nix: don't drag rust toolchain into system closures

PLAN.md

@@ -955,11 +955,9 @@ No `tokio`, no `zbus`, no `dbus-*`.
 
 `flake.nix` ships:
 
-- A **devshell** with rust toolchain (via `rust-overlay` for pinned
-  channel; default to a stable release pinned in
-  `rust-toolchain.toml`), `pkg-config`, `pipewire`'s dev outputs,
-  `clang` (for bindgen if invoked by deps), `socat` (handy for poking
-  the IPC), `jq`.
+- A **devshell** with the Rust toolchain from nixpkgs, `pkg-config`,
+  `pipewire`'s dev outputs, `clang` (for bindgen if invoked by deps),
+  `socat` (handy for poking the IPC), `jq`.
 - A **package** output (`packages.<system>.default`) that builds the
   daemon + CLI with `rustPlatform.buildRustPackage`. v0 uses
   `cargoLock.lockFile`. Crane can come later if incremental builds in

flake.lock

@@ -18,28 +18,7 @@
     },
     "root": {
       "inputs": {
-        "nixpkgs": "nixpkgs",
-        "rust-overlay": "rust-overlay"
-      }
-    },
-    "rust-overlay": {
-      "inputs": {
-        "nixpkgs": [
-          "nixpkgs"
-        ]
-      },
-      "locked": {
-        "lastModified": 1779074409,
-        "narHash": "sha256-6aXy8Ga41iLVM8ibddFU1O5+wYWcBGNEfZzZuL91eIc=",
-        "owner": "oxalica",
-        "repo": "rust-overlay",
-        "rev": "2a77b5b1dc952f214e8102acdef1622b68515560",
-        "type": "github"
-      },
-      "original": {
-        "owner": "oxalica",
-        "repo": "rust-overlay",
-        "type": "github"
+        "nixpkgs": "nixpkgs"
       }
     }
   },

flake.nix

@@ -3,17 +3,12 @@
 
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
-    rust-overlay = {
-      url = "github:oxalica/rust-overlay";
-      inputs.nixpkgs.follows = "nixpkgs";
-    };
   };
 
   outputs =
     {
       self,
       nixpkgs,
-      rust-overlay,
     }:
     let
       systems = [
@@ -26,7 +21,6 @@
         system:
         import nixpkgs {
           inherit system;
-          overlays = [ rust-overlay.overlays.default ];
         };
 
       perSystem =
@@ -34,13 +28,6 @@
         let
           pkgs = pkgsFor system;
 
-          rustToolchain = pkgs.rust-bin.fromRustupToolchainFile ./rust-toolchain.toml;
-
-          rustPlatform = pkgs.makeRustPlatform {
-            cargo = rustToolchain;
-            rustc = rustToolchain;
-          };
-
           # Native libs the audio crates link against.
           nativeAudioBuildInputs = with pkgs; [
             pipewire
@@ -64,8 +51,11 @@
               name = "headroom-dev";
 
               nativeBuildInputs = nativeBuildTools ++ [
-                rustToolchain
+                pkgs.cargo
+                pkgs.clippy
                 pkgs.rust-analyzer
+                pkgs.rustc
+                pkgs.rustfmt
               ];
 
               buildInputs =
@@ -89,7 +79,7 @@
           );
 
           # `nix build` — the final packaged daemon + CLI.
-          headroom = rustPlatform.buildRustPackage (
+          headroom = pkgs.rustPlatform.buildRustPackage (
             {
               pname = "headroom";
               # Pull from the workspace Cargo.toml — the per-crate