--enable-rust with pre-downloaded Rust dependencies fails: no .cargo-checksum.json files

In the FreeBSD port I am downloading the dependencies and supplying digest-0.7.2 libc-0.2.39 directories as they appear on GitHub pointed by the TOR_RUST_DEPENDENCIES environment variable.

The build fails:

error: failed to load source for a dependency on `digest`

Caused by:
  Unable to update registry `https://github.com/rust-lang/crates.io-index`

Caused by:
  failed to update replaced source registry `https://github.com/rust-lang/crates.io-index`

Caused by:
  failed to load checksum `.cargo-checksum.json` of libc v0.2.39

Caused by:
  failed to read `/usr/ports/security/tor/work/tor-0.4.0.5/rs/libc-0.2.39/.cargo-checksum.json`

Is some command supposed to be run that would build .cargo-checksum.json ?

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