Link from error codes to the error message index

[david-christiansen]

Is your enhancement request related to a problem? Please describe.

Some GHC error messages and warnings can be confusing.

Describe the solution you'd like

The Haskell Foundation has started a project to assign unique numbers to each error or warning emitted by GHC, and a community-driven web site that associates documentation with each code. The codes will be part of GHC 9.6. I'd like to see HLS include links to this documentation from messages in some convenient way.

Describe alternatives you've considered

None.

Additional context

All codes emitted by GHC match GHC-(0-9){5}. The message index will have addresses of the form https://errors.haskell.org/messages/GHC-NNNNN. The index is maintained at https://github.com/haskellfoundation/error-message-index/.

[michaelpj]

Need to come back to this when we've got basic 9.6 support.

[michaelpj]

We can actually put this in the Diagnostic: it has a code field and a codeDescription field that's a URI, so perfect.

[michaelpj]

I think we can do this already without structured diagnostics: we can just regex it out of the diagnostic message easily enough.

[michaelpj]

I have some code for this, but annoyingly we don't actually render the error codes already! So it's confusing what to do - do we render them (thus making users see them in the messages), or not (and not be able to pull them out)... 🤔