feat: add halo2_debug package

[ed255]

Introduce a halo2_debug package that fills the need identified in #343

For now it only contains helper types to nicely display prime fields, expressions, lookups and shuffles.

As an example usage I extended the compress_selectors tests:

halo2/halo2_proofs/tests/compress_selectors.rs

Lines 465 to 476 in 458ad70

	assert_eq!(
	"s_add * (l + r - o)",
	format!("{}", expr_disp_names(&cs.gates[0].poly, names))
	);
	assert_eq!(
	"s_mul * (l * r - o)",
	format!("{}", expr_disp_names(&cs.gates[1].poly, names))
	);
	assert_eq!(
	"s_cubed * (l * l * l - o)",
	format!("{}", expr_disp_names(&cs.gates[2].poly, names))
	);

.

NOTE: I just saw this #345 we'll have to converge into the same name 😄

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.56604% with 25 lines in your changes missing coverage. Please review.

Project coverage is 82.28%. Comparing base (80e69b9) to head (5750849).
Report is 1 commits behind head on main.

Files	Patch %	Lines
halo2_middleware/src/circuit.rs	56.25%	14 Missing ⚠️
halo2_debug/src/display.rs	97.27%	6 Missing ⚠️
halo2_backend/src/plonk/circuit.rs	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #346      +/-   ##
==========================================
+ Coverage   81.90%   82.28%   +0.38%     
==========================================
  Files          82       83       +1     
  Lines       17019    17458     +439     
==========================================
+ Hits        13939    14366     +427     
- Misses       3080     3092      +12     

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[adria0]

Why adding a wrapper when we own the Expression type and we code in in an impl?

[ed255]

The reason for the wrapper is to be able to include auxiliary information that allows displaying the expressions with column names.
In the Expression type we don't hold the column names, just the column type and index. Then in ConstraintSystemMid we have a HashMap for column names.
ExprDisp can have a reference to this HashMap so that we can show the columns by name, like in this example

halo2/halo2_proofs/tests/compress_selectors.rs

Lines 465 to 468 in 458ad70

	assert_eq!(
	"s_add * (l + r - o)",
	format!("{}", expr_disp_names(&cs.gates[0].poly, names))
	);

Without these extra data, we could only print generic names like advice0, fixed2, instance3. I think being able to use named columns makes the expression much more readable.

[adria0]

NOTE: I just saw this #345 we'll have to converge into the same name 😄

Renamed to halo2_debug 0c54819

[davidnevadoc]

Love this new feature! LGTM 👍

[davidnevadoc]

We could have this be a bit cleaner if we just had access to F::MODULUS... For now, I think this is as good as it can be done!

[adria0]

Thanks for making the human life easier.
LGTM