Rebuild: Separate evaluation

flake.nix

@@ -153,6 +153,7 @@
                       runtimeInputs = with pkgs; [
                         nix-output-monitor
                         nixos-rebuild
+                        jq
                       ];
                       text = builtins.readFile ./os/rebuild.sh;
                     }

os/rebuild.sh

@@ -24,12 +24,15 @@ then
   exit 2
 fi
 
-info "Building"
-tmpdir="$(mktemp -d)"
+info "Evaluating"
+# Evaluating can take a lot of memory, and Nix doesn't free it until the program ends,
+# which can be limiting on memory-constrained devices. Hence the build step is separate.
+# nix eval doesn't use the eval cache, so we do a nix build --dry-run
 # sudo so the eval cache is shared with nixos-rebuild
-sudo nom build "$self#nixosConfigurations.$HOSTNAME.config.system.build.toplevel" -o "$tmpdir/toplevel" "$@"
-toplevel="$(readlink -f "$tmpdir/toplevel")"
-rm -rf "$tmpdir"
+toplevel=$(time sudo nix build "$self#nixosConfigurations.$HOSTNAME.config.system.build.toplevel" --dry-run --json | jq '.[0].outputs.out' -r)
+
+info "Building"
+sudo nom build "$toplevel" --no-link "$@"
 
 info "Showing diff"
 nvd diff "$(readlink -f /nix/var/nix/profiles/system)" "$toplevel"