Motivation for this change

I'm working to compile the Redox kernel using Nix, which involves using a custom json target (x86_64-unknown-none.json) in buildRustPackage.

At the end of the compile process, the target json path is not "shortened" (relevant Cargo code), causing a failure:

    Finished dev [optimized + debuginfo] target(s) in 42.60s
cp: missing destination file operand after 'target//nix/store/dk55xqrgyavw1gizs9b07p71j7przki0-x86_64-unknown-none.json/debug-tmp/'
Try 'cp --help' for more information.
builder for '/nix/store/dhpb8p1nibwr8hw95rymlipcqvw3swrs-redox-iso-latest-x86_64-unknown-redox.drv' failed with exit code 1
error: build of '/nix/store/dhpb8p1nibwr8hw95rymlipcqvw3swrs-redox-iso-latest-x86_64-unknown-redox.drv' failed

Ping to @Ericson2314 because I've seen you in relevant Rust community discussions. I'm currently using cargo-sysroot to compile to a custom target using Nix. Once I get everything cleaned up and fully declarative, I'll try to figure out how other Nix people can re-use my code for following tutorials like os.phil-opp.com (assuming there isn't an existing solution for a custom cargo sysroot in Nix derivations).

Things done

• added handling for when a buildRustPackage target is defined by a JSON file
• avoided causing existing Rust packages to need to be recompiled

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.