refactor(halo2): Decouple circuit from Midnight types

[hjeljeli32]

Content

This PR improves the Halo2 circuit code structure for by introducing decoupled circuit-local types.

This PR includes:

• Added a circuit-local field wrapper and shared aliases in types.rs:
  • CircuitBaseField, CircuitBase, CircuitCurve
• Replaced duplicated local F/C aliases with shared types from types.rs across:
  • circuit.rs, gadgets.rs, golden/helpers.rs
• Implemented type conversions using From/Into
• Introduced a test-only adapter module (adapters.rs) for STM Merkle path → Halo2 witness conversion.
• Removed utils/mod.rs by inlining field-limb split logic into gadgets.rs
• Unified synthesis error mapping with to_synthesis_error in errors.rs

Pre-submit checklist

• Branch
  • Tests are provided (if possible)
  • Crates versions are updated (if relevant)
  • CHANGELOG file is updated (if relevant)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
• PR
  • All check jobs of the CI have succeeded
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested

Issue(s)

Closes #3037

[github-actions]

Test Results

    5 files  ±0    190 suites  ±0   57m 15s ⏱️ +12s
2 672 tests +1  2 672 ✅ +1  0 💤 ±0  0 ❌ ±0 
9 587 runs  +1  9 587 ✅ +1  0 💤 ±0  0 ❌ ±0 

Results for commit fbf9b8d. ± Comparison against base commit cf9da10.

♻️ This comment has been updated with latest results.

[Copilot]

Pull request overview

This PR refactors the Halo2 circuit code to decouple it from direct Midnight library types. It introduces a circuit-local wrapper type CircuitBaseField and shared type aliases (CircuitBase, CircuitCurve) in types.rs, replaces duplicated per-file type F/C aliases with these shared types, consolidates error-to-synthesis conversion into to_synthesis_error in errors.rs, moves the STM→Halo2 Merkle path adapter from utils/mod.rs into a new adapters.rs, and inlines the field-limb split utilities from utils/mod.rs directly into gadgets.rs.

Changes:

• Introduced CircuitBaseField newtype wrapper and CircuitBase/CircuitCurve shared type aliases in types.rs, replacing direct Midnight type re-exports and per-file type F/C aliases
• Extracted to_synthesis_error from StmCircuit::synthesis_error into errors.rs, sharing the error conversion across circuit.rs and gadgets.rs
• Moved the STM Merkle path→Halo2 witness adapter from utils/mod.rs to the new adapters.rs test-only module, and inlined field-limb split helpers into gadgets.rs

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
mithril-stm/src/circuits/halo2/types.rs	Adds CircuitBaseField wrapper type with arithmetic ops and From/Into conversions; introduces CircuitBase/CircuitCurve aliases; replaces JubjubBase-based type aliases
mithril-stm/src/circuits/halo2/errors.rs	Adds GadgetsFieldModulusParse/GadgetsFieldElementConversion variants; extracts to_synthesis_error function
mithril-stm/src/circuits/halo2/gadgets.rs	Inlines field-limb split helpers from utils/mod.rs; adopts shared F/C aliases from types.rs; uses to_synthesis_error
mithril-stm/src/circuits/halo2/circuit.rs	Adopts shared F/C aliases; inserts .into() conversions at wrapper boundary; uses to_synthesis_error
mithril-stm/src/circuits/halo2/adapters.rs	New test-only file housing the STM→Halo2 Merkle path TryFrom adapter
mithril-stm/src/circuits/halo2/mod.rs	Registers new adapters module; removes utils sub-module; tightens gadgets visibility to pub(crate)
mithril-stm/src/circuits/halo2/golden/helpers.rs	Uses CircuitBaseField via F alias; migrates Midnight type references to use new From/Into path
mithril-stm/src/circuits/halo2/golden/cases/positive.rs	Removes unused use ff::Field import
mithril-stm/src/circuits/halo2/golden/cases/negative.rs	Removes unused use ff::Field import
mithril-stm/src/circuits/halo2/utils/mod.rs	Deleted; content moved to gadgets.rs and adapters.rs

[damrobi]

I left a few minor comments.

[jpraynaud]

LGTM 👍

Just few final comments to be addressed before merging.

[damrobi]

LGTM 👍

[curiecrypt]

LGTM 👍
I think it's better to address the duplicate functionality in the other PR. So, I marked the related conversation as resolved.

It can be merged 🚀