Enforce UintRef slice length checks in release builds

[tob-joe]

Summary

UintRef::copy_from_slice documents that it panics when self.nlimbs() != limbs.len(), but the check used debug_assert!, so optimized builds did not enforce that contract:

crypto-bigint/src/uint/ref_type/slice.rs

Lines 25 to 33 in 4c6f87d

	/// Copy the contents from a limb slice.
	///
	/// # Panics
	/// If `self.nlimbs() != limbs.len()`
	#[inline(always)]
	#[track_caller]
	pub const fn copy_from_slice(&mut self, limbs: &[Limb]) {
	// TODO core::slice::copy_from_slice should eventually be const
	debug_assert!(self.limbs.len() == limbs.len(), "length mismatch");

UintRef::conditional_copy_from_slice has the same runtime length check pattern and its # Panics docs also state a limb-count mismatch panic, though the current wording says rhs.nlimbs() despite taking a limb slice:

crypto-bigint/src/uint/ref_type/slice.rs

Lines 41 to 48 in 4c6f87d

	/// Conditionally copy the contents from a limb slice.
	///
	/// # Panics
	/// If `self.nlimbs() != rhs.nlimbs()`
	#[inline(always)]
	#[track_caller]
	pub const fn conditional_copy_from_slice(&mut self, limbs: &[Limb], copy: Choice) {
	debug_assert!(self.limbs.len() == limbs.len(), "length mismatch");

Fix

Use assert! for both length checks so debug and release builds preserve the documented panic behavior.

Tests

Added regression tests for mismatched lengths in both slice-copy APIs and verified them in release mode:

cargo test --release --all-features uint::ref_type::slice::tests -- --nocapture

---

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 (4996659).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1272   +/-   ##
=======================================
  Coverage   90.99%   91.00%           
=======================================
  Files         189      189           
  Lines       22139    22147    +8     
=======================================
+ Hits        20146    20154    +8     
  Misses       1993     1993           

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