feat: add rosenpass MSRV check

This check requires a specific toolchain version, and to get that, we
introduce oxalica's rust-overlay.

Signed-off-by: wucke13 <wucke13+github@gmail.com>

flake.lock

@@ -59,9 +59,30 @@
         "flake-utils": "flake-utils",
         "nix-vm-test": "nix-vm-test",
         "nixpkgs": "nixpkgs",
+        "rust-overlay": "rust-overlay",
         "treefmt-nix": "treefmt-nix"
       }
     },
+    "rust-overlay": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1744513456,
+        "narHash": "sha256-NLVluTmK8d01Iz+WyarQhwFcXpHEwU7m5hH3YQQFJS0=",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "rev": "730fd8e82799219754418483fabe1844262fd1e2",
+        "type": "github"
+      },
+      "original": {
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "type": "github"
+      }
+    },
     "systems": {
       "locked": {
         "lastModified": 1681028828,

flake.nix

@@ -7,6 +7,10 @@
     nix-vm-test.inputs.nixpkgs.follows = "nixpkgs";
     nix-vm-test.inputs.flake-utils.follows = "flake-utils";
 
+    # for rust nightly with llvm-tools-preview
+    rust-overlay.url = "github:oxalica/rust-overlay";
+    rust-overlay.inputs.nixpkgs.follows = "nixpkgs";
+
     treefmt-nix.url = "github:numtide/treefmt-nix";
     treefmt-nix.inputs.nixpkgs.follows = "nixpkgs";
   };
@@ -17,6 +21,7 @@
       nixpkgs,
       flake-utils,
       nix-vm-test,
+      rust-overlay,
       treefmt-nix,
       ...
     }@inputs:
@@ -91,10 +96,14 @@
             pkgs = import nixpkgs {
               inherit system;
 
-              # apply our own overlay, overriding/inserting our packages as defined in ./pkgs
               overlays = [
+                # apply our own overlay, overriding/inserting our packages as defined in ./pkgs
                 self.overlays.default
+
                 nix-vm-test.overlays.default
+
+                # apply rust-overlay to get specific versions of the rust toolchain for a MSRV check
+                (import rust-overlay)
               ];
             };
 
@@ -168,6 +177,17 @@
                 systemd-rosenpass = pkgs.testers.runNixOSTest ./tests/systemd/rosenpass.nix;
                 systemd-rp = pkgs.testers.runNixOSTest ./tests/systemd/rp.nix;
                 formatting = treefmtEval.config.build.check self;
+                rosenpass-msrv-check =
+                  let
+                    rosenpassCargoToml = pkgs.lib.trivial.importTOML ./rosenpass/Cargo.toml;
+
+                    rustToolchain = pkgs.rust-bin.stable.${rosenpassCargoToml.package.rust-version}.default;
+                    rustPlatform = pkgs.makeRustPlatform {
+                      cargo = rustToolchain;
+                      rustc = rustToolchain;
+                    };
+                  in
+                  pkgs.rosenpass.override { inherit rustPlatform; };
               }
               // pkgs.lib.optionalAttrs (system == "x86_64-linux") (
                 import ./tests/legacy-distro-packaging.nix {