Serialize and deserialize ConstrainedFloats as floats

[benjaminrsherman]

Currently Serde serializes ConstrainedFloats with a PhantomData field and fails to deserialize raw floats. This change causes Serde to treat ConstrainedFloats as if they were a normal float which is being constructed.

I'm not sure if the #[serde(transparent)] is needed with #[repr(transparent)], but I included it just in case.

[olson-sean-k]

Thanks for the PR!

An identical change is slated to land in the 0.4.0 release. It's currently in 2ddc201 on the v0.4.0 branch (but that commit will likely jump around as changes are rebased). That change and similar serialization issues have been discussed in #5.

I've been maintaining upcoming breaking changes on the v0.4.0 branch instead of master, but there's nothing in the repository that points this out. Sorry about that. I'm still not sure exactly how I'd like to manage breaking changes and backporting. Once I know, I'll be sure to document it somewhere.

[olson-sean-k]

After thinking about it some more, I don't think the v0.4.0 branch should be a thing at all. 😝 It probably should've been a feature branch and not some kind of version branch. Let's go ahead and merge this and I'll sort out that branch later.

Thanks again for putting this together.

[bjchambers]

Just dropping in to say this change (or something like it) would be appreciated!

[olson-sean-k]

The previously mentioned change landed on master at 89063f3. However, that change (which ignores the phantom field) still results in serialization as a structure with a single field. Using transparent de/serialization via serde(transparent) isn't sufficient, as it bypasses any constraints applied by proxy types. That can be especially problematic, since the serialized format gives no indication that the data is anything else than a floating-point primitive.

I'm closing this for now, but have opened #36 to track work for transparent de/serialization that enforces constraints. There's also a known issue regarding serde_json (and presumably other formats) wherein non-real values of floating-point primitives cannot be serialized.

Thanks again for the PR and discussion! Sorry I dragged my feet on this one.