Generic reason for custom incompatibility

[konstin]

Restructure the incompatibility into pubgrub internal metadata and a custom external type.

The internal types are:

• not root: The root incompatibility that drives resolution
• no versions: We tried all versions and there is none remaining
• from dependency of: {A, not B} from A -> B
• derived from: {A, not C} from {A, not B} and {B, not C}
• custom: Any incompatibility that originates outside pubgrub. The idea is that users can set M to a custom enum of theirs. Examples:
  • The version would require building the package, but builds are disabled.
  • The package is not available in the cache, but internet access has been disabled.
  • The package has invalid metadata

I'm currently working on adapting the formatting to adapt to the uv of this. This should reduce our diff significantly.

This refactoring is relevant to collapsing custom incompatibilities, but i haven't looked into this properly yet.

[Eh2406]

Sorry for the delay. I just had a chance to look at the diff, and I absolutely love it. Thank you so much! I need to poke at this in an editor before I write up the few nits I have.

[konstin]

Should we create a trait for M: Eq + Clone + Debug + Display?

[Eh2406]

Yes. I think we should have a trait. Although it could be in a follow-up PR. It should, one day, have a method can_merge(&Self, &Self) -> Option<Self> instead of the Eq bound. None would mark that the two incompatibilities should not be merged. And Some(M) would provide the new metadata for the merged incompatibility. That would allow a error like "unavailable because of HTTP 404, with request ID 123456789" to be merged with "unavailable because of HTTP 404, with request ID 987654321" even though they are not structurally equal.