The rules-swift concurrency rule's acceptance check requires every `@unchecked
Sendable` and `nonisolated(unsafe)` to carry an adjacent comment explaining why
the race is acceptable; an unjustified one is a violation. An audit found six
escape hatches whose comment explained the type but not the race:
- The four XPath `FunctionTable` statics (`CoreFunctions`, `StringFunctions`,
`NodeFunctions`, `Evaluator.library`): each is an immutable, build-once lookup
table never mutated after initialization, so concurrent reads race on nothing;
`FunctionTable` is un-`Sendable` only because it stores closures.
- `Validation.Validator`: immutable after `init` (every stored validation list is
a `let`); the erased `AnyValidation` closures are simply not provably `Sendable`.
- The test `ResultBox`: written once by the worker thread before its semaphore
signals and read only after the wait, so the write happens-before the read.
Comments only; no behaviour change. `LoaderBox` and `ElementStorage` were already
justified. swiftformat/swiftlint --strict and the full suite stay green.
A long <xs:attributeGroup ref> chain (ag0 refs ag1 refs ag2 ...) is flat in the document, so flattening a complex type's attributes through it recursed once per link: the stack overflowed past ~6000 links and it was near-cubic before that (~17s at 3000). attributeUses already threads a visited cycle-guard set whose count is the chain depth, so bound it against maximumContentModelExpansionDepth (512): a chain past the bound is rejected with a located error rather than crashed. Behaviour-preserving within the bound, which no realistic schema or the XSTS corpus approaches; verified conformance-neutral over the full W3C XSTS corpus and the full suite. Residual: many distinct over-bound chains still reject in O(n^2) per group (attribute flattening re-done per group); a memo is the linear follow-up.
Two constructs are flat in the schema document, so they escape the parser's element-nesting maxDepth (256) yet make a content model arbitrarily deep once compiled: a <xs:group ref> chain and a complexContent derivation chain (the definitions are siblings). The particle compiler inlines each group reference and composes each derivation once per link, overflowing the stack on a long chain. The expanded depth is now bounded by one counter (XSDContext.expansionDepth, incremented at each compositor level, group-reference expansion, and derivation step) against maximumContentModelExpansionDepth (512); a model past the bound is rejected with a located error, not crashed. The counter lives in a value-type context so it tracks depth not width (no width false positive), and the rejection is harvested into the schema's errors (no silent truncation). Verified conformance-neutral over the full W3C XSTS corpus (all four buckets unchanged), the libxml2 determinism differential unchanged, and the full suite; an adversarial critic found the derivation-chain gap, which this fixes with the same counter.
