I both agree and disagree with a lot of the things that you're saying, but rather than get into all of that, one tiny thing: > setting an environment variable

I both agree and disagree with a lot of the things that you're saying, but rather than get into all of that, one tiny thing: > setting an environment variable that explicitly tells the user not to set it ever in the name of the variable but like... Cargo doesn't be have any other way of making this stuff work correctly. This has nothing to do with Cargo, if you're talking about RUSTC_BOOTSTRAP. This is because the kernel would like to use a stable compiler, but unstable features. This isn't a supported build configuration; the environment variable is an escape hatch for the bootstrap process, and isn't intended for nothing more. It would be, imho, unreasonable to expect Cargo to support something that is explicitly not intended to be supported.