Support additional ways of specifying the error source or backtrace

[shepmaster]

Right now, we only support an underlying error denoted by the field named source and a backtrace by the name backtrace. We should also support a field attribute (#[snafu(source)] / #[snafu(backtrace)]), with an optional boolean to disable the automatic (#[snafu(source(false))] / #[snafu(backtrace(false))])

Precedence wise, it should be:

1. attribute
2. source

[lnicola]

Should the inner error be optional? Right now you get a compile error if you don't have one.

[shepmaster]

Should the inner error be optional

I’m not sure I’m following. If you don’t include a source field, then there will be no inner error.

Do you mean optional as in source: Option<SomeError>? If so, could you expand a bit about why you’d want that and what the use case is?

[lnicola]

In your words :-):

If the original variant had a source field, its context selector will have an implementation of From for a Context

I didn't notice that, and this error got me a little confused, because it's not exactly clear what the issue is:

error[E0277]: the trait bound `Error: std::convert::From<snafu::Context<std::io::Error, Rename<&std::path::Path>>>` is not satisfied
   --> src/main.rs:113:13
    |
113 |             fs::rename(path, cleaned).context(Rename { path })?;
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::convert::From<snafu::Context<std::io::Error, Rename<&std::path::Path>>>` is not implemented for `Error`
    |
    = help: the following implementations were found:
              <Error as std::convert::From<snafu::Context<snafu::NoneError, PathDecode<__T0, __T1>>>>
              <Error as std::convert::From<snafu::Context<snafu::NoneError, Rename<__T0>>>>
              <Error as std::convert::From<snafu::Context<snafu::NoneError, UnsupportedEncoding<__T0>>>>
              <Error as std::convert::From<snafu::Context<uchardet::errors::Error, UnknownEncoding<__T0>>>>
              <Error as std::convert::From<snafu::Context<walkdir::Error, Enumerate>>>
    = note: required by `std::convert::From::from`

I'm not saying you should make that code work, it's just a thing I ran into.

[aramrw]

I have a usecase but maybe I have the wrong idea

#[derive(Debug, Snafu)]
#[snafu(display("{website}:\n  {kind}"))]
pub struct WebsiteError {
    website: SupportedSites,
    kind: WebsiteErrorKind,
    source: fantoccini::error::CmdError,
}

#[derive(Debug, Snafu)]
pub enum WebsiteErrorKind {
    #[snafu(display("the query matched 0 elements on the page.\n  query: '{query}'"))]
    QuerySelectorAll { query: String },
    #[snafu(display(
        "the query only matched {img_count} <img />s when {page_count} were expected.\n  query: '{query}'"
    ))]
    MissingImagesAfterMaxRetries {
        img_count: usize,
        page_count: usize,
        query: String,
    },
}

then I can do this:

//  Result<Vec<Element>, fantoccini::error::CmdError>          
let imgs = self.find_all(Locator::Css(&query)).await.context(WebsiteSnafu {
    website: SupportedSites::MangaFire,
    kind: WebsiteErrorKind::QuerySelectorAll { query },
})?;

but now you cant return errors that dont need a source

// error[E0063]: missing field `source` in initializer of `WebsiteError`
return Err(WebsiteError {
    website: SupportedSites::MangaFire,
    kind: WebsiteErrorKind::QuerySelectorAll { query },
});

ideally you could make source an Option

// only provides a source if .context is used
#[derive(Debug, Snafu)]
#[snafu(display("{website}:\n  {kind}"))]
pub struct WebsiteError {
    website: SupportedSites,
    kind: WebsiteErrorKind,
    source: Option<fantoccini::error::CmdError>,
}