buildRustCrate: Search for matching Cargo.toml in sub directories

[kolloch]

Motivation for this change

When cargo builds a git dependency, it will search for a matching Cargo.toml in any sub directory. buildRustCrate assumes the top-level directory by default which results in the following build failures:

nix-community/crate2nix#33
nix-community/crate2nix#53

In theory, workspace_member could be set to the appropriate sub directory upfront but allowing workspace_member = null to trigger a search is much mroe convenient. This PR will NOT change the default behavior because the current buildRustCrate does not depend on any Cargo.toml being present and we'd like to preserve that.

Things done

• 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):

  nix build -I nixpkgs=. '((import <nixpkgs> {}).callPackage pkgs/build-support/rust/build-rust-crate/test {}).test'
  
  

• 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.

[andir]

@GrahamcOfBorg build buildRustCrateTests

[kolloch]

buildRustCrateTests on x86_64-darwin times out on building rustc itself:

building of '/nix/store/na14i3gf8w4wl60kbhia9gp2a388b21d-rustc-1.41.0.drv' timed out after 3600 seconds

https://logs.nix.ci/?attempt_id=cc2ccc7c-25d3-4115-b4bf-8f068cff8650&key=nixos%2Fnixpkgs.82155

[kolloch]

I think that if I do that, darwin will not rebuild but I'll try ;)

@GrahamcOfBorg build buildRustCrateTests

[andir]

@GrahamcOfBorg build carnix cargo-download

[andir]

The build failure of cargo-download (https://gist.githubusercontent.com/andir/47cefa42982b873e213ab4328419991c/raw/f7bca737264bbb4126f85e69fee25484620bd024/i9n0z4hcqkx9z7vf5pil4hlx83knymva-rust_url-1.6.0.drv.log) seems to be unrelated to this change. It fails due to a newer rustc version (is my first guess).

The tests also look fine and pass (on at least linux).

Thank you for adding this! 👍