execution-plan: Macro for implementing Value on structs

[adamchalmers]

Background

The Value trait lets a Rust type be written to KittyCAD execution plan (KCEP) memory, and read back out of memory. The trait has two methods, into_parts() (convert your type into a vec of KCEP Primitive) and from_parts() (given a vec of KCEP Primitive, reconstruct your type).

Problem

You have to be careful that your into_parts() and from_parts() methods use the same order for the fields of your Rust struct. E.g. if into_parts() outputs a vec with self.length and self.name, then from_parts() must assume its input struct is self.length then self.name.

This is easy, but it's a manual process and a bit error-prone. Additionally, the implementation is pretty boilerplate -- the implementation of Value can easily be recursive if all your fields also impl Value.

Also, if I'm being honest, handwriting implementations of these traits for every single type in our Modeling API is going to be a pretty big timesink, and it's likely that at some point I lose focus and make a mistake, like putting the fields in the wrong order.

Solution

Write a macro which generates an impl of Value for a given type. If the human programmer supplies a list of all fields, we can ensure they're being used in the same order in both from_parts() and into_parts(). And this is actually type-checked -- Rust won't compile the code if you supply an invalid field!

TODO

• This macro only works for structs. Make a similar macro for enums.

[codecov]

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