Expose reqwest's inner error in the fetch function

[tomassedovic]

We were converting the reqwest::Error type directly to span::Error and the rendering chain stopped at that. This ignored the inner (source) error which contained the most useful information to figure out what went wrong.

This gets us from:

Error: error: failed to fetch: error: error decoding response body

to:

Error: error: failed to fetch: error: error decoding response body: missing field `discord` at line 16049 column 1

[tomassedovic]

You can reproduce this on the following commit: 965abda (the underlying issue has been fixed in #423)

$ git checkout 965abda
$ cargo rpg updates 2025h2 --output-file out.html

It will return this error:

Error: error: failed to fetch: error: error decoding response body
   --> /home/shadower/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/spanned-0.6.1/src/error.rs:130:23
    |
130 |                 span: Span::here(),
    |                       ^^^^^^^^^^^^^
    |
  --> crates/rust-project-goals/src/team.rs:17:27
   |
17 |             Err(e) => Err(Error::str(format!("failed to fetch: {e:?}"))),
   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |

That message tells you nothing about what's wrong.

With this PR, we get the following instead:

Error: error: failed to fetch: error: error decoding response body: missing field `discord` at line 16049 column 1
   --> crates/rust-project-goals/src/team.rs:133:13
    |
133 |             spanned::Error::str(format!("{e}{source_message}"))
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
  --> crates/rust-project-goals/src/team.rs:17:27
   |
17 |             Err(e) => Err(Error::str(format!("failed to fetch: {e:?}"))),
   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |

You can see that the first line tells you a bit more about the issue (there's a missing field in the JSON it's trying to convert) and give you slightly better stacktrace (pointing you at the fetch function).

[tomassedovic]

I don't love the spans here, they seem to add more noise than clarity. I also feel constructing a spaned::Error from a formatted string is kind of ugly.

Maybe there's a way to make sure spanned does the right thing with inner errors somehow instead?

[tomassedovic]

Ok, I think this is much better both in terms of the code and the message displayed to the user:

$ cargo rpg updates 2025h2 --output-file out
Error: error: failed to fetch: error: missing field `discord` at line 16049 column 1
   --> crates/rust-project-goals/src/team.rs:129:35
    |
129 |                 .map(|json_error| spanned::Error::str(format!("{json_error}")))
    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
  --> crates/rust-project-goals/src/team.rs:17:27
   |
17 |             Err(e) => Err(Error::str(format!("failed to fetch: {e:?}"))),
   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |

[tomassedovic]

Okay, I'm much happier with this version. Maybe there's a better way to handle this, but we just create a spanned::Error from the source error and the output looks good.

Happy to merge this unless there are objections.

[lqd]

Nice, the output error looks much better.