feat: add support for custom rust toolchains compatible with rustup

flake.lock

@@ -262,9 +262,30 @@
         "crane": "crane",
         "flake-parts": "flake-parts",
         "nixpkgs": "nixpkgs_2",
+        "rust-overlay": "rust-overlay",
         "treefmt-nix": "treefmt-nix"
       }
     },
+    "rust-overlay": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1780197589,
+        "narHash": "sha256-FVCr2Ij/jKf59a4LW481eeOF6rJRreOBrVgW/aUBTrw=",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "rev": "21632e942d89bf1cce4e5a63d7e58a215a0cbfcc",
+        "type": "github"
+      },
+      "original": {
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "type": "github"
+      }
+    },
     "systems": {
       "locked": {
         "lastModified": 1681028828,

flake.nix

@@ -13,6 +13,11 @@
 
     crane.url = "github:ipetkov/crane";
 
+    rust-overlay = {
+      url = "github:oxalica/rust-overlay";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+
     cadquery = {
       url = "github:vinszent/cq-flake";
       # unfortunately dependencies are out of date so this needs it's own nixpkgs

rust/default.nix

@@ -25,10 +25,17 @@
       }: let
         cfg = config.krantz.rust;
 
-        craneLib = inputs.crane.mkLib pkgs;
         src = cfg.srcFiltered;
         manifest = (lib.importTOML "${src}/Cargo.toml").package;
 
+        craneLib =
+          if builtins.pathExists cfg.srcToolchain
+          then
+            (inputs.crane.mkLib (pkgs.extend inputs.rust-overlay.overlays.default)).overrideToolchain (
+              p: p.rust-bin.fromRustupToolchainFile cfg.srcToolchain
+            )
+          else inputs.crane.mkLib pkgs;
+
         buildInputs = cfg.runtimeDeps;
 
         # build dependencies
@@ -55,6 +62,13 @@
             defaultText = lib.literalExpression "config.krantz.rust.craneLib cfg.src";
             example = lib.literalExpression "./.";
           };
+          srcToolchain = lib.mkOption {
+            description = "A path to the rust-toolchain(.toml) file for the repo. Will use the toolchain from nixpkgs if the file does not exist.";
+            type = lib.types.path;
+            default = "${cfg.src}/rust-toolchain.toml";
+            defaultText = lib.literalExpression "cfg.src + \"/rust-toolchain.toml\"";
+            example = lib.literalExpression "./rust-toolchain";
+          };
 
           runtimeDeps = lib.mkOption {
             description = "Packages needed for the app to run.";