fix nix environment

[dbarrosop]

In addition to the issue described in #1292 it also sets correctly RUST_SRC_PATH and patches rustlings lsp to make use of it if available. Otherwise the path for sysroot_src is computed wrongly making rust-analyzer fail.

[luxus]

tested the pr and i was able to use nix develop

[buurro]

I was testing this and cargo run lsp wouldn't run without CoreServices in the devShell buildInputs.

I fixed it with this commit

[dbarrosop]

@buurro that's because we are mixing installing software with nix and non-nix mechanisms. Ideally we'd installing everything with nix and then this wouldn't be a problem. However, as this is a learning tool mixing mechanisms might be ok, it depends a bit on the goal maintainers had when introducing nix here. Would be nice to hear from a maintainer to know what they think.

[shadows-withal]

Not sure what exactly the problem is here, but if you want my rationale, I merged the original PR so that people with Nix installed could just run nix develop and have a shell they could work in. I'm not well versed in "the nix way", so I kinda had to rely on other people to make the changes, which is why I don't fully understand the issue at hand here.

If this PR fixes the flake for people that've had trouble working with this, though, then I'd be inclined to merge it.

[dbarrosop]

Thanks for the clarification, so there are two main problems currently with the nix setup:

1. It doesn't work on Darwin
2. Anything that relies on the NIX_SRC_PATH is also broken (i.e. rust-analyzer)

This PR fixes both issues.

As this is simply about having an easy to deploy environment I think we can forget about the nix way and install everything that is required to build rust applications using cargo so I just pushed another commit to add the required dependencies on macos. I also added rustfmt and clippy as those complement rust-analyzer but we can remove those if you prefer.

[shadows-withal]

Since I can test this on Darwin, and main is indeed broken, and this fixes it, that's good enough for me to merge. Thanks!

[shadows-withal]

@all-contributors please add @dbarrosop for infra

[allcontributors]

@diannasoreil

I've put up a pull request to add @dbarrosop! 🎉