Efficient serialization of programmatically constructed strings

[dtolnay]

A neat idea from @nox that would make a big difference for hyper_serde. We shouldn't need to build the entire string in memory before serializing it.

<nox> Ever thought of doing a SerializeString type parameter, with push_str and methods like that on it?
<nox> Another consequence of doing SerializeString is that we could make that rely on fmt::Write,
<nox> in which case we can do a new default method Serializer::serialize_fmt
<nox> which is better than serializer.serialize_string(stuff.to_string())
<nox> serialize_str(&stuff.to_string()) even.

[nox]

The best design would be an associated type with a default value, implementing the associated trait as a String buffer and an end method calling Serializer::serialize_str. Unfortunately, associated type default values are unstable. https://is.gd/Q1DkCh

[dtolnay]

I would like to have benchmarks before making a decision on this. Sometimes things that are obvious optimizations turn out to be negligible or even detrimental. In this case conceivably we could be taking something that is a tight loop over here and a tight loop over there and turning it into code that jumps back and forth between two loops in a way that makes the CPU sad.

@nox could you come up with a benchmark to show that this is X% better than the existing approach in some reasonable use case?

[nox]

I'm not sure I understand how it could get slower? Which tight loops?

[dtolnay]

#778 convinced me that there is plenty of performance to be gained here.

[nox]

@dtolnay I think so.

[clarfonthey]

Personally, I think that it'd be nice if we could expose the implementations from #778 somehow so that any crate can use it.

[nox]

This can be closed now, right?