rust build breakage

I'm packaging tor for pkgsrc. At some point in the not so far past I had added an option for building with rust, then I forgot about it, and then I tried it again when updating tor to 0.4.4.6, but saw:

> error: could not find `Cargo.toml` in `/scratch/net/tor/work/tor-0.4.4.6` or any parent directory

Compared to a non-rust build the package has the following changes:

configure environment: TOR_RUST_DEPENDENCIES=${WRKDIR}/vendor
configure args: --enable-rust

Did the build method change, or is there a missing file in the distribution tarball, or did something else break?