Fix infinite obligation loop for Send/Sync

Author: HMPerson1

clippy_lints/src/any_coerce.rs

@@ -96,12 +96,15 @@ fn check_unsize_coercion<'tcx>(
     // redo the typechecking for this coercion to see if it required unsizing something to `dyn Any`
     // see https://github.com/rust-lang/rust/blob/cae6efc37d70ab7d353e6ab9ce229d59a65ed643/src/librustc_typeck/check/coercion.rs#L454-L611
     let tcx = infcx.tcx;
+    let coerce_unsized_trait_did = tcx.lang_items().coerce_unsized_trait().unwrap();
+    let unsize_trait_did = tcx.lang_items().unsize_trait().unwrap();
+
     // don't report overflow errors
     let mut selcx = traits::SelectionContext::with_query_mode(&infcx, traits::TraitQueryMode::Canonical);
     let mut queue = VecDeque::new();
     queue.push_back(
         ty::TraitRef::new(
-            tcx.lang_items().coerce_unsized_trait().unwrap(),
+            coerce_unsized_trait_did,
             tcx.mk_substs_trait(src_ty, &[tgt_ty.into()]),
         )
         .to_poly_trait_ref(),
@@ -120,12 +123,14 @@ fn check_unsize_coercion<'tcx>(
             trait_ref.to_poly_trait_predicate(),
         ));
         if let Ok(Some(vtable)) = select_result {
-            // we only care about trait predicates
+            // we only care about trait predicates for these traits
+            let traits = [coerce_unsized_trait_did, unsize_trait_did];
             queue.extend(
                 vtable
                     .nested_obligations()
                     .into_iter()
-                    .filter_map(|oblig| oblig.predicate.to_opt_poly_trait_ref()),
+                    .filter_map(|oblig| oblig.predicate.to_opt_poly_trait_ref())
+                    .filter(|tr| traits.contains(&tr.def_id())),
             );
         }
     }

tests/ui/any_coerce.rs

@@ -29,7 +29,7 @@ struct Unsizeable<T: ?Sized, U: ?Sized, V: ?Sized> {
 }
 
 fn main() {
-    let mut box_any: Box<dyn Any> = Box::new(Foo);
+    let mut box_any: Box<dyn Any + Send> = Box::new(Foo);
     let _: *mut dyn Any = &mut box_any; // LINT
     let _: *mut dyn Any = &mut *box_any; // ok