Description of Changes

To resolve #3875, we added exact-match unique index point lookup support to the C# bindings by introducing and using datastore_index_scan_point_bsatn.

Previously, generated unique index Find() was (in at least one codepath) implemented as:

• A range scan (datastore_index_scan_range_bsatn) over a BTree bound, then
• SingleOrDefault() to collapse the results into a single row.
  When the scan is empty, SingleOrDefault() returns default(T). For value-type rows this can manifest as a default-initialized row instead of “missing”, which is what surfaced as “default-ish row” behavior in views.

Using datastore_index_scan_point_bsatn makes the C# implementation match Rust semantics more closely by performing an exact point lookup and returning:

• null when no rows are found
• the row when exactly one row is found
• (defensively) an error if >1 row is returned (unique index invariant violation)
  Similarly, datastore_delete_by_index_scan_point_bsatn was added and used so deletes-by-unique-key are also exact-match point operations rather than range deletes.

Runtime updates were made to utilize point scan in FindSingle(key) and in both mutable/read-only unique-index paths.

To keep this non-breaking for existing modules, codegen now detects whether the table row is a struct or a class and chooses the appropriate base type:

• Struct rows: Find() returns Row? (Nullable<Row>).
• Class rows: Find() returns Row? (nullable reference, null on miss).

API and ABI breaking changes

This change is non-breaking with respect to row type kinds, because class/record table rows continue to work via RefUniqueIndex/ReadOnlyRefUniqueIndex while struct rows use UniqueIndex/ReadOnlyUniqueIndex.

API surface changes:

• Generated Find() return type is now nullable (Row?) to correctly represent “missing”.

ABI/runtime:

• Requires the point-scan hostcall import (datastore_index_scan_point_bsatn) to be available; the runtime uses point-scan for unique lookup (and point delete for unique delete).

Expected complexity level and risk

Low 2

Testing

• Local testing: repro module + client validate view and direct Find() behavior