[microtonal]

Is this harder to achieve for the Rust ecosystem (due to some properties of Haskell perhaps)?

You can actually do this today. The buildRustCrate function in nixpkgs creates a derivation for a single Rust crate. You can combine this with crate2nix [1] to make derivations for a crate and all its dependencies in a Cargo.lock file. Every compiled crate will be a separate output in the Nix store and you can upload them to a binary cache such as Cachix.

This is what I have been using for CI in many of my Rust projects in combination with Cachix to get very fast builds.

[1] https://github.com/kolloch/crate2nix