a couple clippy::complexity fixes

map_identity
filter_next
option_as_ref_deref
unnecessary_find_map
redundant_slicing
unnecessary_unwrap
bool_comparison
derivable_impls
manual_flatten
needless_borrowed_reference

compiler/rustc_span/src/hygiene.rs

@@ -109,7 +109,7 @@ fn assert_default_hashing_controls<CTX: HashStableContext>(ctx: &CTX, msg: &str)
         // This is the case for instance when building a hash for name mangling.
         // Such configuration must not be used for metadata.
         HashingControls { hash_spans }
-            if hash_spans == !ctx.unstable_opts_incremental_ignore_spans() => {}
+            if hash_spans != ctx.unstable_opts_incremental_ignore_spans() => {}
         other => panic!("Attempted hashing of {msg} with non-default HashingControls: {other:?}"),
     }
 }