Fix variance issues with Mat and MatMut.

[hildebrandmw]

Ensure that Mat and MatMut are invariant in T. Without this change, the two compile-fail tests succeed, which could allow objects with shorter lifetimes to be assigned into a Mat/MatRef and lead to a dangling reference.

This is certainly a corner case since in all our current implementations, the representations T are 'static, but it doesn't hurt to be extra careful here.

[Copilot]

Pull request overview

This PR fixes a variance soundness issue in Mat<T>, MatRef<'a, T>, and MatMut<'a, T> where Mat and MatMut were previously covariant in T due to the bare repr: T value field, allowing unsafe lifetime shrinkage. The fix adds PhantomData markers to enforce correct variance, and adds compile-fail tests to prevent future regression.

Changes:

• Mat<T> is made invariant in T by adding a PhantomData<fn(T) -> T> field.
• MatMut<'a, T> is made invariant in T by changing _lifetime from PhantomData<&'a mut [u8]> to PhantomData<&'a mut T>.
• MatRef<'a, T>'s _lifetime is updated from PhantomData<&'a [u8]> to PhantomData<&'a T> for semantic clarity (variance is unchanged), and compile-fail + inline covariance assertion tests are added.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
diskann-quantization/src/multi_vector/matrix.rs	Adds _invariant to Mat, updates _lifetime in MatRef and MatMut, and adds inline covariance assertion functions for MatRef/MatMut in 'a.
diskann-quantization/tests/compile-fail/multi/mat_invariant.rs	New compile-fail test asserting that Mat<T> is invariant in T.
diskann-quantization/tests/compile-fail/multi/mat_invariant.stderr	Expected compiler output for the mat_invariant compile-fail test.
diskann-quantization/tests/compile-fail/multi/matmut_invariant.rs	New compile-fail test asserting that MatMut<'a, T> is invariant in T.
diskann-quantization/tests/compile-fail/multi/matmut_invariant.stderr	Expected compiler output for the matmut_invariant compile-fail test.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 25.00000% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.45%. Comparing base (9b2fd20) to head (68f4cf3).

Files with missing lines	Patch %	Lines
diskann-quantization/src/multi_vector/matrix.rs	25.00%	15 Missing ⚠️

❌ Your patch status has failed because the patch coverage (25.00%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #806      +/-   ##
==========================================
- Coverage   90.66%   89.45%   -1.21%     
==========================================
  Files         432      432              
  Lines       79433    79452      +19     
==========================================
- Hits        72019    71075     -944     
- Misses       7414     8377     +963     

Flag	Coverage Δ
miri	89.45% <25.00%> (-1.21%)	⬇️
unittests	89.30% <25.00%> (-1.34%)	⬇️

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Files with missing lines	Coverage Δ
diskann-quantization/src/multi_vector/matrix.rs	95.16% <25.00%> (-2.13%)	⬇️

... and 37 files with indirect coverage changes

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