implement flattenTreeSystem

default.nix

@@ -107,6 +107,11 @@ let
   #   }
   flattenTree = tree: import ./flattenTree.nix tree;
 
+  # It turns out that we also need to filter by the supported system. This is
+  # the same thing as able, but that also looks at meta.hydraPlatforms or
+  # meta.platforms if the former is undefined.
+  flattenTreeSystem = system: tree: import ./flattenTreeSystem.nix system tree;
+
   # Returns the structure used by `nix app`
   mkApp =
     { drv
@@ -128,6 +133,7 @@ let
       eachDefaultSystem
       eachSystem
       flattenTree
+      flattenTreeSystem
       mkApp
       simpleFlake
       ;

flattenTreeSystem.nix

@@ -0,0 +1,40 @@
+system: tree:
+let
+  op = sum: path: val:
+    let
+      pathStr = builtins.concatStringsSep "/" path;
+    in
+    # Ignore values that are not attrsets
+    if (builtins.typeOf val) != "set" then
+      sum
+    # For derivations
+    else if val ? type && val.type == "derivation" then
+      let
+        meta = val.meta or { };
+        platforms = meta.hydraPlatforms or meta.platforms or [ ];
+      in
+      # Only include those that target this system
+      if builtins.any (p: p == system) platforms then
+        (sum // {
+          # We used to use the derivation outPath as the key, but that crashes Nix
+          # so fallback on constructing a static key
+          "${pathStr}" = val;
+        })
+      else
+        sum
+    # Recurse into attrsets who have that key
+    else if (val.recurseForDerivations or false) == true then
+      recurse sum path val
+    # Ignore that value as well
+    else
+      sum
+  ;
+
+  recurse = sum: path: val:
+    builtins.foldl'
+      (sum: key: op sum (path ++ [ key ]) val.${key})
+      sum
+      (builtins.attrNames val)
+  ;
+in
+recurse { } [ ] tree

simpleFlake.nix

@@ -55,7 +55,7 @@ let
       legacyPackages = packages;
 
       # Flake expects a flat attrset containing only derivations as values
-      packages = lib.flattenTree packages;
+      packages = lib.flattenTreeSystem system packages;
     }
     //
     (