resolve clippy lints for undocumented private items and locations of …

…hoisted struct definitions

dumpster/src/lib.rs

@@ -337,6 +337,8 @@ pub use dumpster_derive::Collectable;
 /// - `Ok(false)`: The data structure contains no garbage-collected pointers.
 /// - `Err(())`: The data structure was accessed while we checked it for garbage-collected pointers.
 fn contains_gcs<T: Collectable + ?Sized>(x: &T) -> Result<bool, ()> {
+    /// A visitor structure used for determining whether some garbage-collected pointer contains a
+    /// `Gc` in its pointed-to value.
     struct ContainsGcs(bool);
 
     impl Visitor for ContainsGcs {

dumpster_test/src/lib.rs

@@ -162,15 +162,16 @@ fn parallel_loop() {
 }
 
 #[test]
+#[allow(clippy::similar_names)]
 fn unsync_as_ptr() {
+    #[derive(Collectable)]
+    struct B(Gc<Empty>);
+
     let empty = Gc::new(Empty);
     let empty_a = Gc::clone(&empty);
     let empty_ptr = Gc::as_ptr(&empty);
     assert_eq!(empty_ptr, Gc::as_ptr(&empty_a));
 
-    #[derive(Collectable)]
-    struct B(Gc<Empty>);
-
     let b = B(Gc::clone(&empty));
     assert_eq!(empty_ptr, Gc::as_ptr(&b.0));
     let bb = Gc::new(B(Gc::clone(&empty)));