Provide a `finish` method on `IonWriter`s about ion-rust HOT 3 CLOSED

Yeah, this is something I've stubbed my toe on too. I initially held off on it because I had (have!) hope that we'll add an API to re-use a writer's existing resources to start a new stream while avoiding reallocations. That's something I can easily add a separate method for though.

Should finish() return the output so you don't have to remember to pass a &mut?

In the meantime, the writer API has a fn output(&self) -> &Self::Output method (and a mut_output) that lets you get a handle to the output source without dropping the writer.

let mut sink = Vec::new();
let mut writer = TextWriterBuilder::default().build(&mut sink)?;
writer.write_symbol("foo")?;
writer.flush()?;
assert_eq!(writer.output(), "foo".as_bytes());

Not ideal, but often good enough for simple uses like unit tests.

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Should finish() return the output so you don't have to remember to pass a &mut?

Yes, that would be excellent.

In the meantime, the writer API has a fn output(&self) -> &Self::Output method (and a mut_output) that lets you get a handle to the output source without dropping the writer.

This only really works if you don't need to pass along ownership of the sink, which we're often doing. Like you said, good for unit tests, but impractical for real-world code.

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Completed with #720

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