Custom parser error

[stoically]

In my use case I'm providing a library that helps with parsing a specific kind of token stream syntax, namely something HTML/JSX-ish: syn-rsx. I'd like consumers of the library to customize their error behavior depending on what goes wrong while parsing the provided tokens. E.g. if a closing HTML tag is missing there's currently a hard-coded string provided to input.error. Instead I'd like to return an error enum variant such as syn_rsx::Error::MissingClosingTag, ideally with additional context information attached.

AFAIK this currently isn't possible since Parser always expects a syn::Error to be returned, which does not allow to attach custom context information, but maybe I'm missing something?

Possible solutions

Maybe Parser could allow to return everything that implements std::error::Error as an error?

Alternatively syn::Error could allow attaching arbitrary context information? E.g. via adding

• syn::Error::set_context(context: T)
• syn::Error::get_context() -> T

methods?

Possible workaround

Allow consumers of syn_rsx to customize just the error strings that are passed to input.error via some sort of configuration.

Downstream issue: stoically/syn-rsx#12

[dtolnay]

I would prefer not to build anything for this into this library. But there is room in the ecosystem for a different library to implement something for this use case.