Consider `#[must_use]` annotation on `async fn` as also affecting the `Future::Output`

No longer lint against `#[must_use] async fn foo()`.

When encountering a statement that awaits on a `Future`, check if the
`Future`'s parent item is annotated with `#[must_use]` and emit a lint
if so. This effectively makes `must_use` an annotation on the
`Future::Output` instead of only the `Future` itself.

Fix #78149.

2 files changed, 78 insertions, 30 deletions

diff --git a/src/test/ui/lint/unused/unused-async.rs b/src/test/ui/lint/unused/unused-async.rs
index 7d17af11573..eda28dab27f 100644
--- a/src/test/ui/lint/unused/unused-async.rs
+++ b/src/test/ui/lint/unused/unused-async.rs
@@ -1,24 +1,43 @@
 // edition:2018
-// run-pass
-#![allow(dead_code)]
+#![deny(unused_must_use)]
+
 
 #[must_use]
-//~^ WARNING `must_use`
-async fn test() -> i32 {
+async fn foo() -> i32 {
     1
 }
 
+#[must_use]
+fn bar() -> impl std::future::Future<Output=i32> {
+    async {
+        42
+    }
+}
+
+async fn baz() -> i32 {
+    0
+}
 
 struct Wowee {}
 
 impl Wowee {
     #[must_use]
-    //~^ WARNING `must_use`
     async fn test_method() -> i32 {
         1
     }
 }
 
+async fn test() {
+    foo(); //~ ERROR unused return value of `foo` that must be used
+    //~^ ERROR unused implementer of `Future` that must be used
+    foo().await; //~ ERROR unused awaited future returned by `foo` that must be used
+    bar(); //~ ERROR unused return value of `bar` that must be used
+    //~^ ERROR unused implementer of `Future` that must be used
+    bar().await; //~ ERROR unused awaited future returned by `bar` that must be used
+    baz(); //~ ERROR unused implementer of `Future` that must be used
+    baz().await; // ok
+}
+
 /* FIXME(guswynn) update this test when async-fn-in-traits works
 trait Doer {
     #[must_use]
diff --git a/src/test/ui/lint/unused/unused-async.stderr b/src/test/ui/lint/unused/unused-async.stderr
index 6bbc9e2bf00..ae284681720 100644
--- a/src/test/ui/lint/unused/unused-async.stderr
+++ b/src/test/ui/lint/unused/unused-async.stderr
@@ -1,26 +1,55 @@
-warning: `must_use` attribute on `async` functions applies to the anonymous `Future` returned by the function, not the value within
-  --> $DIR/unused-async.rs:5:1
-   |
-LL |   #[must_use]
-   |   ^^^^^^^^^^^
-LL |
-LL | / async fn test() -> i32 {
-LL | |     1
-LL | | }
-   | |_- this attribute does nothing, the `Future`s returned by async functions are already `must_use`
-   |
-   = note: `#[warn(unused_attributes)]` on by default
-
-warning: `must_use` attribute on `async` functions applies to the anonymous `Future` returned by the function, not the value within
-  --> $DIR/unused-async.rs:15:5
-   |
-LL |       #[must_use]
-   |       ^^^^^^^^^^^
-LL |
-LL | /     async fn test_method() -> i32 {
-LL | |         1
-LL | |     }
-   | |_____- this attribute does nothing, the `Future`s returned by async functions are already `must_use`
-
-warning: 2 warnings emitted
+error: unused implementer of `Future` that must be used
+  --> $DIR/unused-async.rs:31:5
+   |
+LL |     foo();
+   |     ^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/unused-async.rs:2:9
+   |
+LL | #![deny(unused_must_use)]
+   |         ^^^^^^^^^^^^^^^
+   = note: futures do nothing unless you `.await` or poll them
+
+error: unused return value of `foo` that must be used
+  --> $DIR/unused-async.rs:31:5
+   |
+LL |     foo();
+   |     ^^^^^
+
+error: unused awaited future returned by `foo` that must be used
+  --> $DIR/unused-async.rs:33:5
+   |
+LL |     foo().await;
+   |     ^^^^^^^^^^^
+
+error: unused implementer of `Future` that must be used
+  --> $DIR/unused-async.rs:34:5
+   |
+LL |     bar();
+   |     ^^^^^
+   |
+   = note: futures do nothing unless you `.await` or poll them
+
+error: unused return value of `bar` that must be used
+  --> $DIR/unused-async.rs:34:5
+   |
+LL |     bar();
+   |     ^^^^^
+
+error: unused awaited future returned by `bar` that must be used
+  --> $DIR/unused-async.rs:36:5
+   |
+LL |     bar().await;
+   |     ^^^^^^^^^^^
+
+error: unused implementer of `Future` that must be used
+  --> $DIR/unused-async.rs:37:5
+   |
+LL |     baz();
+   |     ^^^^^
+   |
+   = note: futures do nothing unless you `.await` or poll them
+
+error: aborting due to 7 previous errors