Nit:

I see mixed usages of &** and as_ref throughout the code base. Out of curiosity, when is one used over the other? The general heuristic I've seen is * and & are recommended for simple stuff; and, once you start de-referencing and re-borrowing, that as_ref is recommended if just to avoid needing to think about whatever container the value* is in.

We used to use as_ref but started to prefer dereferencing (except for Option where you still need as_ref and as_deref) because it's possible that as_ref becomes ambiguous if a new AsRef implementation is added to a type. For example, String has many as_ref methods:

  = note: multiple `impl`s satisfying `String: AsRef<_>` found in the following crates: `alloc`, `std`:
          - impl AsRef<OsStr> for String;
          - impl AsRef<Path> for String;
          - impl AsRef<[u8]> for String;
          - impl AsRef<str> for String;

Oh interesting, that makes sense! Thank you!