CLI support for custom derives

[adamlesinski]

I'd like to continue the discussion started here: #2059 (comment)

Support for adding custom derives exists in the Builder API. I would like to support this in the CLI as well.

I believe that the CLI syntax would need to accept a regex, and not just apply the custom derive to all types. The derive support that is built-in (Copy, Debug, Default, etc) benefits from knowing when they can be applied to a type, based on its fields. Custom derives do not have access to this knowledge, as their requirements are unknown to bindgen.

Therefore the user of the CLI must supply a regex to determine which types get the custom derive.

I was thinking something like: --with-custom-derive zerocopy::AsBytes,zerocopy::FromBytes/Foo.* where the / separator can be anything else we decide on, it just can't be any valid symbol in a type (including :) or a special shell character (;|).

The regex is useful because it also captures anonymous nested structs, whose names are generated from the outer struct.

Example:

struct Foo {
  struct {
    int a;
  } nested;
};

generates both Foo and Foo__bindgen_ty_1. The regex Foo.* would capture both types.

[spease]

This would be useful to derive strum::*, Serialize, and Deserialize for enum types as well. It would also be useful if there were a way to specify the type rather than have it apply to every type (the current PR currently doesn't seem to have a way to be selective).