vcvars locates a Visual Studio / Build Tools install via vswhere and loads the MSVC toolchain (vcvars*.bat) into the current process, so C extensions build under an mswin Ruby without first opening a "Developer Command Prompt". It provides a library API (Vcvars.activate!), a Rake integration (require "vcvars/rake"), a `vcvars exec -- <cmd>` runner, a `vcvars doctor` that diagnoses the classic MSVC extension-build failures, a `vcvars env` shell-env emitter, and a `vcvars new` scaffolder for MSVC-ready extension gems. It is "ridk enable", but for MSVC. Pure Ruby, no compiler required to install.