Silence dropping_references lint.

[ehuss]

Rust 1.71 added a dropping_references lint which detects a drop() of a reference. This was firing in some default trait functions which were using that to avoid the unused variables lint. The workaround here is to just bind them to a wildcard pattern. An alternate solution would be to add #[allow(unused_variables)]. Another alternate solution would to use _ prefixes, but I assume that isn't desirable since that affects the documentation.