Migrate from flake-utils to flake-parts

- Replace flake-utils with flake-parts and nix-systems/default
- Add nixfmt-rfc-style formatter
- Update README.md Nix section to document packages.default

Author: renatoGarcia

README.md

@@ -167,7 +167,12 @@ Icecream-cpp can be included as a flakes input:
 inputs.icecream-cpp.url = "github:renatoGarcia/icecream-cpp";
 ```
 
-The flake defines an overlay for use when importing `nixpkgs`:
+The package can be referenced directly:
+```Nix
+inputs.icecream-cpp.packages.${system}.default
+```
+
+Alternatively, the flake provides an overlay for use when importing `nixpkgs`:
 ```Nix
 import nixpkgs {
   system = "x86_64-linux";

ci_images/flake.nix

@@ -7,68 +7,100 @@
     nixpkgs_23_11.url = "nixpkgs/nixos-23.11";
   };
 
-  outputs = { self, nixpkgs_24_11, nixpkgs_24_05, nixpkgs_23_11 }:
-  let
-    lib = nixpkgs_24_11.lib;
+  outputs =
+    {
+      self,
+      nixpkgs_24_11,
+      nixpkgs_24_05,
+      nixpkgs_23_11,
+    }:
+    let
+      lib = nixpkgs_24_11.lib;
 
-    ci-docker = { pkgs, stdenv, tagname }:
-      pkgs.dockerTools.buildLayeredImage {
-        name = "renatogarcia/icecream-ci";
-        tag = tagname;
+      ci-docker =
+        {
+          pkgs,
+          stdenv,
+          tagname,
+        }:
+        pkgs.dockerTools.buildLayeredImage {
+          name = "renatogarcia/icecream-ci";
+          tag = tagname;
 
-        contents = with pkgs; [
-          busybox
-          cmake
-          conan
-          gnumake
-          stdenv.cc
-        ];
+          contents = with pkgs; [
+            busybox
+            cmake
+            conan
+            gnumake
+            stdenv.cc
+          ];
 
-        # Required by clang using mktemp and failing when /tmp doesn't exist.
-        extraCommands = ''
+          # Required by clang using mktemp and failing when /tmp doesn't exist.
+          extraCommands = ''
             mkdir -p tmp
-        '';
+          '';
 
-        config = {
-          WorkingDir = "/home";
-          Env =
-             # Required so that the linker can find the libc++ when compiling with clang
-            lib.lists.optionals (stdenv.cc.libcxx != null) [
-              "NIX_CC_WRAPPER_TARGET_HOST_x86_64_unknown_linux_gnu=1"
-              (
-                let
-                  # clang8 depends on libc++abi besides libc++
-                  findLibcxxabi = depsList: lib.lists.findFirst (drv: drv.pname == "libcxxabi") null depsList;
-                  libcxxabi = findLibcxxabi (lib.lists.remove null stdenv.cc.libcxx.buildInputs);
-                in
-                  if libcxxabi != null
-                  then "NIX_LDFLAGS=-L${stdenv.cc.libcxx}/lib/ -L${libcxxabi.out}/lib"
-                  else "NIX_LDFLAGS=-L${stdenv.cc.libcxx}/lib/"
-              )
-            ];
+          config = {
+            WorkingDir = "/home";
+            Env =
+              # Required so that the linker can find the libc++ when compiling with clang
+              lib.lists.optionals (stdenv.cc.libcxx != null) [
+                "NIX_CC_WRAPPER_TARGET_HOST_x86_64_unknown_linux_gnu=1"
+                (
+                  let
+                    # clang8 depends on libc++abi besides libc++
+                    findLibcxxabi = depsList: lib.lists.findFirst (drv: drv.pname == "libcxxabi") null depsList;
+                    libcxxabi = findLibcxxabi (lib.lists.remove null stdenv.cc.libcxx.buildInputs);
+                  in
+                  if libcxxabi != null then
+                    "NIX_LDFLAGS=-L${stdenv.cc.libcxx}/lib/ -L${libcxxabi.out}/lib"
+                  else
+                    "NIX_LDFLAGS=-L${stdenv.cc.libcxx}/lib/"
+                )
+              ];
+          };
         };
-      };
 
-    pkgs_24_11 = import nixpkgs_24_11 {
-      system = "x86_64-linux";
-    };
+      pkgs_24_11 = import nixpkgs_24_11 {
+        system = "x86_64-linux";
+      };
 
-    pkgs_24_05 = import nixpkgs_24_05 {
-      system = "x86_64-linux";
-    };
+      pkgs_24_05 = import nixpkgs_24_05 {
+        system = "x86_64-linux";
+      };
 
-    pkgs_23_11 = import nixpkgs_23_11 {
-      system = "x86_64-linux";
-    };
+      pkgs_23_11 = import nixpkgs_23_11 {
+        system = "x86_64-linux";
+      };
 
-  in
-  {
-    packages.x86_64-linux = {
-      ci-gcc6 = ci-docker { pkgs = pkgs_24_11; stdenv = pkgs_24_05.gcc6Stdenv; tagname = "gcc6-3"; };
-      ci-gcc9 = ci-docker { pkgs = pkgs_24_11; stdenv = pkgs_24_11.gcc9Stdenv; tagname = "gcc9-3"; };
-      ci-gcc14 = ci-docker { pkgs = pkgs_24_11; stdenv = pkgs_24_11.gcc14Stdenv; tagname = "gcc14-3"; };
-      ci-clang8 = ci-docker { pkgs = pkgs_24_11; stdenv = pkgs_23_11.llvmPackages_8.libcxxStdenv; tagname = "clang8-3"; };
-      ci-clang19 = ci-docker { pkgs = pkgs_24_11; stdenv = pkgs_24_11.llvmPackages_19.libcxxStdenv; tagname = "clang19-3"; };
+    in
+    {
+      packages.x86_64-linux = {
+        ci-gcc6 = ci-docker {
+          pkgs = pkgs_24_11;
+          stdenv = pkgs_24_05.gcc6Stdenv;
+          tagname = "gcc6-3";
+        };
+        ci-gcc9 = ci-docker {
+          pkgs = pkgs_24_11;
+          stdenv = pkgs_24_11.gcc9Stdenv;
+          tagname = "gcc9-3";
+        };
+        ci-gcc14 = ci-docker {
+          pkgs = pkgs_24_11;
+          stdenv = pkgs_24_11.gcc14Stdenv;
+          tagname = "gcc14-3";
+        };
+        ci-clang8 = ci-docker {
+          pkgs = pkgs_24_11;
+          stdenv = pkgs_23_11.llvmPackages_8.libcxxStdenv;
+          tagname = "clang8-3";
+        };
+        ci-clang19 = ci-docker {
+          pkgs = pkgs_24_11;
+          stdenv = pkgs_24_11.llvmPackages_19.libcxxStdenv;
+          tagname = "clang19-3";
+        };
+      };
     };
-  };
 }

example_project/flake.nix

@@ -10,9 +10,11 @@
     };
   };
 
-  outputs = inputs@{ self, nixpkgs, ... }:
+  outputs =
+    inputs@{ self, nixpkgs, ... }:
     let
-      application = { pkgs }:
+      application =
+        { pkgs }:
         pkgs.stdenv.mkDerivation {
           pname = "icecream-example";
           version = "1.0";
@@ -27,14 +29,17 @@
           ];
         };
 
-      pkgs = (import nixpkgs {
-        system = "x86_64-linux";
-        overlays = [
-          inputs.icecream-cpp.overlays.default
-        ];
-      });
+      pkgs = (
+        import nixpkgs {
+          system = "x86_64-linux";
+          overlays = [
+            inputs.icecream-cpp.overlays.default
+          ];
+        }
+      );
 
-    in {
+    in
+    {
       packages.x86_64-linux = {
         default = application { inherit pkgs; };
       };