deprecate mkApp in favor of direct definitions

Now that nixpkgs has its own idea of `passthru.exePath` under the
name `meta.mainProgram`, deprecate our solution in favor of the new
standard.

This subsequently removes the value of which `mkApp` brings,
and therefore a full deprecation for its removal has begun.

README.md

@@ -105,7 +105,10 @@ eachSystem allSystems (system: { hello = 42; })
           default = hello;
         };
         apps = rec {
-          hello = flake-utils.lib.mkApp { drv = self.packages.${system}.hello; };
+          hello = {
+            type = "app";
+            program = "${nixpkgs.lib.getExe self.packages.${system}.hello}";
+          };
           default = hello;
         };
       }
@@ -121,6 +124,12 @@ manageable parts.
 
 ### `mkApp { drv, name ? drv.pname or drv.name, exePath ? drv.passthru.exePath or "/bin/${name}"`
 
+> **DEPRECATED**
+>
+> `mkApp` has been deprecated in favor of direct definitions.
+>
+> Example: `{ type = "app"; program = "${nixpkgs.lib.getExe drv}"; }`
+
 A small utility that builds the structure expected by the special `apps` and `defaultApp` prefixes.
 
 

examples/each-system/flake.nix

@@ -12,7 +12,10 @@
           default = hello;
         };
         apps = rec {
-          hello = flake-utils.lib.mkApp { drv = self.packages.${system}.hello; };
+          hello = {
+            type = "app";
+            program = "${nixpkgs.lib.getExe self.packages.${system}.hello}";
+          };
           default = hello;
         };
       }

lib.nix

@@ -202,11 +202,19 @@ let
     recursiveUpdate output (import subflake inputs)) { };
 
   # Returns the structure used by `nix app`
-  mkApp =
+  mkApp = let
+    # Pulled from nixpkgs.lib
+    warn =
+      if builtins.elem (builtins.getEnv "NIX_ABORT_ON_WARN") ["1" "true" "yes"]
+      then msg: builtins.trace "warning: ${msg}" (abort "NIX_ABORT_ON_WARN=true; warnings are treated as unrecoverable errors.")
+      else msg: builtins.trace "warning: ${msg}";
+  in
     { drv
     , name ? drv.pname or drv.name
     , exePath ? drv.passthru.exePath or "/bin/${name}"
     }:
+    warn
+      "`mkApp` has been deprecated in favor of direct definitions. Replace definitions of `mkApp { drv = ...; }` with `{ type = \"app\"; program = \"\${nixpkgs.lib.getExe drv}\"; }` to remove this warning. Use of `passthru.exePath` inside derivations should be replaced with nixpkgs' `meta.mainProgram` instead."
     {
       type = "app";
       program = "${drv}${exePath}";