Make List::tree_hash_root infallible by computing hashes with pending updates

[Copilot]

Implementation Summary

Successfully implemented infallible tree_hash_root for List and Vector that handles pending updates without requiring apply_updates() to be called first.

Changes Made

• Added methods to ImmList trait to enable tree traversal (tree(), depth(), packing_depth())
• Implemented new tree_hash_root method in Interface that:
  • Computes tree hashes with pending updates
  • Uses cached hashes for subtrees without updates for efficiency
  • Handles both packed and regular leaves
  • Handles Zero trees with pending updates (empty lists with pushes)
  • Recursively traverses the tree
• Updated List::tree_hash_root and Vector::tree_hash_root:
  • Removed assertions about pending updates
  • Delegated to new interface method
• Implemented trait methods for ListInner and VectorInner
• Added comprehensive tests (11 tests total):
  • Empty list with pending push
  • Large lists with updates at different depths
  • Multiple pending pushes
  • Vector with pending updates
  • Packed types with multiple updates in same leaf
  • Single element list
  • All elements updated
• Addressed all code review feedback
• Ran cargo fmt to format code

Testing

✅ All 282 tests pass (7 new tests added):

• Tree hash and SSZ tests
• Operations tests
• Rebase tests
• Comprehensive pending updates tests covering edge cases

Coverage Improvements

Added tests for:

• Empty lists transitioning to non-empty via pending updates
• Large lists exercising deeper tree structures
• Multiple simultaneous pending pushes
• Vector-specific pending update paths
• Packed leaf edge cases with multiple updates
• Complete list update scenarios

Original prompt

I want you to update the tree_hash_root method on List so that it is infallible, i.e. remove the assert about pending updates. To do this safely, I think the best approach is to make use of the pending updates while hashing. Add a new tree_hash_root method to interface.rs which can handle getting hashes from the pending updates or from cached within the tree as appropriate. You may need to add some methods to the ImmList trait in order to enable tree traversal from this generic code.

After your changes all of the tests run by cargo test --release should pass. While you are making changes you can quickly check that your code compiles with cargo check.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.90647% with 46 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.94%. Comparing base (4d88c72) to head (a0c1d0b).

Files with missing lines	Patch %	Lines
src/interface.rs	64.80%	44 Missing ⚠️
src/list.rs	85.71%	1 Missing ⚠️
src/vector.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #92      +/-   ##
==========================================
- Coverage   70.15%   69.94%   -0.22%     
==========================================
  Files          22       22              
  Lines        1280     1414     +134     
==========================================
+ Hits          898      989      +91     
- Misses        382      425      +43     

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[michaelsproul]

@copilot Please run cargo fmt and try to increase coverage by looking at the coverage report from coverrals (or run cargo-tarpaulin locally)

[Copilot]

@copilot Please run cargo fmt and try to increase coverage by looking at the coverage report from coverrals (or run cargo-tarpaulin locally)

Done! I've:

1. Run cargo fmt - committed formatting changes in 506cbfe
2. Added 7 comprehensive tests to increase coverage in b38cdef:
   • Empty list with pending push (tests Zero tree with updates)
   • Large lists (exercises deeper tree structures)
   • Multiple pending pushes
   • Vector with pending updates
   • Packed types with multiple updates in same leaf
   • Single element list
   • All elements updated

All 282 tests now pass. The new tests cover edge cases like empty lists transitioning to non-empty via pending updates, and Zero tree handling which wasn't previously tested.