If you have a serious proposal, then I encourage someone to seriously propose it. Cargo is an understaffed open source project that, like the rest of the Rust project, relies largely on volunteers. However, gesturing to unspecified internal weird mechanisms does not strike me as a serious proposal worthy of consideration by anyone, so I'd suggest working on that first.
> The sandbox should not have network access, but cargo needs network access to download the package in the first place.
If you have a serious proposal, then I encourage someone to seriously propose it. Cargo is an understaffed open source project that, like the rest of the Rust project, relies largely on volunteers. However, gesturing to unspecified internal weird mechanisms does not strike me as a serious proposal worthy of consideration by anyone, so I'd suggest working on that first.
> The sandbox should not have network access, but cargo needs network access to download the package in the first place.
Naturally. Use `cargo fetch` to download a package locally without invoking any build step: https://doc.rust-lang.org/cargo/commands/cargo-fetch.html