From 1d9d2590b1b8e175a6be796202f23ad833789f2a Mon Sep 17 00:00:00 2001
From: Aleksandr <nerodono0@gmail.com>
Date: Thu, 22 Aug 2024 18:41:22 +0300
Subject: [PATCH 1/2] feat: add `build.rs` for additional build-time checks of
 layout

---
 build.rs | 68 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)
 create mode 100644 build.rs

diff --git a/build.rs b/build.rs
new file mode 100644
index 0000000..e34b659
--- /dev/null
+++ b/build.rs
@@ -0,0 +1,68 @@
+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
+    );
+}
+
+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() {
+    test_ptr_layouts();
+}

From 83e9f7be6eb60ac1def6af956372a097db8a01e6 Mon Sep 17 00:00:00 2001
From: Aleksandr <nerodono0@gmail.com>
Date: Thu, 22 Aug 2024 18:54:31 +0300
Subject: [PATCH 2/2] chore: comments on exhaustiveness

---
 build.rs | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/build.rs b/build.rs
index e34b659..ae65dfc 100644
--- a/build.rs
+++ b/build.rs
@@ -22,6 +22,10 @@ fn layout_broken(what: &str) {
     );
 }
 
+/// 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
     {
@@ -64,5 +68,9 @@ fn test_ptr_layouts() {
 }
 
 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();
 }