unicode/utf16 validates; disclosed-divergence generalizes
Full text of the July 18, 2026 announcement, condensed in the go2cs News Archive.
Phase-4 package #5. unicode/utf16
validates its own Go test suite in C# — 8 tests agreeing outright against go test -json, plus one
honestly disclosed. The structural twin of the very first validated package (unicode/utf8), it round-trips
UTF-16 encode/decode with results checked by reflect.DeepEqual — exercised here through the converted
reflection bridge — and all eight correctness tests match verdict for verdict.
Its significance is what the ninth test demonstrates. TestAllocationsDecode asserts that Decode returns
its []rune with zero heap allocations — a result Go reaches only through compiler escape analysis, so
the test guards itself with testenv.SkipIfOptimizationOff. The managed runtime provably cannot match it: a
returned slice<rune> is always a heap allocation, no matter how the method is written. This is the same
allocation-model divergence bytes and strings disclosed a day earlier — and unicode/utf16 is the
first package to reuse the disclosed-divergence manifest
as a general tool rather than a two-package special case. Its go2cs_test_disclosures.json pins one
alloc-profile row by exact failure signature ("Decode allocated "), while the separate TestDecode
independently proves the decoded output is correct — so the disclosure covers exactly the allocation
profile and nothing else. A mechanism that generalizes cleanly to the next package is a mechanism that was
designed right.
Phase-4 package #5 · unicode/utf16 8 + 1 disclosed (alloc-profile) · reproduce from a clone via
Try it yourself