clippy::complexity fixes

filter_next
needless_question_mark
bind_instead_of_map
manual_find
derivable_impls
map_identity
redundant_slicing
skip_while_next
unnecessary_unwrap
needless_bool

compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs

@@ -488,7 +488,7 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
                         // If this empty region is from a universe that can
                         // name the placeholder, then the placeholder is
                         // larger; otherwise, the only ancestor is `'static`.
-                        if a_ui.can_name(placeholder.universe) { true } else { false }
+                        return a_ui.can_name(placeholder.universe);
                     }
                 }
             }

compiler/rustc_infer/src/infer/undo_log.rs

@@ -87,18 +87,12 @@ impl<'tcx> Rollback<UndoLog<'tcx>> for InferCtxtInner<'tcx> {
 
 /// The combined undo log for all the various unification tables. For each change to the storage
 /// for any kind of inference variable, we record an UndoLog entry in the vector here.
-#[derive(Clone)]
+#[derive(Clone, Default)]
 pub(crate) struct InferCtxtUndoLogs<'tcx> {
     logs: Vec<UndoLog<'tcx>>,
     num_open_snapshots: usize,
 }
 
-impl Default for InferCtxtUndoLogs<'_> {
-    fn default() -> Self {
-        Self { logs: Default::default(), num_open_snapshots: Default::default() }
-    }
-}
-
 /// The UndoLogs trait defines how we undo a particular kind of action (of type T). We can undo any
 /// action that is convertible into an UndoLog (per the From impls above).
 impl<'tcx, T> UndoLogs<T> for InferCtxtUndoLogs<'tcx>