add swift 5.4.2

runtimes/default.nix

@@ -17,4 +17,5 @@ args: {
     "ruby" = import ./ruby.nix args;
     "zig" = import ./zig.nix args;
     "vlang" = import ./vlang.nix args;
+    "swift" = import ./swift.nix args;
 }

runtimes/ruby.nix

@@ -1,6 +1,6 @@
 {pkgs, piston, ...}:
 let 
-    pkg = pkgs.ruby_3_1; # ruby 3.1 stable from unstable channel
+    pkg = pkgs.ruby_3_1;
 in piston.mkRuntime {
     language = "ruby";
     version = pkg.version;

runtimes/swift.nix

@@ -0,0 +1,27 @@
+{pkgs, piston, ...}:
+let
+    pkg = pkgs.swift;
+in piston.mkRuntime {
+    language = "swift";
+    version = "5.4.2";
+    aliases = [];
+
+    run = ''
+        ls ${pkg}
+        ${pkg}/bin/swift -module-cache-path . "$@"
+    '';
+
+    tests = [
+        (piston.mkTest {
+            files = {
+                "test.swift" = ''
+                    print("OK");
+                '';
+            };
+            args = [];
+            stdin = "";
+            packages = [];
+            main = "test.swift";
+        })
+    ];
+}