Infallible BodyError guidance

[kellytk]

When should Infallible be used for IntoResponse's BodyError and why is it not the default recommendation?

[davidpdrsn]

The BodyError associated type only exists to make impl IntoResponse work, without it you would have to specify the exact body type. As such you cannot pick whatever error type you want. It must always match the Body associated type. For example if you're using axum::body::Body the error must be hyper::Error because that how the trait is implemented in hyper. So you should always set it to

type BodyError = <Self::Body as axum::body::HttpBody>::Error;

Then rust can infer the error type from the body without you having to specify it.

Generally the error type will only be infallible if you're using Full or Empty as the body type.

[kellytk]

Thank you for the helpful information. The documentation note on Full states:

When you know your response always contains exactly one chunk.

How can that fact be ascertained?

[davidpdrsn]

If your body is not empty and is not a stream, then you probably want full. For example sending some JSON requires having all the JSON ready upfront. Thus it can be sent in one chunk.

[jplatte]

To be clear, streaming serialization of JSON is absolutely possible, it's just not something serde_json supports (outside of sync io::Writers).