`//X` compiles to `descendant-or-self::node()/child::X`, which materializes
every node of the subtree as an intermediate context before selecting the X
children of each. Fuse that pair into a single `descendant::X` step at compile
time: the two select the same node-set, so evaluation skips the intermediate.
The fusion is transparent except for a positional predicate: `//x[1]` is the
first x child of EACH parent, not the first x in document order, so it must
not become `descendant::x[1]`. Apply the rewrite only when every predicate is
provably non-positional: its value is a boolean, node-set, or string (never a
number, which XPath reads as position() = value) and it nowhere references
position() or last(). Anything not provably both is left unfused, so a result
can never change.
Measured xpath ~10x to ~7x slower than libxml2 on the 20k-item generated
corpus via a same-state A/B, identical results. XPathDescendantFusionTests
pins the positional forms keeping their per-parent meaning, the non-positional
forms selecting across the subtree, and composition with trailing and nested
steps. An adversarial differential review (fused versus a spec-literal unfused
reference over nine attack suites) found no query whose result changes. The
boolean-function allowlist carries a documented invariant: every entry must be
position- and size-independent, since the position scan is syntactic.
