Improve error messages for boxed trait objects in tuples

Author: varkor

src/librustc_lint/unused.rs

@@ -145,8 +145,8 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnusedResults {
             match ty.sty {
                 ty::Adt(..) if ty.is_box() => {
                     let boxed_ty = ty.boxed_ty();
-                    let descr_pre_path = format!("{}boxed ", descr_pre_path);
-                    check_must_use_ty(cx, boxed_ty, expr, span, &descr_pre_path, descr_post_path)
+                    let descr_pre_path = &format!("{}boxed ", descr_pre_path);
+                    check_must_use_ty(cx, boxed_ty, expr, span, descr_pre_path, descr_post_path)
                 }
                 ty::Adt(def, _) => {
                     check_must_use_def(cx, def.did, span, descr_pre_path, descr_post_path)
@@ -157,8 +157,8 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnusedResults {
                         if let ty::Predicate::Trait(ref poly_trait_predicate) = predicate {
                             let trait_ref = poly_trait_predicate.skip_binder().trait_ref;
                             let def_id = trait_ref.def_id;
-                            let descr_pre = format!("{}implementer of ", descr_pre_path);
-                            if check_must_use_def(cx, def_id, span, &descr_pre, descr_post_path) {
+                            let descr_pre = &format!("{}implementer of ", descr_pre_path);
+                            if check_must_use_def(cx, def_id, span, descr_pre, descr_post_path) {
                                 has_emitted = true;
                                 break;
                             }
@@ -171,7 +171,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnusedResults {
                     for predicate in binder.skip_binder().iter() {
                         if let ty::ExistentialPredicate::Trait(ref trait_ref) = predicate {
                             let def_id = trait_ref.def_id;
-                            let descr_post = " trait object";
+                            let descr_post = &format!(" trait object{}", descr_post_path);
                             if check_must_use_def(cx, def_id, span, descr_pre_path, descr_post) {
                                 has_emitted = true;
                                 break;

src/test/ui/lint/must_use-trait.rs

@@ -21,7 +21,19 @@ fn get_boxed_critical() -> Box<dyn Critical> {
     Box::new(Anon {})
 }
 
+fn get_nested_boxed_critical() -> Box<Box<dyn Critical>> {
+    Box::new(Box::new(Anon {}))
+}
+
+fn get_critical_tuple() -> (u32, Box<dyn Critical>, impl Critical, ()) {
+    (0, get_boxed_critical(), get_critical(), ())
+}
+
 fn main() {
     get_critical(); //~ ERROR unused implementer of `Critical` that must be used
     get_boxed_critical(); //~ ERROR unused boxed `Critical` trait object that must be used
+    get_nested_boxed_critical();
+    //~^ ERROR unused boxed boxed `Critical` trait object that must be used
+    get_critical_tuple(); //~ ERROR unused boxed `Critical` trait object in tuple element 1
+    //~^ ERROR unused implementer of `Critical` in tuple element 2
 }

src/test/ui/lint/must_use-trait.stderr

@@ -1,5 +1,5 @@
 error: unused implementer of `Critical` that must be used
-  --> $DIR/must_use-trait.rs:25:5
+  --> $DIR/must_use-trait.rs:33:5
    |
 LL |     get_critical();
    |     ^^^^^^^^^^^^^^^
@@ -11,10 +11,28 @@ LL | #![deny(unused_must_use)]
    |         ^^^^^^^^^^^^^^^
 
 error: unused boxed `Critical` trait object that must be used
-  --> $DIR/must_use-trait.rs:26:5
+  --> $DIR/must_use-trait.rs:34:5
    |
 LL |     get_boxed_critical();
    |     ^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: unused boxed boxed `Critical` trait object that must be used
+  --> $DIR/must_use-trait.rs:35:5
+   |
+LL |     get_nested_boxed_critical();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: unused boxed `Critical` trait object in tuple element 1 that must be used
+  --> $DIR/must_use-trait.rs:37:5
+   |
+LL |     get_critical_tuple();
+   |     ^^^^^^^^^^^^^^^^^^^^^
+
+error: unused implementer of `Critical` in tuple element 2 that must be used
+  --> $DIR/must_use-trait.rs:37:5
+   |
+LL |     get_critical_tuple();
+   |     ^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 5 previous errors