Rust has 3 "platform support" tiers (effectively - guaranteed to work, guaranteed to build, supposed to work). However, these are (obviously) defined only for some of the target triples. This project defines "Tier-4" (which is normally not a thing) unstable support for Windows Vista-and-prior
tiers 1-3 are policies[0] for in-tree targets, so by saying tier 4 they mean one implemented in a fork. Though that kind of skips over targets that can get away with just a custom target spec[1] and not modifying the source.
