lib: add a TaggedUnion helper to create C ABI compatible tagged unions

src/lib/struct.zig

@@ -0,0 +1,31 @@
+const std = @import("std");
+const Target = @import("target.zig").Target;
+
+pub fn Struct(
+    comptime target: Target,
+    comptime Zig: type,
+) type {
+    return switch (target) {
+        .zig => Zig,
+        .c => c: {
+            const info = @typeInfo(Zig).@"struct";
+            var fields: [info.fields.len]std.builtin.Type.StructField = undefined;
+            for (info.fields, 0..) |field, i| {
+                fields[i] = .{
+                    .name = field.name,
+                    .type = field.type,
+                    .default_value_ptr = field.default_value_ptr,
+                    .is_comptime = field.is_comptime,
+                    .alignment = field.alignment,
+                };
+            }
+
+            break :c @Type(.{ .@"struct" = .{
+                .layout = .@"extern",
+                .fields = &fields,
+                .decls = &.{},
+                .is_tuple = info.is_tuple,
+            } });
+        },
+    };
+}