Backlog#1845 step 8 conclusion. The plan called for folding iam::Error into a policy::Error #[from] wrapper and deleting the hand-written mapping. Measurement rejected the fold: the duplicated variants have ~220 construction/match sites (about 140 in production) across iam and the admin handlers - all auth-critical - and the alias route is blocked by the orphan rule (iam's From<IamStorageError> and io conversions cannot be implemented for a foreign type). Meanwhile the drift risk the fold targeted is already compiler-covered: the From match is exhaustive with no catch-all, so any new policy variant fails the build until mapped.
What remains of the step, delivered: the six dead policy variants are gone (previous commit), the grouped lossy arm is down to the two variants actually produced, a doc comment on the From impl records the verdict with the evidence, and a new totality test constructs one representative of every policy::error::Error variant and asserts the conversion preserves the rendered message - so the mapping is now pinned loss-free in both directions the classifiers care about.
Ref rustfs/backlog#1845
Zhengchao An
45f706b274