day-toolchain — ONE place that knows where host toolchains and SDKs live, shared by the
day CLI and by crate build scripts (day-winui-sys, every day-piece-*/day-tweak-* that
compiles its own native shim, and the scaffolds day new generates).
Two rules govern every lookup here (docs/environment.md):
- An environment variable always wins. Each function documents its override(s).
- No literal install paths. Default locations are derived from the platform's own
environment (
%ProgramFiles%,$HOME,%LOCALAPPDATA%) — never a hardwiredC:\…, so relocated installs (Windows Kits onD:, a portable SDK) work by setting one var.
Functions that are meant to be called from build scripts have _for_build_script variants
that also emit the matching cargo:rerun-if-env-changed= lines, so changing an override
re-runs the script instead of silently keeping stale results.