> it wants control of the source tree
Um no. I have a Maven project with some Rust code, with pom.xml and Cargo.toml in the same folder. That works fine. Cargo DGAF that it's being invoked from another build system instead of directly, nor that the Rust sources are in a different folder than usual (because, again, there is both Rust and Java code in this project). It Just Works™, as Cargo usually does.