self) -> impl Future<Output = ()> {
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | fn explicit<'a, 'b>(_: &'a i32, _: &'b i32) -> impl Future<Output = i32> + 'a + 'b {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-help: make the function `async` and remove the return type
+help: make the function `async` and return the output of the future directly
    |
-LL |     async fn empty_fut(&self)  {
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | async fn explicit<'a, 'b>(_: &'a i32, _: &'b i32) -> i32 {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 help: move the body of the async block to the enclosing function
    |
-LL |     fn empty_fut(&self) -> impl Future<Output = ()> {}
-   |                                                     ^^
+LL | fn explicit<'a, 'b>(_: &'a i32, _: &'b i32) -> impl Future<Output = i32> + 'a + 'b { 42 }
+   |                                                                                    ^^^^^^
 
 error: aborting due to 6 previous errors