lib.systems: remove supported, replace with flakeExposed

Since the list only gates the platforms the nixpkgs flake exposes
packages to build on, the `hydra` label made little sense. It was also
only used for this purpose, so the `tier*` attributes were largely
unnecessary.

To reflect the intention more accurately, we expose
`lib.systems.flakeExposed` and use it to gate flake.nix's system list.

3 files changed, 39 insertions, 27 deletions

diff --git a/lib/systems/default.nix b/lib/systems/default.nix
index 7ddd5b8a58129..e02890b113861 100644
--- a/lib/systems/default.nix
+++ b/lib/systems/default.nix
@@ -8,7 +8,16 @@ 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; };
+
+  /* List of all Nix system doubles the nixpkgs flake will expose the package set
+     for. All systems listed here must be supported by nixpkgs as `localSystem`.
+
+     **Warning**: This attribute is considered experimental and is subject to change.
+  */
+  flakeExposed = import ./flake-systems.nix { };
+
+  # TODO(@sternenseemann): remove before 21.11
+  supported = throw "2022-05-23: Use lib.systems.flakeExposed instead of lib.systems.supported.hydra, as lib.systems.supported has been removed";
 
   # Elaborate a `localSystem` or `crossSystem` so that it contains everything
   # necessary.
diff --git a/lib/systems/flake-systems.nix b/lib/systems/flake-systems.nix
new file mode 100644
index 0000000000000..74124c32e8369
--- /dev/null
+++ b/lib/systems/flake-systems.nix
@@ -0,0 +1,29 @@
+# See [RFC 46] for mandated platform support and ../../pkgs/stdenv for
+# implemented platform support. This list is mainly descriptive, i.e. all
+# system doubles for platforms where nixpkgs can do native compiliation
+# reasonably well are included.
+#
+# [RFC 46]: https://github.com/NixOS/rfcs/blob/master/rfcs/0046-platform-support-tiers.md
+{ }:
+
+[
+  # Tier 1
+  "x86_64-linux"
+  # Tier 2
+  "aarch64-linux"
+  "x86_64-darwin"
+  # Tier 3
+  "armv6l-linux"
+  "armv7l-linux"
+  "i686-linux"
+  "mipsel-linux"
+
+  # Other platforms with sufficient support in stdenv which is not formally
+  # mandated by their platform tier.
+  "aarch64-darwin"
+  "armv5tel-linux"
+  "powerpc64le-linux"
+  "riscv64-linux"
+
+  # "x86_64-freebsd" is excluded because it is mostly broken
+]
diff --git a/lib/systems/supported.nix b/lib/systems/supported.nix
deleted file mode 100644
index a1c038a5c8bc0..0000000000000
--- a/lib/systems/supported.nix
+++ /dev/null
@@ -1,26 +0,0 @@
-# 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 ++ [
-    "aarch64-darwin"
-  ];
-
-  tier1 = [
-    "x86_64-linux"
-  ];
-
-  tier2 = [
-    "aarch64-linux"
-    "x86_64-darwin"
-  ];
-
-  tier3 = [
-    "armv6l-linux"
-    "armv7l-linux"
-    "i686-linux"
-    "mipsel-linux"
-  ];
-}