Reowork 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

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

     |
2038 |         // Make sure this predicate is refering to either an `Unsize` or `CoerceUnsized` trait,
     |                                        ^^^^^^^^
     |
rerun tidy with `--extra-checks=spellcheck --bless` to fix typos
tidy [extra_checks]: checks with external tool 'typos' failed
tidy [extra_checks]: FAIL
tidy: The following check failed: extra_checks
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 /node/bin/npm --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1549:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1280:29

Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:02:30
  local time: Sat Oct 18 09:02:09 UTC 2025
  network time: Sat, 18 Oct 2025 09:02:09 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"

[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