extendable-data

A rust macro that allows you to specify data that can be "extended" or inherited from. With data, I mean specifically enum, struct and union.

Why not composition/traits/some other method

This project started because I was using the very nice logos library and I wanted to define 2 lexers with some of the same base tokens, but extended. I could not find a proper way to do this that was not simply copy-pasting the parts of the enum that I needed, so I set about bodging it with macro abuse. I then extended the approach I used for enums to also support structs and unions.

How to Use

Simply, define the base enum (A) that you want to use. Then add the #[extendable_data] attribute to it. This will automatically generate a new macro called extend_from_A (or you can specify a name in the attribute arguments). Now, use this new macro for the extended enum B.

Example

use extendable_data::extendable_data;

#[extendable_data(extend_a)]
enum A {
	One,
	Two,
	Three
}

In a crate (we'll use X here) that has proc-macro set to true, then:

use X::extend_a;

#[extend_a]
enum B {
	Four,
	Five,
	Six
}
 |
 V
enum B {
	One,
	Two,
	Three,
	Four,
	Five,
	Six
}

Any attributes and generics used in the definitions for enums A and B are combined and copied over. For the name and visibility, only those of B are used and are directly copied over.

Any documentation written for A is used for the resulting procedural macro, while any documentation for B is used for the resulting data structure.

Filter

You can provide a list of fields to filter when using the extend_from_* macros. It works as follows:

#[extend_a(filter(Two))]
enum B {    
	Four
}
 |
 V
enum B {
	One,
	Three,
	Four
}

You can also use the filter to specify specific attributes to remove from the original data structure. (For example, derive if it this new version should no longer derive).

If you are using the merge_on_conflict argument (see below) and you want to remove an attribute from a field, you MUST force conflict resolution by writing the field again:

#[extendable_data]
enum A {
	#[attr]
	One,
	Two
}

...

#[extend_from_A(filter(attr))]
enum B {
	One
}
 |
 V
enum B {
	One,
	Two
}

Conflict Resolution

By default, if a name conflict occurs (I.E, you have a field X in both A and B), then the field in B will completely overwrite the field in A. Optionally, you can pass the merge_on_conflict argument to extend_from_* to make the library attempt to merge the fields. Merging is only be possible in enums, because nested structs and unions are not supported in Rust.

Structs

As opposed to enums and unions, not all types of structs make sense to combine together. As such, the following design decisions were made:

• Combining two named structs just generates a new named structs.
• Combining two unnamed structs just generates a new unnamed struct.
• Combining two unit structs just generates a new unit struct.
• Combining a unit struct with anything else will generate the other type of struct (so a unit with a named will generate a named), regardless of which is the "parent".

Technically you could allow the combining of named and unnamed structs, or have the parent matter more when combining with unit structs, but the former would promote even more ugly coding habits than this library already does, and the latter seemed a less common use-case.