add(zig-interface): dependency to check interface contracts at comptime

The described interfaces for `Widget` and `Layout` are now defined and
correspondingly checked at comptime.

src/layout.zig

@@ -22,9 +22,18 @@ pub fn Layout(comptime Event: type, comptime Renderer: type) type {
         @compileError("Provided user event `Event` for `Layout(comptime Event: type)` is not of type `union(enum)`.");
     }
     const Events = std.ArrayList(Event);
-    return struct {
+    const Type = struct {
         const LayoutType = @This();
+        const Element = union(enum) {
+            layout: LayoutType,
+            widget: @import("widget.zig").Widget(Event, Renderer),
+        };
         const Ptr = usize;
+        pub const Interface = @import("interface").Interface(.{
+            .handle = fn (anytype, Event) anyerror!*Events,
+            .render = fn (anytype, *Renderer) anyerror!void,
+            .deinit = fn (anytype) void,
+        }, .{});
 
         const VTable = struct {
             handle: *const fn (this: *LayoutType, event: Event) anyerror!*Events,
@@ -79,9 +88,17 @@ pub fn Layout(comptime Event: type, comptime Renderer: type) type {
         }
 
         // import and export of `Layout` implementations
-        pub const HStack = @import("layout/HStack.zig").Layout(Event, Renderer);
-        pub const VStack = @import("layout/VStack.zig").Layout(Event, Renderer);
-        pub const Padding = @import("layout/Padding.zig").Layout(Event, Renderer);
-        pub const Framing = @import("layout/Framing.zig").Layout(Event, Renderer);
+        pub const HStack = @import("layout/HStack.zig").Layout(Event, Element, Renderer);
+        pub const VStack = @import("layout/VStack.zig").Layout(Event, Element, Renderer);
+        pub const Padding = @import("layout/Padding.zig").Layout(Event, Element, Renderer);
+        pub const Framing = @import("layout/Framing.zig").Layout(Event, Element, Renderer);
     };
+    // test widget implementation satisfies the interface
+    comptime Type.Interface.satisfiedBy(Type);
+    // TODO: there is a dependency loop (due to the necessary `Layout` type for the `Element`)
+    comptime Type.Interface.satisfiedBy(Type.HStack);
+    comptime Type.Interface.satisfiedBy(Type.VStack);
+    comptime Type.Interface.satisfiedBy(Type.Padding);
+    comptime Type.Interface.satisfiedBy(Type.Framing);
+    return Type;
 }