flake: Add attic.nix-versions

flake/nix-versions.nix

@@ -0,0 +1,52 @@
+{ lib, flake-parts-lib, ... }:
+let
+  inherit (lib)
+    mkOption
+    types
+    ;
+  inherit (flake-parts-lib)
+    mkPerSystemOption
+    ;
+in
+{
+  options = {
+    perSystem = mkPerSystemOption {
+      options.attic.nix-versions = {
+        versions = mkOption {
+          type = types.attrsOf types.package;
+          default = {};
+        };
+        manifestFile = mkOption {
+          type = types.package;
+        };
+      };
+    };
+  };
+
+  config = {
+    perSystem = { self', pkgs, config, ... }: let
+      cfg = config.attic.nix-versions;
+    in {
+      attic.nix-versions = {
+        versions = {
+          default = pkgs.nix;
+          "2.20" = pkgs.nixVersions.nix_2_20;
+          "2.24" = pkgs.nixVersions.nix_2_24;
+        };
+
+        manifestFile = let
+          manifest = lib.mapAttrs (_: nix: {
+            inherit nix;
+            shellHook = ''
+              export NIX_INCLUDE_PATH="${lib.getDev nix}/include"
+              export NIX_CFLAGS_COMPILE="-isystem $NIX_INCLUDE_PATH $NIX_CFLAGS_COMPILE"
+              export NIX_LDFLAGS="-L${nix}/lib $NIX_LDFLAGS"
+              export PKG_CONFIG_PATH="${lib.getDev nix}/lib/pkgconfig:$PKG_CONFIG_PATH"
+              export PATH="${lib.getBin nix}/bin:$PATH"
+            '';
+          }) cfg.versions;
+        in pkgs.writeText "nix-versions.json" (builtins.toJSON manifest);
+      };
+    };
+  };
+}

justfile

@@ -0,0 +1,19 @@
+set positional-arguments
+
+here := env_var_or_default("JUST_INVOCATION_DIR", invocation_directory())
+base := `pwd`
+
+#@echo "here: {{ here }}"
+#@echo "base: {{ base }}"
+
+# List available targets
+list:
+	@just --list --unsorted
+
+# Run a command with an alternative Nix version
+with-nix version *command:
+	set -e; \
+		hook="$(jq -e -r '.[$version].shellHook' --arg version "{{ version }}" < "$NIX_VERSIONS" || (>&2 echo "Version {{ version }} doesn't exist"; exit 1))"; \
+		eval "$hook"; \
+		CARGO_TARGET_DIR="{{ base }}/target/nix-{{ version }}" \
+		{{ command }}