Rollup merge of rust-lang#111491 - compiler-errors:nested-fut-must-use, r=wesleywiser

Dont check `must_use` on nested `impl Future` from fn

Fixes (but does not close, per beta policy) rust-lang#111484

Also fixes a `FIXME` left in the code about (presumably) false-positives on non-async `#[must_use] fn() -> impl Future` cases, though if that's not desirable to include in the beta backport then I can certainly revert it.

Beta nominating as it fixes a beta ICE.

Author: Dylan-DPC

compiler/rustc_lint/src/unused.rs

@@ -103,8 +103,10 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
             && let ty = cx.typeck_results().expr_ty(&await_expr)
             && let ty::Alias(ty::Opaque, ty::AliasTy { def_id: future_def_id, .. }) = ty.kind()
             && cx.tcx.ty_is_opaque_future(ty)
-            // FIXME: This also includes non-async fns that return `impl Future`.
             && let async_fn_def_id = cx.tcx.parent(*future_def_id)
+            && matches!(cx.tcx.def_kind(async_fn_def_id), DefKind::Fn | DefKind::AssocFn)
+            // Check that this `impl Future` actually comes from an `async fn`
+            && cx.tcx.asyncness(async_fn_def_id).is_async()
             && check_must_use_def(
                 cx,
                 async_fn_def_id,

tests/ui/lint/unused/auxiliary/must-use-foreign.rs

@@ -0,0 +1,12 @@
+// edition:2021
+
+use std::future::Future;
+
+pub struct Manager;
+
+impl Manager {
+    #[must_use]
+    pub async fn new() -> (Self, impl Future<Output = ()>) {
+        (Manager, async {})
+    }
+}

tests/ui/lint/unused/must-use-foreign.rs

@@ -0,0 +1,15 @@
+// edition:2021
+// aux-build:must-use-foreign.rs
+// check-pass
+
+extern crate must_use_foreign;
+
+use must_use_foreign::Manager;
+
+async fn async_main() {
+    Manager::new().await.1.await;
+}
+
+fn main() {
+    let _ = async_main();
+}

tests/ui/lint/unused/unused-async.rs

@@ -33,7 +33,7 @@ async fn test() {
     foo().await; //~ ERROR unused output of 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 output of future returned by `bar` that must be used
+    bar().await; // ok, it's not an async fn
     baz(); //~ ERROR unused implementer of `Future` that must be used
     baz().await; // ok
 }

tests/ui/lint/unused/unused-async.stderr

@@ -52,17 +52,6 @@ help: use `let _ = ...` to ignore the resulting value
 LL |     let _ = bar();
    |     +++++++
 
-error: unused output of future returned by `bar` that must be used
-  --> $DIR/unused-async.rs:36:5
-   |
-LL |     bar().await;
-   |     ^^^^^^^^^^^
-   |
-help: use `let _ = ...` to ignore the resulting value
-   |
-LL |     let _ = bar().await;
-   |     +++++++
-
 error: unused implementer of `Future` that must be used
   --> $DIR/unused-async.rs:37:5
    |
@@ -71,5 +60,5 @@ LL |     baz();
    |
    = note: futures do nothing unless you `.await` or poll them
 
-error: aborting due to 7 previous errors
+error: aborting due to 6 previous errors