Small updates to MockProver

[zemse]

• expose InstanceValue::value() -> this is an helpful util to get the underlying value, though InstanceValue is public already
• expose region data -> this is helpful to get the column annotations
• add conversion for Selector -> Column -> this is helpful to cast selector as a fixed column

These changes allow something like this https://github.com/zemse/halo2-utils

[CPerezz]

• expose InstanceValue::value() -> this is an helpful util to get the underlying value, though InstanceValue is public already
  This looks good to me.
• expose region data -> this is helpful to get the column annotations.
  I don't think access to the annotations in this way is needed. Annotations are there only for debugging purposes for the MockProver but nothing else. I don't see why you need access to them.
  Same for a lot of the other fields. I think giving access to the Region internals can be problematic.
• add conversion for Selector -> Column -> this is helpful to cast selector as a fixed column:
  Due to selector-compression, not sure how efficient/secure this is.
• record the last row assigned -> useful to estimate value of k:
  Already addressed in the comments of the PR.

[CPerezz]

Why would it be possible to cast this? How are you planning to create an Advice or Instance column from this?

If this stays here, it should only be valid to do:

Suggested change

	impl From<Selector> for Column<Any> {
	impl From<Selector> for Column<Fixed> {

And not only that, This cast is also bad in the sense that selector_compression will happen and I'm not sure how will affect this castings done here.
I'd remove this and not allow to do this type of cast just to be safe.

Tagging @ed255 @han0110

[zemse]

I want to use the name_column annotation util added in #109. However it only works for advice, fixed and instance. Currently it is not possible to annotate a selector column.

This is an example where it is currently failing https://github.com/zemse/halo2-utils/blob/d36fdf2036f4300bfc2559e754b7b68909f63044/src/example_circuit.rs#L70

[CPerezz]

That's correct. But there's an issue with that.
Notice that when we do keygen, we call the fn compress_selectors.
If you have compression enabled, the annotation could be erased. Otherwise, if we prevent compression, we're not matching the circuit we will end up with.

Hence why this is an issue.

[zemse]

Got it, I have removed this change by reverting the commit

[zemse]

Thanks for review

I don't think access to the annotations in this way is needed. Annotations are there only for debugging purposes for the MockProver but nothing else. I don't see why you need access to them.

If user is annotating the columns for debugging, that can be reused to print all column assignments (for a different debugging approach), while those columns can be labeled accordingly. Here is an example of how that would look.

[han0110]

LGTM

[CPerezz]

LGTM!