Comment 18 for bug 1957932

Before even starting to address the various points further, I must ask
whether they're showstopper for the *rustc* MIR.

I ask because some of the concerns raised here are irrelevant for rustc
itself. For instance, the X-Cargo-Built-Using is not only not used by
the rustc packaging at all, it would also not be used by Rust packages
entering main since, under the proposed amended rules, those packages
would first vendor all their dependencies.

We intend to implement all tooling changes that are required for a
wider Rust ecosystem support in main, but this starts with having the
compiler!