Introduce enter_forall to supercede instantiate_binder_with_placeholders

[BoxyUwU]

r? @lcnr

Long term we'd like to experiment with decrementing the universe count after "exiting" binders so that we do not end up creating infer vars in non-root universes even when they logically reside in the root universe. The fact that we dont do this currently results in a number of issues in the new trait solver where we consider goals to be ambiguous because otherwise it would require lowering the universe of an infer var. i.e. the goal ?x.0 eq <T as Trait<?y.1>>::Assoc where the alias is rigid would not be able to instantiate ?x with the alias as there would be a universe error.

This PR is the first-ish sort of step towards being able to implement this as eventually we would want to decrement the universe in enter_forall. Unfortunately its Difficult to actually implement decrementing universes nicely so this is a separate step which moves us closer to the long term goal ✨

[rustbot]

Some changes occurred to the core trait solver

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

Type relation code was changed

cc @compiler-errors, @lcnr

[bors]

☔ The latest upstream changes (presumably #120620) made this pull request unmergeable. Please resolve the merge conflicts.

[lcnr]

some nits, then r=me. Please make sure to remove all FIXME(tree_universes) from enter_forall calls

[BoxyUwU]

@bors r=lcnr rollup=never

[bors]

📌 Commit b6f0a77 has been approved by lcnr

It is now in the queue for this repository.

[lcnr]

@bors r-

[compiler-errors]

it would be nice if this pr got a description too :> i know what it's doing, but others may not

[BoxyUwU]

I KNEW I FORGOT SOMETHING

[lcnr]

r=me after ci

[BoxyUwU]

@bors r=lcnr rollup=never

[bors]

📌 Commit f867742 has been approved by lcnr

It is now in the queue for this repository.

[bors]

⌛ Testing commit f867742 with merge c29082f...

[bors]

☀️ Test successful - checks-actions
Approved by: lcnr
Pushing c29082f to master...

[rust-timer]

Finished benchmarking commit (c29082f): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

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This benchmark run did not return any relevant results for this metric.

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Bootstrap: 662.316s -> 662.416s (0.02%)
Artifact size: 308.26 MiB -> 308.28 MiB (0.01%)