lib: add list of supported systems (#140428)

Adds the first 3 tiers of RFC0046 that are being used in flake.nix.

flake.nix

@@ -11,15 +11,7 @@
 
       lib = import ./lib;
 
-      systems = [
-        "x86_64-linux"
-        "i686-linux"
-        "x86_64-darwin"
-        "aarch64-linux"
-        "armv6l-linux"
-        "armv7l-linux"
-        "aarch64-darwin"
-      ];
+      systems = lib.systems.supported.hydra;
 
       forAllSystems = f: lib.genAttrs systems (system: f system);
 

lib/systems/default.nix

@@ -8,6 +8,7 @@ rec {
   platforms = import ./platforms.nix { inherit lib; };
   examples = import ./examples.nix { inherit lib; };
   architectures = import ./architectures.nix { inherit lib; };
+  supported = import ./supported.nix { inherit lib; };
 
   # Elaborate a `localSystem` or `crossSystem` so that it contains everything
   # necessary.

lib/systems/supported.nix

@@ -0,0 +1,24 @@
+# Supported systems according to RFC0046's definition.
+#
+# https://github.com/NixOS/rfcs/blob/master/rfcs/0046-platform-support-tiers.md
+{ lib }:
+rec {
+  # List of systems that are built by Hydra.
+  hydra = tier1 ++ tier2 ++ tier3;
+
+  tier1 = [
+    "x86_64-linux"
+  ];
+
+  tier2 = [
+    "aarch64-linux"
+    "x86_64-darwin"
+  ];
+
+  tier3 = [
+    "armv6l-linux"
+    "armv7l-linux"
+    "i686-linux"
+    "mipsel-linux"
+  ];
+}