But cargo already isn’t in Rust. There are AFAIK no cargo-specific concepts that have leaked into Rust, the language. It is very straightforward to build Rust code without using cargo, for example with a makefile.
The reason other tools are slow to add support for rust is because cargo is so ubiquitous in the rust ecosystem that there is little point (I’d estimate that >99% of Rust code is built using Cargo), and not because of any technical impossibility.
It is my fondest hope that the Linux folks will finally beat the cargo out of Rust.