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_passes/src/dead.rs

@@ -787,7 +787,6 @@ impl<'tcx> DeadVisitor<'tcx> {
         let mut dead_codes = dead_codes
             .iter()
             .filter(|v| !v.name.as_str().starts_with('_'))
-            .map(|v| v)
             .collect::<Vec<&DeadVariant>>();
         if dead_codes.is_empty() {
             return;