Show dedicated hint for `--unsafe-first-match` with PyTorch index

[charliermarsh]

This is so common that we should special-case it for a list of packages (like requests): astral-sh/rye#1282 (comment)

[charliermarsh]

@zanieb -- For this one, do you think it's sufficient to show this if: we fail to resolve (can't find a valid version), and the failing package came from the first index, i.e., there are more indexes remaining? Is that too noisy?

[zanieb]

Yeah, I think the minimal idea is that if we have a NoVersions incompatibility, there are multiple indexes, and you're using first-match we should hint. This is so common that I don't mind it being noisy to start. I think we should implement this ASAP, I started looking into it but need to figure out what index information is available to the reporter and what we have to thread

I have a few improvements in mind but am not sure what information is available, e.g.:

• Confirm that we didn't check all of the indexes for the package (i.e., as you suggested)
• Confirm if the package is available on a subsequent index
• Confirm if there is a version in the range available on a subsequent index

I think with these we could provide really helpful messages.

[charliermarsh]

Confirm if the package is available on a subsequent index

This is the hard one, because with --first-match, we don't even check subsequent indexes. So we'd either have to (1) start checking subsequent indexes, or (2) make new network requests in the error reporter.

[zanieb]

I figured that might be the case, but we can try to solve that later.

I think it'd be okay to have a low-priority pool checking the subsequent indexes in the background then finish the checks if needed for a no-solution error? 🤷‍♀️