added deployment module option for nix copy --substitute-on-destination

README.md

@@ -163,8 +163,10 @@ The default is an empty set, meaning that the nix configuration is inherited fro
 
 `buildOnly` makes morph skip the "push" and "switch" steps for the given host, even if "morph deploy" or "morph push" is executed. (default: false)
 
+`substituteOnDestination` Sets the `--substitute-on-destination` flag on nix copy, allowing for the deployment target to use substitutes. See `nix copy --help`. (default: false)
 
-Example usage of `nixConfig` and `deployment.buildOnly`:
+
+Example usage of `nixConfig` and deployment module options:
 ```
 network = {
     nixConfig = {
@@ -176,6 +178,9 @@ machine1 = { ... }: {
     deployment.buildOnly = true;
 };
 
+machine2 = { ... }: {
+    deployment.substituteOnDestination = true;
+};
 ```
 
 

data/eval-machines.nix

@@ -72,7 +72,7 @@ rec {
 
     machines =
       flip mapAttrs nodes (n: v': let v = scrubOptionValue v'; in
-        { inherit (v.config.deployment) targetHost secrets healthChecks buildOnly;
+        { inherit (v.config.deployment) targetHost secrets healthChecks buildOnly substituteOnDestination;
           name = n;
           nixosRelease = v.config.system.nixos.release or (removeSuffix v.config.system.nixos.version.suffix v.config.system.nixos.version);
           nixConfig = mapAttrs

data/options.nix

@@ -172,6 +172,16 @@ in
       '';
     };
 
+    substituteOnDestination = mkOption {
+      type = bool;
+      default = false;
+      description = ''
+        Sets the `--substitute-on-destination` flag on nix copy,
+        allowing for the deployment target to use substitutes.
+        See `nix copy --help`.
+      '';
+    };
+
     secrets = mkOption {
       default = {};
       example = {

nix/nix.go

@@ -24,6 +24,7 @@ type Host struct {
 	TargetHost   string
 	Secrets      map[string]secrets.Secret
 	BuildOnly    bool
+	SubstituteOnDestination bool
 	NixConfig    map[string]string
 }
 
@@ -259,6 +260,9 @@ func Push(ctx *ssh.SSHContext, host Host, paths ...string) (err error) {
 			"--to", "ssh://" + userArg + host.TargetHost + keyArg,
 		}
 		args = append(args, options...)
+		if host.SubstituteOnDestination {
+			args = append(args, "--substitute-on-destination")
+		}
 
 		cmd := exec.Command(
 			"nix", args...,