How to set up Bazel with local Rust crates?

[thmsbinder]

I want to set up a hermetic bazel build with Rust toolchain, pre-fetching all the dependencies and passing them via --distdir pointing to a local directory. Rust crates are served from crates.io, via URLs of the form

https://static.crates.io/crates/$CRATE_NAME/$CRATE_VERSION/download

With bazel's distdir conventions, all crates are identified under the key download (the basename of the URL) and their hash, so there need to be as many distdirs as there are crates. That's technically correct and will work, but practically infeasible, since this would involve --distdir d1 ... --distdir d999, with only one file named download in each distdir.

I also tried to use a rewrite rule with --downloader_config, but it is only intended for URLs (http or https). It's not possible to rewrite to a local file (i.e. rewrite https:// to file://).

Question is, what's the best way to keep the Rust dependencies local and manageable, ideally in a single local directory where all crates appear under a meaningful name, e.g. crates/${CRATE_NAME}__${CRATE_VERSION}.crate?

[thmsbinder]

cc @meteorcloudy @UebelAndre