nix: use /var/tmp/ during builds

modules/base/nix.nix

@@ -50,6 +50,19 @@
     frequency = config.nix.gc.dates;
   };
 
+  # Nix uses /tmp/ (tmpfs) during builds by default. This may cause 'No space
+  # left on device' errors with limited system memory or during big builds. Set
+  # the Nix daemon to use /var/tmp/ instead. Note that /var/tmp/ should ideally
+  # be on the same filesystem as /nix/store/ for faster copying of files.
+  # https://github.com/NixOS/nixpkgs/issues/54707
+  #
+  # NOTE: This does not change the directory for builds during `nixos-rebuild`.
+  # See overlays/nixos-rebuild.nix for workaround.
+  # https://github.com/NixOS/nixpkgs/issues/293114
+  systemd.services.nix-daemon = {
+    environment.TMPDIR = "/var/tmp";
+  };
+
   # Run unpatched dynamic binaries on NixOS.
   # https://github.com/Mic92/nix-ld
   programs.nix-ld.enable = true;

overlays/default.nix

@@ -1,5 +1,6 @@
 {...}: {
   imports = [
     ./neovim.nix
+    ./nixos-rebuild.nix
   ];
 }

overlays/nixos-rebuild.nix

@@ -0,0 +1,25 @@
+{...}: {
+  # The Nix daemon's temporary build directory is changed from /tmp/ to
+  # /var/tmp in modules/base/nix.nix, but it is only respected by `nix build`,
+  # not `nixos-rebuild`.
+  # This overlay wraps `nixos-rebuild` to explicitly set TMPDIR=/var/tmp.
+  # https://github.com/NixOS/nixpkgs/issues/293114
+  nixpkgs.overlays = [
+    (final: prev: {
+      # `overrideAttrs`, instead of simply overriding the `nixos-rebuild`
+      # package, to ensure `nixos-rebuild.override`, which is used in NixOS,
+      # works and is overridden.
+      # https://wiki.nixos.org/wiki/Nix_Cookbook#Wrapping_packages
+      # TODO: There must be a better way to do this?
+      nixos-rebuild = prev.nixos-rebuild.overrideAttrs (oldAttrs: {
+        nativeBuildInputs = oldAttrs.nativeBuildInputs ++ [prev.makeWrapper];
+        postInstall =
+          oldAttrs.postInstall
+          + ''
+            wrapProgram $out/bin/nixos-rebuild \
+              --set TMPDIR /var/tmp
+          '';
+      });
+    })
+  ];
+}