allow types to specify the kind of serializer they should be used with or carry state

[nikomatsakis]

Right now, the serialization interface requires that the type to be serialized be generic with respect to any sort of serializer:

trait Serializable { fn serialize<S: Serializer>(&self, s: &s); }

Unfortunately, some types require context to be serialized, such as ty::t or span. When doing the original design I had intended that this state would be carried in the serializer, but of course if you must write the serialization function generically that didn't work, so we ended up with ast_encode.rs which does some manual serialization for any type that touches ty::t. At first this wasn't a lot of stuff but of course it's growing and it's annoying.

One way to solve this is to modify the Serializable trait as follows:

trait Serializable<S: Serializer> { fn serialize(&self, s: &s); }

This allows a type to implement Serializable only for specific serializers:

impl ty::t: Serializable<RustcSerializer> { ... }

To really make this work, though, it is also necessary for the auto_serialize code to permit generation of an impl that is specialized to a particular serializer, since any struct which contains ty::t values will also only work with RustcSerializer.

Another option would be to extend the Serializable trait with some kind of state and carry that around, but I think that winds up just being more type parameters without really adding any sort of flexibility.

@erickt has done preliminary work but it's hitting an ICE somewhere in the static methods implementation. I haven't tracked down the cause.

[nikomatsakis]

This is a prereq to #3738, since the RustcSerializer could store a reference to the interner.

[nikomatsakis]

Also a prereq to fixing #1972.

[pnkfelix]

It is a little tricky to be sure given the alpha-renaming that has occurred since this bug was filed, but I think the design proposed by Niko in the description has been implemented.

The version of src/libstd/serialization.rs from 8 months ago had the old Serializable trait for stage0, and the Serializable<S: Serializer> trait for stages > 0.

The current version of src/libextra/serialize.rs has the latter renamed as Encodable<S:Encoder> trait. It is available in all stages.

So I think we should be able to close this.

I guess I'll want to double check whether the feature is actually being tested. And whether we can indeed fix #3738 and #1972 now. :)

[nikomatsakis]

This is not fully implemented because the deriving mode does not allow you to specify the serializer.

[huonw]

@nikomatsakis I opened #7229 about that (it suggests a possible solution).

[emberian]

Visiting for triage; nothing to add.

[catamorphism]

De-milestoning, just a bug

[alexcrichton]

Closing, this appears to be the current design of the Encodable trait