Several fixes for Nix build (#2389)

* Fixed a bug for build via nix, when c3c-checks and c3c-debug didn't have a git hash

* Got rid of python3 requirement for checks in nix as it's no longer needed

* Now c3c properly displays it's build date in nix build

* Add more platforms for checks in nix build

* Add installation and compilation on Nix to README

README.md

@@ -264,6 +264,38 @@ cd c3c-git
 makepkg -si
 ```
 
+#### Installing via Nix
+
+You can access `c3c` via [flake.nix](./flake.nix), which will contain the latest commit of the compiler. To add `c3c` to your `flake.nix`, do the following:
+```nix
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs?ref=nixpkgs-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+    c3c.url = "github:c3lang/c3c";
+    # Those are desired if you don't want to copy extra nixpkgs
+    c3c.inputs = {
+      nixpkgs.follows = "nixpkgs";
+      flake-utils.follows = "flake-utils";
+    };
+  };
+
+  outputs = { self, ... } @ inputs: inputs.flake-utils.lib.eachDefaultSystem (system: 
+    let 
+      pkgs = import inputs.nixpkgs { inherit system; };
+      c3c = inputs.c3c.packages.${system}.c3c;
+    in 
+    {
+      devShells.default = pkgs.mkShell {
+        buildInputs = [
+          pkgs.c3c
+        ];
+      };
+    }
+  );
+}
+```
+
 #### Building via Docker
 
 You can build `c3c` using an Ubuntu container. By default, the script will build through Ubuntu 22.04. You can specify the version by passing the `UBUNTU_VERSION` environment variable.
@@ -405,6 +437,14 @@ After compilation, the `c3c` binary will be located in the `build` directory. Yo
 
 6. To install the compiler globally: `sudo cmake --install build`
 
+#### Compiling on NixOS
+
+1. Enter nix shell, by typing `nix develop` in root directory
+2. Configure cmake via `cmake . -Bbuild $=C3_CMAKE_FLAGS`. Note: passing `C3_CMAKE_FLAGS` is needed in due to generate `compile_commands.json` and find missing libs.
+4. Build it `cmake --build build`
+5. Test it out: `./build/c3c -V`
+6. If you use `clangd` lsp server for your editor, it is recommended to make a symbolic link to `compile_command.json` in the root: `ln -s ./build/compile_commands.json compile_commands.json`
+
 #### Compiling on other Linux / Unix variants
 
 1. Install CMake.

flake.nix

@@ -6,29 +6,27 @@
     flake-utils.url = "github:numtide/flake-utils";
   };
 
-  outputs = { self, ... } @ inputs: inputs.flake-utils.lib.eachDefaultSystem 
+  outputs = { self, ... }@inputs: inputs.flake-utils.lib.eachDefaultSystem 
   (system: 
     let pkgs = import inputs.nixpkgs { inherit system; }; 
-        call = set: pkgs.callPackage ./nix/default.nix ( 
-          set // { 
-            rev = self.rev or "unknown"; 
-          } 
-        );
+        c3cBuild = set: pkgs.callPackage ./nix/default.nix (set // { 
+          rev = self.rev or "unknown"; 
+        });
     in {
       packages = {
         default = self.packages.${system}.c3c;
 
-        c3c = call {};
+        c3c = c3cBuild {};
         
-        c3c-checks = pkgs.callPackage ./nix/default.nix { 
+        c3c-checks = c3cBuild { 
           checks = true; 
         };
 
-        c3c-debug = pkgs.callPackage ./nix/default.nix { 
+        c3c-debug = c3cBuild { 
           debug = true; 
         };
 
-        c3c-debug-checks = pkgs.callPackage ./nix/default.nix { 
+        c3c-debug-checks = c3cBuild { 
           debug = true; 
           checks = true; 
         };

nix/default.nix

@@ -2,12 +2,11 @@
   lib,
   llvmPackages,
   cmake,
-  python3,
   curl,
   libxml2,
   libffi,
   xar,
-  rev ? "unknown",
+  rev,
   debug ? false,
   checks ? false,
 }: let 
@@ -16,8 +15,8 @@
   inherit (lib.lists) findFirst;
   inherit (lib.asserts) assertMsg;
   inherit (lib.strings) hasInfix splitString removeSuffix removePrefix optionalString;
-in llvmPackages.stdenv.mkDerivation (finalAttrs: {
-
+in llvmPackages.stdenv.mkDerivation (_: 
+{
   pname = "c3c${optionalString debug "-debug"}";
 
   version = let
@@ -28,13 +27,19 @@ in llvmPackages.stdenv.mkDerivation (finalAttrs: {
 
   src = ../.;
   
-  cmakeBuildType = if debug then "Debug" else "Release";
-  
-  postPatch = ''
-    substituteInPlace git_hash.cmake \
-      --replace-fail "\''${GIT_HASH}" "${rev}"
+  # Here we substitute GIT_HASH which is not set for cmake in nix builds.
+  # Similar situation is with __DATE__ and __TIME__ macros, which are
+  # set to "Jan 01 1980 00:00:00" by default.
+  patchPhase = ''
+    substituteInPlace git_hash.cmake --replace-fail "\''${GIT_HASH}" "${rev}"
+
+    local FILE_NAMES="$(find src -type f)"
+    substituteInPlace $FILE_NAMES --replace-quiet "__DATE__" "\"$(date '+%b %d %Y')\""
+    substituteInPlace $FILE_NAMES --replace-quiet "__TIME__" "\"$(date '+%T')\""
   '';
 
+  cmakeBuildType = if debug then "Debug" else "Release";
+
   cmakeFlags = [
     "-DC3_ENABLE_CLANGD_LSP=${if debug then "ON" else "OFF"}"
     "-DC3_LLD_DIR=${llvmPackages.lld.lib}/lib"
@@ -54,17 +59,25 @@ in llvmPackages.stdenv.mkDerivation (finalAttrs: {
     libffi
   ] ++ lib.optionals llvmPackages.stdenv.hostPlatform.isDarwin [ xar ];
 
-  nativeCheckInputs = [ python3 ];
-
-  doCheck = llvmPackages.stdenv.system == "x86_64-linux" && checks;
+  doCheck = checks && lib.elem llvmPackages.stdenv.system [
+    "x86_64-linux"
+    "x86_64-darwin"
+    "aarch64-darwin"
+  ];
 
+  # In check phase we preserve BUILD directory as
+  # we need to return to it before install phase
   checkPhase = ''
-    runHook preCheck
-    ( cd ../resources/testproject; ../../build/c3c build --trust=full )
-    ( cd ../test; ../build/c3c compile-run -O1 src/test_suite_runner.c3 -- ../build/c3c test_suite )
-    runHook postCheck
-  '';
+    local BUILD_DIR=$(pwd)
+    
+    cd ../resources/testproject
+    ../../build/c3c build --trust=full
 
+    cd ../../test
+    ../build/c3c compile-run -O1 src/test_suite_runner.c3 -- ../build/c3c ./test_suite
+    
+    cd $BUILD_DIR
+  '';
 
   meta = with lib; {
     description = "Compiler for the C3 language";