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.

6 files changed, 51 insertions, 30 deletions

diff --git a/flake.nix b/flake.nix
index c48d6c68e5834..8c0403adc4a34 100644
--- a/flake.nix
+++ b/flake.nix
@@ -11,9 +11,7 @@
 
       lib = import ./lib;
 
-      systems = lib.systems.supported.hydra;
-
-      forAllSystems = f: lib.genAttrs systems (system: f system);
+      forAllSystems = f: lib.genAttrs lib.systems.flakeExposed (system: f system);
 
     in
     {
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"
-  ];
-}
diff --git a/nixos/doc/manual/from_md/release-notes/rl-2205.section.xml b/nixos/doc/manual/from_md/release-notes/rl-2205.section.xml
index a660e28af04aa..b3d6b6a1b956b 100644
--- a/nixos/doc/manual/from_md/release-notes/rl-2205.section.xml
+++ b/nixos/doc/manual/from_md/release-notes/rl-2205.section.xml
@@ -585,6 +585,15 @@
       </listitem>
       <listitem>
         <para>
+          <literal>lib.systems.supported</literal> has been removed, as
+          it was overengineered for determining the systems to support
+          in the nixpkgs flake. The list of systems exposed by the
+          nixpkgs flake can now be accessed as
+          <literal>lib.systems.flakeExposed</literal>.
+        </para>
+      </listitem>
+      <listitem>
+        <para>
           For new installations
           <literal>virtualisation.oci-containers.backend</literal> is
           now set to <literal>podman</literal> by default. If you still
diff --git a/nixos/doc/manual/release-notes/rl-2205.section.md b/nixos/doc/manual/release-notes/rl-2205.section.md
index 96b138aa82e2d..f8dc78eac7ebf 100644
--- a/nixos/doc/manual/release-notes/rl-2205.section.md
+++ b/nixos/doc/manual/release-notes/rl-2205.section.md
@@ -193,6 +193,8 @@ In addition to numerous new and upgraded packages, this release has the followin
   This is to improve compatibility with `libcontainer` based software such as Podman and Skopeo
   which assumes they have ownership over `/etc/containers`.
 
+- `lib.systems.supported` has been removed, as it was overengineered for determining the systems to support in the nixpkgs flake. The list of systems exposed by the nixpkgs flake can now be accessed as `lib.systems.flakeExposed`.
+
 - For new installations `virtualisation.oci-containers.backend` is now set to `podman` by default.
   If you still want to use Docker on systems where `system.stateVersion` is set to to `"22.05"` set `virtualisation.oci-containers.backend = "docker";`.Old systems with older `stateVersion`s stay with "docker".