Add nix flake to scripts/nix/ (nushell#16848)

I saw nushell#10945 and added some notes in the `scripts/nix/README.md` file on
how to use it properly in the non-root directory. Closes nushell#10352.

---

Allows including in another flake using the `...?dir=scripts/nix` option
on the flake url. Some common uses in the repo:

```console
$ # Build nushell
$ nix build ./scripts/nix
$ # Enter the development shell (sadly it's bash, use .envrc for nu)
$ nix develop ./scripts/nix
$ # Enable direnv for this project
$ "use flake ./scripts/nix" | save -a ./.envrc
```

Everything else should be nicely extracted from the
`rust-toolchain.toml` and `Cargo.toml`s around the repo, making changes
outside the nix directory require almost no intervention, unless
dependencies change or a new plugin is added.

---

<!--
Thank you for improving Nushell!
Please, read our contributing guide:
https://github.com/nushell/nushell/blob/main/CONTRIBUTING.md
-->

## Release notes summary - What our users need to know
<!--
This section will be included as part of our release notes. See the
contributing guide for more details.
Please include only details relevant for users here. Motivation and
technical details can be added above or below this section.

You may leave this section blank until your PR is finalized. Ask a core
team member if you need help filling this section.
-->

## Tasks after submitting
<!-- Remove any tasks which aren't relevant for your PR, or add your own
-->
- [ ] Update the
[documentation](https://github.com/nushell/nushell.github.io)

Author: KaiSforza

.gitignore

@@ -54,5 +54,9 @@ tarpaulin-report.html
 .direnv/
 .envrc
 
+# nix
+/result
+/scripts/nix/result
+
 # pre-commit-hooks
 .pre-commit-config.yaml

scripts/nix/README.md

@@ -0,0 +1,112 @@
+# Nix Flake for Nushell
+
+Placed in the `scripts/nix` directory to keep it nicely out of the root of the repo, but also allow
+including and building with the latest sources. Due to it not being in the root most nix commands
+will need to be told the location of the flake:
+
+```console
+$ nix run ./scripts/nix
+$ nix build ./scripts/nix
+$ nix flake update --flake ./scripts/nix
+```
+
+## Including in another flake
+
+```nix
+# flake.nix
+{
+  inputs = {
+    nixpkgs.url = "...";
+    nushell = {
+      url = "github:nushell/nushell?dir=scripts/nix";
+      inputs.nixpkgs.follows = "nixpkgs"; # optional
+    };
+  };
+
+  outputs = inputs: {
+    # ...
+    # I would recommend using a `for-each-system` function of some kind here.
+    packages.x86_64-linux = {
+      nushell = inputs.nushell.packages.x86_64-linux.nushell;
+    };
+  }
+}
+```
+
+### Adding the hash to `version`
+
+The `inputs.nushell.rev` variable can be used to get the commit hash in the
+output of `version`:
+
+```nix
+{
+# ...
+      nushell = inputs.nushell.packages.x86_64-linux.nushell.override {
+        NU_COMMIT_HASH = inputs.nushell.rev;
+      };
+# ...
+}
+```
+
+## Using the Overlay
+
+This flake also includes an overlay to allow using the new nushell and plugins
+without modifying every instance of `nushell` in the configuration. This is a
+more complex example than the one above, allowing multiple systems and showing
+an example of a `nixosConfiguration` that uses the flakes `nushell`.
+
+```nix
+{
+  outputs = inputs:
+  let
+      systems = [
+        # List of systems to enable, for example:
+        "x86_64-linux"
+        "aarch64-darwin"
+      ];
+      # And for packages or other system-specific inputs, use a function that
+      # overlays overlays set during the nixpkgs import:
+      forEachSystem =
+        f:
+        inputs.nixpkgs.lib.genAttrs systems (
+          system:
+          f {
+            pkgs = import inputs.nixpkgs {
+              inherit system;
+              overlays = [ inputs.nixpkgs.overlays.default ];
+            }
+          }
+        );
+  in
+  {
+    packages = forEachSystem ({pkgs, ...}: {inherit nushell;});
+    # For a nixos configuration, add it to the `nixpkgs.overlays`. If a nixos
+    # configuration is all that is required, this is the only part needed
+    nixosConfiguration.yourHost = inputs.nixpkgs.lib.nixosSystem {
+      system = "...";
+      modules = [
+        {
+          nixpkgs.overlays = [
+            inputs.nushell.overlays.default
+          ];
+        }
+        ./configuration.nix
+        # ... other modules
+      ];
+    };
+  };
+}
+```
+
+## Using Without Flakes
+
+The package can be built using the `scripts/nix/default.nix` file:
+
+```nix
+let
+  pkgs = import <nixpkgs> { };
+in
+{
+  nushell = pkgs.callPackage ./scripts/nix { };
+}
+```

scripts/nix/default.nix

@@ -0,0 +1,123 @@
+# Generic builder for nushell and its included plugins.
+{
+  stdenv,
+  lib,
+  rustPlatform,
+  # Allows overriding for the plugins
+  pname ? "nushell",
+  # Use this for the cargo file that the plugin has
+  extraCargo ? ../../Cargo.toml,
+  withDefaultFeatures ? true,
+  additionalFeatures ? (p: p),
+  # Package dependencies
+  pkg-config,
+  openssl,
+  curlMinimal,
+  python3,
+  xorg,
+  nghttp2,
+  libgit2,
+  zstd,
+  zlib,
+  # Mostly args and things that might be different for plugins
+  nativeBuildInputs ? [
+    pkg-config
+  ]
+  ++ lib.optionals (withDefaultFeatures && stdenv.hostPlatform.isLinux) [ python3 ]
+  ++ lib.optionals stdenv.cc.isClang [ rustPlatform.bindgenHook ],
+  buildInputs ? [
+    zstd
+  ]
+  ++ lib.optionals stdenv.hostPlatform.isDarwin [ zlib ]
+  ++ lib.optionals (withDefaultFeatures && stdenv.hostPlatform.isLinux) [ xorg.libX11 ]
+  ++ lib.optionals (withDefaultFeatures && stdenv.hostPlatform.isDarwin) [
+    nghttp2
+    libgit2
+  ],
+  checkInputs ?
+    lib.optionals stdenv.hostPlatform.isDarwin [ curlMinimal ]
+    ++ lib.optionals stdenv.hostPlatform.isLinux [ openssl ],
+
+  # Fixed in #16914
+  buildAndTestSubdir ? ".",
+  checkFlags ? [ ],
+  buildType ? "release",
+  checkType ? buildType,
+  ...
+}@args:
+let
+  inherit (builtins)
+    fromTOML
+    readFile
+    head
+    ;
+  inherit (lib.fileset)
+    toSource
+    intersection
+    unions
+    difference
+    ;
+  root = ../..;
+  cargoToml = fromTOML (readFile (root + /Cargo.toml));
+  # Allow for extra Cargo.toml files to be passed for crates
+  extraToml = fromTOML (readFile extraCargo);
+in
+rustPlatform.buildRustPackage {
+  inherit
+    pname
+    nativeBuildInputs
+    buildInputs
+    checkInputs
+    buildAndTestSubdir
+    checkFlags
+    buildType
+    checkType
+    ;
+  inherit (cargoToml.package) version;
+
+  # Crates will need to build with their Cargo.toml
+  src = toSource {
+    inherit root;
+    fileset = (
+      intersection root (
+        difference (unions (
+          map (p: root + p) [
+            /Cargo.toml
+            /Cargo.lock
+            /src
+            /crates
+            /tests
+            /assets
+            /benches
+            /toolkit
+            /scripts
+          ]
+        )) ./.
+      )
+    );
+  };
+
+  cargoLock = {
+    lockFile = root + /Cargo.lock;
+    # Required for some of the dependencies in this repo
+    allowBuiltinFetchGit = true;
+  };
+
+  buildNoDefaultFeatures = !withDefaultFeatures;
+  buildFeatures = additionalFeatures [ ];
+
+  # Builds the plugins for the test using the right profile, otherwise the
+  # plugins cannot be found.
+  NUSHELL_CARGO_PROFILE = checkType;
+  NU_TEST_LOCALE_OVERRIDE = "en_US.UTF-8";
+  preCheck = ''
+    export HOME=$(mktemp -d)
+  '';
+  meta = {
+    description = extraToml.package.description or cargoToml.package.description;
+    homepage =
+      if cargoToml == extraToml then cargoToml.package.homepage else extraToml.package.repository or null;
+    license = lib.licenses.mit;
+    mainProgram = (head (extraToml.bin or [ extraToml.package ])).name or pname;
+  };
+}