This does not yet handle the case of mixed deref patterns with normal
constructors; it'll ICE in `Constructor::is_covered_by`. That'll be
fixed in a later commit.
Since deref patterns on boxes will be lowered differently, I'll be
making a separate test file for them. This makes sure we're still
testing the generic `Deref(Mut)::deref(_mut)`-based lowering.
This handles using deref patterns to choose the correct match arm. This
does not handle bindings or guards.
Co-authored-by: Deadbeef <ent3rm4n@gmail.com>
