WIP feat: add supported systems filter

lib/buildTree.nix

@@ -1,5 +1,72 @@
 { path
 , args
+, supportedSystems ? [
+    "aarch64-darwin"
+    "aarch64-genode"
+    "aarch64-linux"
+    "aarch64-netbsd"
+    "aarch64-none"
+    "aarch64_be-none"
+    "arm-none"
+    "armv5tel-linux"
+    "armv6l-linux"
+    "armv6l-netbsd"
+    "armv6l-none"
+    "armv7a-darwin"
+    "armv7a-linux"
+    "armv7a-netbsd"
+    "armv7l-linux"
+    "armv7l-netbsd"
+    "avr-none"
+    "i686-cygwin"
+    "i686-darwin"
+    "i686-freebsd"
+    "i686-genode"
+    "i686-linux"
+    "i686-netbsd"
+    "i686-none"
+    "i686-openbsd"
+    "i686-windows"
+    "js-ghcjs"
+    "m68k-linux"
+    "m68k-netbsd"
+    "m68k-none"
+    "mips64el-linux"
+    "mipsel-linux"
+    "mipsel-netbsd"
+    "mmix-mmixware"
+    "msp430-none"
+    "or1k-none"
+    "powerpc-netbsd"
+    "powerpc-none"
+    "powerpc64-linux"
+    "powerpc64le-linux"
+    "powerpcle-none"
+    "riscv32-linux"
+    "riscv32-netbsd"
+    "riscv32-none"
+    "riscv64-linux"
+    "riscv64-netbsd"
+    "riscv64-none"
+    "s390-linux"
+    "s390-none"
+    "s390x-linux"
+    "s390x-none"
+    "vc4-none"
+    "wasm32-wasi"
+    "wasm64-wasi"
+    "x86_64-cygwin"
+    "x86_64-darwin"
+    "x86_64-freebsd"
+    "x86_64-genode"
+    "x86_64-linux"
+    "x86_64-netbsd"
+    "x86_64-none"
+    "x86_64-openbsd"
+    "x86_64-redox"
+    "x86_64-solaris"
+    "x86_64-windows"
+  ]
 }:
 let
   # copied from <nixpkgs/lib>
@@ -96,7 +163,16 @@ let
   callBuildWith = args: path:
     import "${toString path}/BUILD.nix" args;
 
-  root = { self = readBuildTree path; } // args;
+  hasSupport = kernel: builtins.foldl' (x: y: x || y) false (map (i: builtins.match (".+-" + kernel) i != null) supportedSystems);
+  hasLinuxSupport = hasSupport "linux";
+  hasDarwinSupport = hasSupport "darwin";
+
+  root = {
+    self = readBuildTree path // { inherit hasLinuxSupport hasDarwinSupport; };
+    inherit supportedSystems;
+  } // args;
+
+  filterPackages = import ./filterPackages.nix { inherit supportedSystems; };
 
   callBuild = callBuildWith root;
 
@@ -113,7 +189,7 @@ let
       ;
       subDirs = lib.filterAttrs dirFilter dir;
 
-      buildAttrs = if dir ? "BUILD.nix" then callBuild path else { };
+      buildAttrs = if dir ? "BUILD.nix" then filterPackages supportedSystems (callBuild path) else { };
 
       op = name: _: readBuildTree "${toString path}/${toString name}";
 

lib/filterPackages.nix

@@ -0,0 +1,34 @@
+{ supportedSystems }:
+system: packages:
+let
+  # Adopted from nixpkgs.lib
+  inherit (builtins) listToAttrs concatMap attrNames filter;
+  nameValuePair = name: value: { inherit name value; };
+  intersectLists = e: filter (x: builtins.elem x e);
+  filterAttrs = pred: set:
+    listToAttrs (
+      concatMap
+        (name:
+          let v = set.${name}; in
+          if pred name v then [ (nameValuePair name v) ] else [ ]
+        )
+        (attrNames set)
+    );
+
+  # Everything that nix flake check requires for the packages output
+  sieve = n: v:
+    with v;
+    let
+      inherit (builtins) isAttrs;
+      isDerivation = x: isAttrs x && x ? type && x.type == "derivation";
+      isBroken = meta.broken or false;
+      platforms = meta.platforms or supportedSystems;
+      badPlatforms = meta.badPlatforms or [ ];
+    in
+    # check for isDerivation, so this is independently useful of
+      # flattenTree, which also does filter on derivations
+    (isDerivation v && !isBroken && (builtins.length (intersectLists supportedSystems platforms) > 0) &&
+    !(builtins.elem system badPlatforms)) || (!isDerivation v)
+  ;
+in
+filterAttrs sieve packages