Rework unsizing coercions in the new solver

[jdonszelmann]

Replaces #141926, contains:

• a commit adding tests that fail before this work
• the two commits from the previous PR
• a commit in which these tests are fixed
• finally, a fixup for an in my opinion rather large regression in diagnostics. It's still not perfect, but better?

I hope this is roughly what you had in mind

r? @lcnr

[rustbot]

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor

changes to inspect_obligations.rs

cc @compiler-errors, @lcnr

[BoxyUwU]

can you split the old solver custom fulfillment loop out into its own function? named something that makes it clear that its old solver specific too

[lcnr]

please add comments to the added tests explaining why we've added them and also link to the relevant issues

View changes since this review

[lcnr]

why does this one not use revisions?

[jdonszelmann]

good question 🤔 it should get some :)

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.