build(nix): enable rustfmt check+formatter

with this, the gh actions ci will fail if submitted code does not adhere
to rustfmt rules.

additionally, running `nix fmt` will also format rust code.

because we want rules only supported by nightly rustfmt, and nixpkgs
doesn't have a nightly toolchain, we need to pull in another flake
input: fenix.

flake.lock

@@ -1,5 +1,26 @@
 {
   "nodes": {
+    "fenix": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ],
+        "rust-analyzer-src": "rust-analyzer-src"
+      },
+      "locked": {
+        "lastModified": 1716791293,
+        "narHash": "sha256-zxFADJsXhODHH9rrF7gXjN+BYVkMoNY2NpLyjGQDOJA=",
+        "owner": "nix-community",
+        "repo": "fenix",
+        "rev": "9af557bccdfa8fb6a425661c33dbae46afef0afa",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-community",
+        "repo": "fenix",
+        "type": "github"
+      }
+    },
     "flake-compat": {
       "flake": false,
       "locked": {
@@ -64,12 +85,30 @@
     },
     "root": {
       "inputs": {
+        "fenix": "fenix",
         "flake-compat": "flake-compat",
         "flake-parts": "flake-parts",
         "nixpkgs": "nixpkgs",
         "treefmt-nix": "treefmt-nix"
       }
     },
+    "rust-analyzer-src": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1716714715,
+        "narHash": "sha256-xJ5vzVr5Et6wxHRQLOYicsa0gN2bwm4mj6AFHc8ugJc=",
+        "owner": "rust-lang",
+        "repo": "rust-analyzer",
+        "rev": "71a816a90facb6546a0a06010da17598e11812f7",
+        "type": "github"
+      },
+      "original": {
+        "owner": "rust-lang",
+        "ref": "nightly",
+        "repo": "rust-analyzer",
+        "type": "github"
+      }
+    },
     "treefmt-nix": {
       "inputs": {
         "nixpkgs": [

flake.nix

@@ -11,6 +11,11 @@
       inputs.nixpkgs.follows = "nixpkgs";
     };
 
+    fenix = {
+      url = "github:nix-community/fenix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+
     flake-compat = {
       url = "github:edolstra/flake-compat";
       flake = false;
@@ -23,6 +28,7 @@
       nixpkgs,
       flake-parts,
       treefmt-nix,
+      fenix,
       flake-compat,
       ...
     }:
@@ -41,8 +47,12 @@
           self',
           config,
           pkgs,
+          system,
           ...
         }:
+        let
+          rustfmtToml = builtins.fromTOML (builtins.readFile ./rustfmt.toml);
+        in
         {
           packages.default = config.packages.bcachefs-tools;
           packages.bcachefs-tools = pkgs.callPackage ./build.nix { };
@@ -74,6 +84,9 @@
 
             programs = {
               nixfmt-rfc-style.enable = true;
+              rustfmt.edition = rustfmtToml.edition;
+              rustfmt.enable = true;
+              rustfmt.package = fenix.packages.${system}.default.rustfmt;
             };
           };
         };