warn on must_use use on async fn's

compiler/rustc_passes/src/check_attr.rs

@@ -112,6 +112,7 @@ impl CheckAttrVisitor<'tcx> {
                     self.check_default_method_body_is_const(attr, span, target)
                 }
                 sym::must_not_suspend => self.check_must_not_suspend(&attr, span, target),
+                sym::must_use => self.check_must_use(hir_id, &attr, span, target),
                 sym::rustc_const_unstable
                 | sym::rustc_const_stable
                 | sym::unstable
@@ -1046,6 +1047,37 @@ impl CheckAttrVisitor<'tcx> {
         is_valid
     }
 
+    /// Warns against some misuses of `#[must_use]`
+    fn check_must_use(
+        &self,
+        hir_id: HirId,
+        attr: &Attribute,
+        span: &Span,
+        _target: Target,
+    ) -> bool {
+        let node = self.tcx.hir().get(hir_id);
+        if let Some(fn_node) = node.fn_kind() {
+            if let rustc_hir::IsAsync::Async = fn_node.asyncness() {
+                self.tcx.struct_span_lint_hir(UNUSED_ATTRIBUTES, hir_id, attr.span, |lint| {
+                    lint.build(
+                        "`must_use` attribute on `async` functions \
+                              applies to the anonymous `Future` returned by the \
+                              function, not the value within",
+                    )
+                    .span_label(
+                        *span,
+                        "this attribute does nothing, the `Future`s \
+                                returned by async functions are already `must_use`",
+                    )
+                    .emit();
+                });
+            }
+        }
+
+        // For now, its always valid
+        true
+    }
+
     /// Checks if `#[must_not_suspend]` is applied to a function. Returns `true` if valid.
     fn check_must_not_suspend(&self, attr: &Attribute, span: &Span, target: Target) -> bool {
         match target {

src/test/ui/lint/unused/unused-async.rs

@@ -0,0 +1,43 @@
+// edition:2018
+// run-pass
+#![allow(dead_code)]
+
+#[must_use]
+//~^ WARNING `must_use`
+async fn test() -> i32 {
+    1
+}
+
+
+struct Wowee {}
+
+impl Wowee {
+    #[must_use]
+    //~^ WARNING `must_use`
+    async fn test_method() -> i32 {
+        1
+    }
+}
+
+/* FIXME(guswynn) update this test when async-fn-in-traits works
+trait Doer {
+    #[must_use]
+    async fn test_trait_method() -> i32;
+    WARNING must_use
+    async fn test_other_trait() -> i32;
+}
+
+impl Doer for Wowee {
+    async fn test_trait_method() -> i32 {
+        1
+    }
+    #[must_use]
+    async fn test_other_trait() -> i32 {
+        WARNING must_use
+        1
+    }
+}
+*/
+
+fn main() {
+}

src/test/ui/lint/unused/unused-async.stderr

@@ -0,0 +1,26 @@
+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
+