`aws_smithy_types::Document` should have an implementation of the `serde::Serializer` trait

[MinisculeGirraffe]

aws_smithy_types::Document already has #[derive(Serialize, Deserialize)] behind a feature flag, but that only lets you serialize the Document itself. it doesn't let you convert an arbitrary struct to a Document directly.

smithy-rs/rust-runtime/aws-smithy-types/src/document.rs

Lines 10 to 14 in a0d46cd

	#[cfg(any(
	all(aws_sdk_unstable, feature = "serde-deserialize"),
	all(aws_sdk_unstable, feature = "serde-serialize")
	))]
	use serde;

I'd like to be able to do:

  let doc = aws_smithy_types::document::to_document(&my_struct)?;
  let my_struct: MyStruct = aws_smithy_types::document::from_document(doc)?;

Basically the same thing serde_json does with serde_json::to_value / serde_json::from_value, but targeting Document instead of Value.

This would also allow for interoperability between the type types without client facing code.

Right now the workaround i'm using is to round-trip through serde_json::Value and then manually convert:

  let json_value = serde_json::to_value(&my_struct)?;
  let doc = json_to_document(json_value); // hand-rolled conversion function

This works but it's clunky and allocates an intermediate representation for no reason.

It would be a significantly more ergonomic client experience to provide a Serializer implementation analogous to serde_json to allow for conversion from arbitrary types directly.
https://github.com/serde-rs/json/blob/master/src/value/ser.rs#L58