make an overlay and call it from everywhere

flake.nix

@@ -5,9 +5,30 @@
     nixpkgs.url = "github:NixOS/nixpkgs/release-21.05";
     nixos-hardware.url = "github:NixOS/nixos-hardware";
     home-manager.url = "github:nix-community/home-manager/release-21.05";
+
+    tmux = {
+      url = "github:tmux/tmux";
+      flake = false;
+    };
   };
 
   outputs = inputs: {
+    overlay = final: prev: {
+      tmux = prev.tmux.overrideAttrs (attrs:
+        attrs // {
+          src = inputs.tmux;
+
+          # macOS does some weird stuff with locales and character widths.
+          # Practically, that means that without extra support tmux will
+          # behave weirdly around multi-byte characters like emoji. Enabling
+          # utf8proc support should backfill the right tables so that tmux
+          # can get the correct character widths.
+          buildInputs = attrs.buildInputs ++ [ prev.utf8proc ];
+          configureFlags = attrs.configureFlags ++ [ "--enable-utf8proc" ];
+        }
+      );
+    };
+
     nixosConfigurations.torch = inputs.nixpkgs.lib.nixosSystem {
       system = "x86_64-linux";
       modules = [

machines/torch/default.nix

@@ -11,11 +11,12 @@ inputs:
     "${inputs.nixos-hardware}/apple/macbook-pro/12-1"
 
     # system setup
+    (import ../../nixos/nix inputs)
+
     ../../nixos/bluetooth
     ../../nixos/brian
     ../../nixos/gnome-keyring
     ../../nixos/networkmanager
-    ../../nixos/nix
     ../../nixos/upower
     ../../nixos/xmonad
   ];

machines/vbox-dev/default.nix

@@ -9,8 +9,9 @@ inputs:
     [ # Include the results of the hardware scan.
       ./hardware-configuration.nix
 
+      (import ../../nixos/nix inputs)
+
       ../../nixos/brian
-      ../../nixos/nix
       ../../nixos/xmonad
       ../../nixos/xserver
     ];

nixos/nix/default.nix

@@ -1,3 +1,4 @@
+inputs:
 { pkgs, ... }: {
   nix = {
     # TODO: might have to disable this to get home-manager working
@@ -7,5 +8,8 @@
     '';
   };
 
-  nixpkgs.config.allowUnfree = true;
+  nixpkgs = {
+    config.allowUnfree = true;
+    overlays = [ inputs.self.overlay ];
+  };
 }