feat: add build.rs for additional build-time checks of layout

build.rs

@@ -0,0 +1,76 @@
+use std::ptr;
+
+#[allow(dead_code)]
+struct Sample(usize);
+
+trait SomeTrait {
+    fn call_me(&self) -> bool {
+        true
+    }
+}
+
+impl SomeTrait for Sample {}
+
+fn layout_broken(what: &str) {
+    panic!(
+        concat!(
+            "Assumptions on layout are broken, this crate relies on ",
+            "`unsafe code guidelines` layout specification, ",
+            "now layout of {:?} is broken, report about it on github"
+        ),
+        what
+    );
+}
+
+/// Tests some layout assumptions, these cases as of now:
+///
+/// 1. data pointer and vtable pointer location of trait objects
+/// 2. data pointer and size location of slice objects
+fn test_ptr_layouts() {
+    // Test for dyn object
+    {
+        #[repr(C)]
+        struct DynObj {
+            data_ptr: *const u8,
+            vtable: *const u8,
+        }
+
+        let sample = Box::new(Sample(100));
+        let data_ptr = Box::into_raw(sample);
+
+        let trait_obj: *const dyn SomeTrait = data_ptr;
+        let dyn_obj_repr: DynObj = unsafe { ptr::read(ptr::addr_of!(trait_obj) as *const DynObj) };
+
+        if dyn_obj_repr.data_ptr != data_ptr as *const u8 {
+            layout_broken("trait objects");
+        }
+        let out = unsafe { Box::from_raw(data_ptr) };
+        out.call_me();
+    }
+
+    // Test for slice object
+    {
+        let array = [1, 2, 3];
+        let slice: &[u8] = &array;
+
+        #[repr(C)]
+        struct Slice {
+            data_ptr: *const u8,
+            size: usize,
+        }
+
+        let slice_repr: Slice = unsafe { ptr::read(ptr::addr_of!(slice) as *const Slice) };
+
+        if slice_repr.data_ptr != slice.as_ptr() || slice_repr.size != slice.len() {
+            layout_broken("slices");
+        }
+    }
+}
+
+fn main() {
+    // NOTE: this will not protect from every possible case,
+    // for example, rust may add one more fat pointer type which this test
+    // will not check, host layout may be different from target layout,
+    // and probably more.
+    test_ptr_layouts();
+}