Allow must_use on associated types

Author: compiler-errors

compiler/rustc_lint/src/unused.rs

@@ -290,8 +290,15 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                         .map(|inner| MustUsePath::Pinned(Box::new(inner)))
                 }
                 ty::Adt(def, _) => is_def_must_use(cx, def.did(), span),
-                ty::Alias(ty::Opaque | ty::Projection, ty::AliasTy { def_id: def, .. }) => {
-                    elaborate(cx.tcx, cx.tcx.explicit_item_self_bounds(def).iter_identity_copied())
+                ty::Alias(ty::Opaque | ty::Projection, ty::AliasTy { def_id, .. }) => {
+                    if let Some(def) = is_def_must_use(cx, def_id, span) {
+                        Some(def)
+                    } else {
+                        // Look at the bounds for a `#[must_use]` trait.
+                        elaborate(
+                            cx.tcx,
+                            cx.tcx.explicit_item_self_bounds(def_id).iter_identity_copied(),
+                        )
                         // We only care about self bounds for the impl-trait
                         .filter_only_self()
                         .find_map(|(pred, _span)| {
@@ -307,6 +314,7 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                             }
                         })
                         .map(|inner| MustUsePath::Opaque(Box::new(inner)))
+                    }
                 }
                 ty::Dynamic(binders, _, _) => binders.iter().find_map(|predicate| {
                     if let ty::ExistentialPredicate::Trait(ref trait_ref) = predicate.skip_binder()

compiler/rustc_passes/src/check_attr.rs

@@ -1560,6 +1560,16 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
             return;
         }
 
+        // `#[must_use]` can be applied to an associated type in a trait, so that it will
+        // warn on unused associated types in generic contexts.
+        if let Target::AssocTy = target
+            && let parent_def_id = self.tcx.hir_get_parent_item(hir_id).def_id
+            && let containing_item = self.tcx.hir_expect_item(parent_def_id)
+            && let hir::ItemKind::Trait(..) = containing_item.kind
+        {
+            return;
+        }
+
         let article = match target {
             Target::ExternCrate
             | Target::Enum