[diskann-utils] Simplify StridedView.

[Mark Hildebrand (hildebrandmw)]

We apparently don't need owned or mutable StridedViews. This struct was getting in the way of simplifying the Matrix type, so I decided to do the following:

1. Collapse everything into a single Strided<'a, T> struct.
2. Ensure that the layout for a Strided is well formed.
3. Shrink the memory representation from 40 bytes to 32 bytes.
4. Implement a custom Rows iterator so we no longer panic if the column-stride is zero.

Note that currently, From<MatrixView<'a, T>> can panic. This is because MatrixView and friends don't actually guarantee that nrows * ncols does not overflow usize::MAX. I have another PR in the pipeline to fix that, but ran into a chicken and egg problem with Strided preventing that PR from making progress.

[Copilot]

🔵 Needs a closer look

It introduces a new unsafe, pointer-based core view type and manual auto-trait impls, which warrants careful human review despite largely mechanical downstream updates.

Pull request overview

This PR refactors diskann-utils’ strided matrix view abstraction by collapsing prior owned/mutable view variants into a single borrowed Strided<'a, T> with an explicit validated Layout, and updates downstream quantization/benchmark/provider code to use the new API.

Changes:

• Replaced StridedView/MutStridedView/StridedBase with Strided<'a, T>, plus a Layout validator and a non-panicking Rows iterator.
• Updated PQ training, k-means, transposed tables, providers error bridging, and benchmark recall code to use Strided::try_from_data(...), rows(), and new accessors.
• Introduced a small internal helper for slice-to-NonNull conversion to support the new representation.

File summaries

File	Description
diskann-utils/src/strided.rs	Replaces the old strided view types with Strided<'a, T>, adds Layout validation, new element/row accessors, and a custom Rows iterator; updates tests accordingly.
diskann-utils/src/lib.rs	Adds the new internal module to support Strided’s representation.
diskann-utils/src/internal.rs	Adds an internal helper for creating NonNull from a slice base pointer.
diskann-quantization/src/product/train.rs	Switches PQ training chunk views from StridedView to Strided and uses rows() instead of row_iter().
diskann-quantization/src/product/tables/transposed/table.rs	Updates pivot slicing to construct Strided via try_from_data.
diskann-quantization/src/product/tables/transposed/pivots.rs	Updates APIs and call sites to accept Strided and uses rows() / row_unchecked() access.
diskann-quantization/src/multi_vector/block_transposed.rs	Updates block-transpose construction to accept Strided and uses row_unchecked().
diskann-quantization/src/algorithms/kmeans/plusplus.rs	Updates kmeans++ helpers to consume Strided and uses row_or_panic() for copying selected points.
diskann-quantization/src/algorithms/kmeans/lloyds.rs	Updates Lloyd’s implementation to consume Strided and iterate with rows().
diskann-providers/src/model/pq/strided.rs	Updates error-bridging to the new non-generic TryFromError type.
diskann-benchmark-core/src/recall.rs	Updates optional distance matrix handling to accept Strided and uses row_or_panic().

Review details

Suppressed comments (2)

diskann-utils/src/strided.rs:329

• The doc comment says TryFromError is for Strided::new, but there is no new constructor; the fallible constructor is try_from_data. This makes rustdoc misleading for callers.

/// Errors for [`Strided::new`].
#[derive(Debug, Error)]

diskann-utils/src/strided.rs:353

• The From<MatrixView> impl can panic via .expect(...). From is expected to be infallible; if this conversion is genuinely fallible (overflow or length mismatch), prefer TryFrom<MatrixView> (and/or keep From only after MatrixView invariants are strengthened).

 matrix.nrows(),
matrix.ncols(),
matrix.ncols(),
)
.expect("this will be made infallible in the future")

• Files reviewed: 11/11 changed files
• Comments generated: 3
• Review effort level: Lite

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Codecov Report

❌ Patch coverage is 99.69512% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.53%. Comparing base (600c2b9) to head (e941c51).

Files with missing lines	Patch %	Lines
diskann-quantization/src/product/train.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #1376 +/- ##
==========================================
- Coverage 91.55% 91.53% -0.02% 
==========================================
Files 521 522 +1 Lines 100302 100195 -107 ==========================================
- Hits 91828 91716 -112 - Misses 8474 8479 +5 

Flag	Coverage Δ
miri	91.53% <99.69%> (-0.02%)	⬇️
unittests	91.21% <99.69%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
diskann-benchmark-core/src/recall.rs	99.06% <100.00%> (ø)
diskann-providers/src/model/pq/strided.rs	100.00% <100.00%> (ø)
...skann-quantization/src/algorithms/kmeans/lloyds.rs	99.23% <100.00%> (ø)
...ann-quantization/src/algorithms/kmeans/plusplus.rs	99.48% <100.00%> (+<0.01%)	⬆️
...-quantization/src/multi_vector/block_transposed.rs	98.79% <100.00%> (ø)
...antization/src/product/tables/transposed/pivots.rs	98.68% <100.00%> (-0.02%)	⬇️
...uantization/src/product/tables/transposed/table.rs	99.28% <100.00%> (ø)
diskann-utils/src/internal.rs	100.00% <100.00%> (ø)
diskann-utils/src/lib.rs	100.00% <ø> (ø)
diskann-utils/src/strided.rs	100.00% <100.00%> (ø)
... and 1 more

... and 6 files with indirect coverage changes

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