Rust and C have differences around compilation units: Rust's already tend to be much larger than C on average, because the entire crate (aka tree of modules) is the compilation unit in Rust, as opposed to the file-based (okay not if you're on some weird architecture) compilation unit of C.

Unity builds are useful for C programs because they tend to reduce header processing overhead, whereas Rust does not have the preprocessor or header files at all.

They also can help with reducing the number of object files (down to one from many), so that the linker has less work to do, this is already sort of done (though not to literally one) due to what I mentioned above.

In general, the conventional advice is to do the exact opposite: breaking large Rust projects into more, smaller compilation units can help do less "spurious" rebuilding, so smaller changes have less overall impact.

Basically, Rust's compile time issues lie elsewhere.

Can you explain why a unity build would help? Conventional wisdom is that Rust compilation is slow in part because it has too few translation units (one per crate, plus codegen units which only sometimes work), not too many.