Add assert_eq_align! macro

Author: nvzqz

src/assert_eq_align.rs

@@ -0,0 +1,45 @@
+/// Asserts that types are equal in alignment.
+///
+/// This is useful when ensuring that pointer arithmetic is done correctly, or
+/// when [FFI] requires a type to have the same alignment as some foreign type.
+///
+/// # Examples
+///
+/// A `usize` has the same alignment as any pointer type:
+///
+/// ```
+/// # #[macro_use] extern crate static_assertions; fn main() {}
+/// assert_eq_align!(usize, *const u8, *mut u8);
+/// ```
+///
+/// The following passes because `[i32; 4]` has the same alignment as `i32`:
+///
+/// ```
+/// # #[macro_use] extern crate static_assertions; fn main() {}
+/// assert_eq_align!([i32; 4], i32);
+/// ```
+///
+/// The following example fails to compile because `i32x4` explicitly has 4
+/// times the alignment as `[i32; 4]`:
+///
+/// ```compile_fail
+/// # #[macro_use] extern crate static_assertions; fn main() {}
+/// # #[allow(non_camel_case_types)]
+/// #[repr(align(16))]
+/// struct i32x4([i32; 4]);
+///
+/// assert_eq_align!(i32x4, [i32; 4]);
+/// ```
+///
+/// [FFI]: https://en.wikipedia.org/wiki/Foreign_function_interface
+#[macro_export]
+macro_rules! assert_eq_align {
+    ($x:ty, $($xs:ty),+ $(,)?) => {
+        const _: fn() = || {
+            // Assigned instance must match the annotated type or else it will
+            // fail to compile
+            use $crate::_core::mem::align_of;
+            $(let _: [(); align_of::<$x>()] = [(); align_of::<$xs>()];)+
+        };
+    };
+}

src/lib.rs

@@ -66,6 +66,7 @@
 pub extern crate core as _core;
 
 mod assert_cfg;
+mod assert_eq_align;
 mod assert_eq_size;
 mod assert_fields;
 mod assert_impl;