Prototype making geo-traits an optional dependency; support both 0.2 and 0.3 versions

[kylebarron]

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• I added an entry to CHANGES.md if knowledge of this change could be valuable to users.
• I ran cargo fmt

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This seems to work fine on the read side, but the write side is fully based on geo-traits now, so it's not clear how to make it optional. If geo-traits is required, then how do we choose which version to call from write_wkt?

Ref #148 (comment)

[michaelkirk]

If geo-traits is required, then how do we choose which version to call from write_wkt?

I think you'd need a version of the functions for each version of the trait you're integrating with.

Maybe something like:

mod to_wkt {
    #[cfg(feature=geo_traits_0_2)]
    mod  geo_traits_0_2_to_wkt;
    #[cfg(feature=geo_traits_0_2)]
    pub use geo_traits_0_2_to_wkt::*;

    #[cfg(feature=geo_traits_0_3)]
    mod  geo_traits_0_3_to_wkt;
    #[cfg(feature=geo_traits_0_3)]
    pub use geo_traits_0_3_to_wkt::*;
 }

Note this violates the convention that features are additive, and seems like it still might be a breaking change?
To be truly non-breaking, I think I'd actually expect something like

mod to_wkt {
    mod  geo_traits_0_2_to_wkt;
    pub use geo_traits_0_2_to_wkt::*;

    #[cfg(feature=geo_traits_0_3)]
    mod  geo_traits_0_3_to_wkt;
 }

And then the caller would have to explicitly call to_wkt::geo_traits_0_3_to_wkt::write_geometry

Like I said, given how much work this is, it feels mostly academic at this point.

[kylebarron]

Like I said, given how much work this is, it feels mostly academic at this point.

What do you think is the right path forward here?

For my own purposes, one path forward is a wkb release with geo-traits 0.2, waiting for a wkb release with geo-traits 0.3, and then we can wait a similar amount of time here for a new release with geo-traits 0.3. All reading and writing in the wkb crate is very integrated into geo-traits, and so there's no way to make that an optional dependency there.