Features to support optional-numeric-index #3058

What is the name of your project?
optional-numeric-index

Please provide a link to your project (GitHub repository, crates.io page, etc).
https://github.com/sebschmi/optional-numeric-index

What features would you like from zerocopy?

• Allow types to specify custom valid byte patterns.
  The optional-numeric-index crate provides macros to create custom types to index e.g. vectors. These come in pairs, where one is the normal index type and one is an optional index type. For a compact representation, the bit-pattern that corresponds to the maximum value of the index is reserved for a None value. Hence, it is invalid for the normal index type.
  Currently, zerocopy has no way of handling that, as it does not allow to manually restrict the valid bit-patterns of a type. Deriving TryFromBytes for one of these custom index types would allow the maximum value as a valid bit pattern. What our project would need is a way to pass a filter function to #[derive(TryFromBytes)] that can reject bit patterns as needed.