Improve let_underscore_lock

- lint if the lock was in a nested pattern
- lint if the lock is inside a `Result<Lock, _>`

compiler/rustc_lint/messages.ftl

@@ -345,6 +345,9 @@ lint_multiple_supertrait_upcastable = `{$ident}` is object-safe and has multiple
 lint_node_source = `forbid` level set here
     .note = {$reason}
 
+lint_non_binding_let_multi_drop_fn =
+    consider immediately dropping the value using `drop(..)` after the `let` statement
+
 lint_non_binding_let_multi_suggestion =
     consider immediately dropping the value