types_escaping_snapshot cleanup finalization

[Mark-Simulacrum]

#83185 deleted types_escaping_snapshot in compiler/rustc_infer/src/infer/type_variable.rs, but there are several comments remaining referencing that function, some of which seem to indicate no longer necessary operations. This code, to my knowledge, is pretty finicky, so we will want to be careful, but there may be some performance (or at least cleanup) wins left there.

cc @jyn514

[jyn514]

I'm not sure what's going on with that code. The comment on impl sv::SnapshotVecDelegate for Delegate { says

// In fact, we could almost just remove the
// SnapshotVec entirely, except that we would have to
// reproduce some of its logic, since we want to know which
// type variables have been instantiated since the snapshot
// was started, so we can implement types_escaping_snapshot.

but when I try to remove the impl I get lots of errors:

error[E0277]: the trait bound `type_variable::Delegate: SnapshotVecDelegate` is not satisfied
   --> compiler/rustc_infer/src/infer/type_variable.rs:319:10
    |
319 |     ) -> sv::SnapshotVec<Delegate, &mut Vec<TypeVariableData>, &mut InferCtxtUndoLogs<'tcx>> {
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `SnapshotVecDelegate` is not implemented for `type_variable::Delegate`
    | 
   ::: /home/joshua/.local/lib/cargo/registry/src/github.com-1ecc6299db9ec823/ena-0.14.0/src/snapshot_vec.rs:111:8
    |
111 |     D: SnapshotVecDelegate,
    |        ------------------- required by this bound in `SnapshotVec`

including from functions that are still used, like values().

The comment was added in 57a593f - @nikomatsakis do any obvious cleanups come to mind now that types_escaping_snapshot no longer exists?

[jyn514]

I did remove

rust/compiler/rustc_infer/src/infer/type_variable.rs

Line 227 in 6bed7e1

self.undo_log.push(Instantiate { vid });

without any test failures. Maybe self.undo_log can be removed altogether?

[nikomatsakis]

Hmm

[nikomatsakis]

I don't think you can remove the undo_log, that's needed for probes and other things, but I might be forgetting something. There might be some simplifications possible.