diff --git a/library/modules/std/objc_block.sx b/library/modules/std/objc_block.sx
index 09d73cb..0d0f3cc 100644
--- a/library/modules/std/objc_block.sx
+++ b/library/modules/std/objc_block.sx
@@ -107,3 +107,70 @@ impl Into(Block) for Closure(bool) -> void {
         };
     }
 }
+
+// Comptime builder for the generic `Into(Block) for Closure(..$args)
+// -> $R` impl body. Receives the closure's per-position pack types
+// (`args`, a comptime `[]Type`) and the closure's return type
+// (`$ret`), emits source that:
+//
+//   1. Declares a nested `__invoke` `callconv(.c)` trampoline whose
+//      signature matches the per-shape Apple Block ABI: first arg is
+//      `block_self: *Block`, then the pack types verbatim. The body
+//      reconstructs a typed fn-pointer from `block_self.sx_fn`,
+//      prepends `block_self.sx_env`, and tail-calls the sx closure.
+//   2. Returns a stack Block literal whose `invoke` slot points at
+//      the nested trampoline via `xx @__invoke`.
+//
+// The host impl wraps the emitted source via `#insert
+// build_block_convert($args, $R);`. Per-call-shape monomorphisation
+// of the impl body re-runs this builder with the concrete types
+// bound, so each closure shape gets its own dedicated trampoline.
+//
+// Void return type emits `typed_fn(...)`; non-void emits
+// `return typed_fn(...)`.
+build_block_convert :: (args: []Type, $ret: Type) -> string {
+    ret_name := type_name(ret);
+    code := "__invoke :: (block_self: *Block";
+    i : s64 = 0;
+    while i < args.len {
+        code = concat(code, ", arg");
+        code = concat(code, int_to_string(i));
+        code = concat(code, ": ");
+        code = concat(code, type_name(args[i]));
+        i = i + 1;
+    }
+    code = concat(code, ") -> ");
+    code = concat(code, ret_name);
+    code = concat(code, " callconv(.c) { typed_fn : (*void");
+    i = 0;
+    while i < args.len {
+        code = concat(code, ", ");
+        code = concat(code, type_name(args[i]));
+        i = i + 1;
+    }
+    code = concat(code, ") -> ");
+    code = concat(code, ret_name);
+    code = concat(code, " = xx block_self.sx_fn; ");
+    if ret_name == "void" {
+        code = concat(code, "typed_fn(block_self.sx_env");
+    } else {
+        code = concat(code, "return typed_fn(block_self.sx_env");
+    }
+    i = 0;
+    while i < args.len {
+        code = concat(code, ", arg");
+        code = concat(code, int_to_string(i));
+        i = i + 1;
+    }
+    code = concat(code, "); } ");
+    code = concat(code, "return .{ ");
+    code = concat(code, "isa = @_NSConcreteStackBlock, ");
+    code = concat(code, "flags = 0, ");
+    code = concat(code, "reserved = 0, ");
+    code = concat(code, "invoke = xx @__invoke, ");
+    code = concat(code, "descriptor = xx @__sx_block_descriptor, ");
+    code = concat(code, "sx_env = self.env, ");
+    code = concat(code, "sx_fn = self.fn_ptr, ");
+    code = concat(code, "};");
+    return code;
+}