Rollup merge of #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) #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.

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,