Error conversion in tokio-serde: Why does this work?

[jwodder]

I'm trying to get tokio-serde to work in a program I'm writing, and along the way I've found that I don't understand why the example code compiles. According to the documentation, tokio-serde's Framed can only serve as a Sink or Stream if Codec::Error can be converted Into Transport::Error. In the case of, say, the server.rs example:

• The Transport is a FramedRead, whose Error is the Error of its Decoder, with Decoder here being LengthDelimitedCodec, which has LengthDelimitedCodecError as its Error, so Transport::Error is LengthDelimitedCodecError
• The Codec is tokio_serde::formats::Json, whose Error is json_serde::Error.

Now, I see that serde_json::Error implements conversion to std::io::Error, but how do we get from that to a LengthDelimitedCodecError? I must be missing something in the documentation for one of these types, but I (obviously) don't see what.

[H2CO3]

@jwodder

The Transport is a FramedRead, whose Error is the Error of its Decoder, with Decoder here being LengthDelimitedCodec, which has LengthDelimitedCodecError as its Error, so Transport::Error is LengthDelimitedCodecError

It isn't; it's io::Error: docs

[jwodder]

(Not sure if that's the docs link you meant to provide, but this seems like a better link.)

I see. So LengthDelimitedCodecError isn't a "top-level" Error type but rather used as the error payload to io::Error::new(). Odd.