It is quite common for big projects with Rust code to vendor their dependencies. rust2rpm should be able to scan for vendored dependencies and generate Provides: bundled(crate(foo)) = 1.2.3 lines for the bundled dependencies.

Note that while it's generally in a vendor subdirectory in the same directory as Cargo.toml, that isn't necessarily guaranteed. But at least at a first pass, we can start here...