Syntax extension for converting enums to/from integers

[erickt]

Converting an enum to and from an integer is repetitive and brittle. So instead we should add the traits ToInt/FromInt/ToUint/FromUint, and add a syntax extension to automatically generate implementations of those traits for a given enum.

[nikomatsakis]

+1

[graydon]

The 'to' side is currently automatic, foo as uint. The 'from' side is trickier since it has to only consider the valid constructors. In any case, while this is a real problem I do not feel it's best approached via syntax extension. More like an additional dimension of the deriving thing.

[sanxiyn]

This is #2132?

[erickt]

That only captures the ToInt side of things. Trying to convert an integer into an enum can be prone to error because you need to make sure the definition of an enum is sharing the same integer bindings as the conversion code. It's easy to add a new variant and forget to update the converters.

[erickt]

Ack, I completely misread #2132. Yes you are right, they are the same. This deriving method @graydon mentioned is the modern way of generating this kind of code.

[graydon]

I suppose I did not know when I wrote that comment that deriving is implemented as a syntax extension. In that case, yeah, let's merge #2132 with this and say this is just a new deriving mode. Nominating for feature complete.

[graydon]

accepted for feature-complete milestone

[emberian]

Related to #7080

[catamorphism]

Bug triage. Milestone looks good to me.