Create assert_field_offsets! macro

src/assert_field_offsets.rs

@@ -0,0 +1,75 @@
+/// Asserts that the type has the given field offsets.
+///
+/// Due to implementation details, this only works for [`Sized`] types.
+///
+/// # Safety
+///
+/// This is implemented by creating a dangling reference to an uninitialized
+/// memory. Currently, this works on supported Rust versions (as of this
+/// writing: 1.37 and 1.38). However, a test that is expected to compile today
+/// may not in the future.
+///
+/// # Examples
+///
+/// Given some type whose fields can't be re-ordered because of `#[repr(C)]`:
+///
+/// ```
+/// #[repr(C)]
+/// struct Foo {
+///     start: u32,
+///     end:   u64,
+/// }
+/// ```
+///
+/// The following example fails to compile because, despite `start` being 4
+/// bytes, `end` is aligned to 8 bytes and thus requires an offset of 8.
+///
+/// ```compile_fail
+/// # #[macro_use] extern crate static_assertions;
+/// # #[repr(C)] struct Foo { start: u32, end: u64 }
+/// assert_field_offsets!(Foo { end: == 4 });
+/// ```
+///
+/// However, the correct assumptions are upheld below:
+///
+/// ```
+/// # #[macro_use] extern crate static_assertions;
+/// # #[repr(C)] struct Foo { start: u32, end: u64 }
+/// assert_field_offsets!(Foo {
+///     start: == 0,
+///     end:   == 8,
+/// });
+/// ```
+///
+/// This macro even works with tuple `struct`s:
+///
+/// ```
+/// # #[macro_use] extern crate static_assertions;
+/// struct Bar(u32, u32);
+///
+/// assert_field_offsets!(Bar { 1: == 4 });
+/// ```
+///
+/// [`Sized`]: https://doc.rust-lang.org/std/marker/trait.Sized.html
+#[macro_export]
+macro_rules! assert_field_offsets {
+    ($t:ty { $($f:tt: == $o:expr),+ $(,)? }) => {
+        $(
+            const _: [(); $o] = [(); unsafe {
+                // FIXME: Creating a reference to uninitialized memory is
+                // undefined behavior, but does that apply at compile time or
+                // only at run time?
+                // TODO: Make this work with unsized types
+                union Cast<T: 'static> {
+                    ptr: &'static T,
+                    addr: usize,
+                }
+
+                let align = $crate::_core::mem::align_of::<$t>();
+                let ptr: &$t = Cast { addr: align }.ptr;
+
+                Cast { ptr: &ptr.$f }.addr - align
+            }];
+        )+
+    };
+}

src/lib.rs

@@ -79,6 +79,7 @@ mod assert_cfg;
 mod assert_eq_align;
 mod assert_eq_size;
 mod assert_fields;
+mod assert_field_offsets;
 mod assert_impl;
 mod assert_obj_safe;
 mod assert_type;

tests/field_offsets.rs

@@ -0,0 +1,14 @@
+#![no_std]
+#![allow(dead_code)]
+#![deny(unsafe_code)]
+
+#[macro_use]
+extern crate static_assertions;
+
+#[repr(C)]
+struct Foo(u32, u64);
+
+assert_field_offsets!(Foo {
+    0: == 0,
+    1: == 8,
+});