simplify nix settings

- Don't auto-optimise-store by default
    - We should avoid arbitrary "sane defaults" in this template, as nixpkgs already has pretty sane defaults nowadays
- Simplify registry+nixPath
    - Instead of using a file as indirection (to avoid staleness), indirect it through the registry using `flake:nixpkgs`
    - Add a comment about NixOS/nixpkgs#254405, which all this by default for nixpkgs
- Disable channels and flake registry by default

minimal/nixos/configuration.nix

@@ -40,26 +40,21 @@
     };
   };
 
-  # This will add each flake input as a registry
-  # To make nix3 commands consistent with your flake
-  nix.registry = (lib.mapAttrs (_: flake: {inherit flake;})) ((lib.filterAttrs (_: lib.isType "flake")) inputs);
-
-  # This will additionally add your inputs to the system's legacy channels
-  # Making legacy nix commands consistent as well, awesome!
-  nix.nixPath = ["/etc/nix/path"];
-  environment.etc =
-    lib.mapAttrs'
-    (name: value: {
-      name = "nix/path/${name}";
-      value.source = value.flake;
-    })
-    config.nix.registry;
+  nix = let
+    flakeInputs = lib.filterAttrs (_: lib.isType "flake") inputs;
+  in {
+    settings = {
+      # Enable flakes and new 'nix' command
+      experimental-features = "nix-command flakes";
+      # Opinionated: disable global registry
+      flake-registry = "";
+    };
+    # Opinionated: disable channels
+    channel.enable = false;
 
-  nix.settings = {
-    # Enable flakes and new 'nix' command
-    experimental-features = "nix-command flakes";
-    # Deduplicate and optimize nix store
-    auto-optimise-store = true;
+    # Opinionated: make flake registry and nix path match flake inputs
+    registry = lib.mapAttrs (_: flake: {inherit flake;}) flakeInputs;
+    nixPath = lib.mapAttrsToList (n: _: "${n}=flake:${n}") flakeInputs;
   };
 
   # FIXME: Add the rest of your current configuration

standard/nixos/configuration.nix

@@ -49,26 +49,21 @@
     };
   };
 
-  # This will add each flake input as a registry
-  # To make nix3 commands consistent with your flake
-  nix.registry = (lib.mapAttrs (_: flake: {inherit flake;})) ((lib.filterAttrs (_: lib.isType "flake")) inputs);
-
-  # This will additionally add your inputs to the system's legacy channels
-  # Making legacy nix commands consistent as well, awesome!
-  nix.nixPath = ["/etc/nix/path"];
-  environment.etc =
-    lib.mapAttrs'
-    (name: value: {
-      name = "nix/path/${name}";
-      value.source = value.flake;
-    })
-    config.nix.registry;
+  nix = let
+    flakeInputs = lib.filterAttrs (_: lib.isType "flake") inputs;
+  in {
+    settings = {
+      # Enable flakes and new 'nix' command
+      experimental-features = "nix-command flakes";
+      # Opinionated: disable global registry
+      flake-registry = "";
+    };
+    # Opinionated: disable channels
+    channel.enable = false;
 
-  nix.settings = {
-    # Enable flakes and new 'nix' command
-    experimental-features = "nix-command flakes";
-    # Deduplicate and optimize nix store
-    auto-optimise-store = true;
+    # Opinionated: make flake registry and nix path match flake inputs
+    registry = lib.mapAttrs (_: flake: {inherit flake;}) flakeInputs;
+    nixPath = lib.mapAttrsToList (n: _: "${n}=flake:${n}") flakeInputs;
   };
 
   # FIXME: Add the rest of your current configuration