Add Nix flake for direnv (shader-slang#6635)

* Add Nix flake for direnv

* Use Sai's suggestion for the `description`

* Make `.envrc` optional

* Move Nix docs to their own section

* Tweak wording

* Tweak wording more

* Add `nixfmt`

---------

Co-authored-by: Sai Praveen Bangaru <31557731+saipraveenb25@users.noreply.github.com>
Co-authored-by: Ellie Hermaszewska <ellieh@nvidia.com>

Author: 3 people

.gitignore

@@ -12,6 +12,8 @@
 *.zip
 *.ini
 *.DS_store
+.direnv
+.envrc
 .gdb_history
 .vimspector
 .vimspector.json

docs/building.md

@@ -311,6 +311,20 @@ cmake --preset vs2022 --fresh -A arm64 -DSLANG_GENERATORS_PATH=generators/bin
 cmake --build --preset release
 ```
 
+### Nix
+
+This repository contains a [Nix](https://nixos.org/)
+[flake](https://wiki.nixos.org/wiki/Flakes) (not officially supported or
+tested), which provides the necessary prerequisites for local development. Also,
+if you use [direnv](https://direnv.net/), you can run the following commands to
+have the Nix environment automatically activate when you enter your clone of
+this repository:
+
+```bash
+echo 'use flake' >> .envrc
+direnv allow
+```
+
 ## Building with an older CMake
 
 Because older CMake versions don't support all the features we want to use in

flake.lock

@@ -0,0 +1,33 @@
+{
+  description = "The Slang Shading Language and Compiler";
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+  outputs =
+    {
+      self,
+      nixpkgs,
+      flake-utils,
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+      in
+      {
+        devShell =
+          with pkgs;
+          mkShell {
+            buildInputs = [
+              cmake
+              llvm
+              ninja
+              nixfmt-rfc-style
+              python3
+              xorg.libX11
+            ];
+          };
+      }
+    );
+}