Handle wide power-of-two divisors in exact division

[tob-joe]

Summary

UintRef::div_exact could panic when the divisor was wider than the dividend and had exactly as many trailing zeros as the dividend precision. In that case, exact division should report inexact division rather than indexing or shifting past the dividend.

Fix

ensure_trailing_zeros now handles zero-count requests that cover the full dividend width by checking whether the whole dividend is zero, instead of indexing the limb at nlimbs.

After the trailing-zero exactness check, div_exact now uses an unbounded right shift so shifting by the full dividend precision produces zero instead of panicking.

Tests

Added a regression test covering exact division of U128::ONE by a U256 power-of-two divisor at bit position U128::BITS.

Verified with:

cargo test --all-features --test uint div_exact_wide_power_of_two_divisor_is_inexact -- --exact

---

This work was completed by Trail of Bits as part of the Patch The Planet project in collaboration with OpenAI. The issue was identified primarily by the Codex coding agent, and manually reviewed before submission.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.00%. Comparing base (4c6f87d) to head (809148c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1271   +/-   ##
=======================================
  Coverage   90.99%   91.00%           
=======================================
  Files         189      189           
  Lines       22139    22141    +2     
=======================================
+ Hits        20146    20150    +4     
+ Misses       1993     1991    -2     

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