fix: preserve escaped mapped pointers in the V3 buffer example

examples/buffer_suballocation/main.v

@@ -128,10 +128,8 @@ fn run() ! {
 	require_success(allocator.map(mut first_allocation, &first_mapped), 'map first staging buffer')!
 	require_success(allocator.map(mut second_allocation, &second_mapped),
 		'map second staging buffer')!
-	unsafe {
-		*(&u8(first_mapped)) = 21
-		*(&u8(second_mapped)) = 42
-	}
+	store_mapped_byte(first_mapped, 21)
+	store_mapped_byte(second_mapped, 42)
 	require_success(allocator.flush(first_allocation), 'flush first upload buffer')!
 	require_success(allocator.flush(second_allocation), 'flush second upload buffer')!
 	assert usize(second_mapped) - usize(first_mapped) == second_allocation.offset - first_allocation.offset

examples/buffer_suballocation/mapped_bytes.v

@@ -0,0 +1,9 @@
+module main
+
+// Pass the pointer value into a fresh local scope: vlang/v#31481 otherwise
+// casts a heap-promoted voidptr slot's address instead of its stored value.
+fn store_mapped_byte(mapped voidptr, value u8) {
+	unsafe {
+		*(&u8(mapped)) = value
+	}
+}

examples/buffer_suballocation/mapped_bytes_test.v

@@ -0,0 +1,16 @@
+module main
+
+fn escaped_mapped_byte_store(bytes []u8, value u8) &voidptr {
+	mut mapped := voidptr(bytes.data)
+	store_mapped_byte(mapped, value)
+	return &mapped
+}
+
+fn test_mapped_byte_store_preserves_an_escaped_pointer() {
+	for value in [u8(0), 21, 255] {
+		mut bytes := [u8(7), 8, 9, 10]
+		pointer_slot := escaped_mapped_byte_store(bytes, value)
+		assert bytes == [value, u8(8), 9, 10]
+		assert unsafe { *pointer_slot } == bytes.data
+	}
+}