Rustc is very tightly coupled with cargo. Nothing stops you from calling rustc yourself, but you basically need to reverse engineer what cargo is doing. My ref

Rustc is very tightly coupled with cargo. Nothing stops you from calling rustc yourself, but you basically need to reverse engineer what cargo is doing. My reference for this is https://github.com/bazelbuild/rules_rust https://github.com/bazelbuild/rules_rust which needs to pass a bunch of CARGO_ envvars to get stuff to build, among various other required conventions.