Allow devenv up to work in downstream flakes

README.md

@@ -96,6 +96,10 @@ provided by this flake's outputs:
       devShells = element-nix-flakes.outputs.composeShell [
         "synapse"
       ];
+      # Use the `setupPackages` function to configure `devenv up`
+      # to use the `services` and `processes` defined in the
+      # "synapse" project module.
+      packages = element-nix-flakes.outputs.setupPackages "synapse";
     };
 }
 ```
@@ -104,6 +108,12 @@ Note that `composeShell` takes a list as an argument. You can provide multiple
 project names in this list, and `composeShell` will build a development shell
 dependencies from all projects combined together.
 
+`setupPackages` is another function provided by the `nix-flakes` flake, which
+must be called (and its output set to `packages`) in order to utilise the
+`devenv up` functionality. This will start any `processes` and `services`
+defined in the project module - for instance, starting up the project and
+any database/redis/language servers etc needed for development.
+
 ## Development
 
 ### Adding a new project

flake.nix

@@ -8,7 +8,7 @@
     rust-overlay.url = "github:oxalica/rust-overlay";
   };
 
-  outputs = inputs@{ devenv, nixpkgs, rust-overlay, systems, ... }:
+  outputs = inputs@{ self, devenv, nixpkgs, rust-overlay, systems, ... }:
     let
       # The local directory where project-specific flakes are stored.
       projectFlakesDirectory = ./project-flakes;
@@ -98,5 +98,17 @@
           };
         }
       );
+      # Define a function `setupPackages` that downstream flakes can use to correctly
+      # configure the `devenv up` functionality.
+      #
+      # This function creates an entry intended to be inserted into a downstream flake's
+      # `packages` flake output.
+      #
+      # See the README for how to use this in a downstream flake.
+      setupPackages = projectName: forEachSystem (system:
+        {
+          devenv-up = self.devShells.${system}.${projectName}.config.procfileScript;
+        }
+      );
     };
 }