Re-slice the downcast view instead of re-validating it

[emilk]

Related

• Make TypedArray public #23
• Follow-ups from the 0.6 post-merge reviews #55

What

TypedArray::slice sliced the arrow array and then re-ran L::downcast on the result. For a nested type that recurses into the children, and where a child carries a validity buffer it rescans that bitmap — work the whole array was already validated against, and which slicing cannot invalidate (a slice reaches a subset of the rows).

LogicalType::slice_typed(typed, offset, length) -> Option<Self::Typed> is the new hook: the view of the sliced rows, or None to fall back to downcast. The default is None, so a hand-written LogicalType impl keeps compiling and keeps today's behavior — the hook is additive.

Every encoding in the crate overrides it:

Encoding	How
leaves (primitives, strings, binary, …)	the arrow array slices itself
Option<L>, newtypes, As<T, Repr>	forward to the inner type
List, LargeList, ListView, Map, Dictionary, Run	move the parent's window, share the child view
FixedSizeList	move the child window too — arrow re-bases the values on slice
AnyList, AnyUtf8, AnyBinary	dispatch per variant

Measured with the new slice bench (first commit, so the before/after is reproducible):

slice of i64 (1M)                        34ns  ->  34ns   (already cheap)
slice of Utf8 (100k)                     44ns  ->  38ns
slice of List<i64> (100k rows)           66ns  ->  51ns
slice of List<List<i64>> (100k rows)     85ns  ->  60ns
slice of List<i64>, item null buffer   1.82µs  ->  42ns   (~43x)

The last row is the case worth having: non-nullable items whose child array carries a validity buffer, so downcast scans it on every slice.

Testing

slice_agrees_with_a_full_downcast cross-checks the fast path against TypedArray::try_new(array.as_arrow().slice(…)) for 22 encodings, over five windows each (whole, prefix, suffix, one row, empty), and asserts that slicing an already-sliced array lands where slicing once does — the case that catches a fast path which restarts the window rather than composing it.

cargo test --all-features, cargo clippy --all-targets --all-features -- -D warnings, cargo fmt --check pass.

Compatibility

Additive: slice_typed has a default, so external LogicalType impls are unaffected. No public signature changes.

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