Support optional generation

[ice1000]

Support generating only selected family of functions.

[ice1000]

This is required by tikv/raft-rs#218

[ice1000]

QAQ

[hicqu]

I doubt that whether we really need so many options or not. I only like MSG, CLR, HAS, and TKE in person. NEW can be replaced with Default, and GET, SET and MUT are useless because we can touch the internal fields directly.

[ice1000]

The process of eliminating wrapper usages should be step by step and this (relatively) fine grained config can help us do that.

[hicqu]

Or can we merge these 3 as one option?

[ice1000]

@hicqu I guess you've missed the point of this PR. What I was trying to do is to introduce a mechanism which can gradually reduce the wrapper functions. The ultimate goal is passing a GenOpt::empty() to the wrapper generator. Currently we're generating all of them so we can quickly launch prost support and my plan is to replace all usages of the generated wrappers with direct access. These options allow me to eliminate the generated wrappers step by step. That's all. 😉

[hicqu]

So after all wrappers are removed, should we remove these options also?

[ice1000]

We can! And I believe we're supposed to do so.

[hicqu]

@nrc PTAL. kvproto is blocked on raft-rs, and raft-rs is blocked on here.

[nrc]

Looks good, a few small comments inline

[ice1000]

@nrc something else to consider:

We may need to provide an option that "generates getters that are not of primitive types".

[ice1000]

Because for non-primitive types, they become Option<Bla>, where we're supposed to provide a getter that uses a default value for them.

[ice1000]

I start doubting the idea of eliminating all getters. I'm here to propose another design of this library (unrelated to this PR):

• Generate non-trivial getters (which unwrap_or_else for you)
• Generate trivial getters with #[deprecated]
• Provide options to eliminate deprecated stuffs

PTAL @nrc