error[E0793]: reference to packed field is unaligned
  --> $DIR/unaligned_references_external_macro.rs:5:1
   |
LL | / unaligned_references_external_crate::mac! {
LL | |     #[repr(packed)]
LL | |     pub struct X {
LL | |         pub field: u16
LL | |     }
LL | | }
   | |_^
   |
   = note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
   = note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
   = note: this error originates in the macro `unaligned_references_external_crate::mac` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

For more information about this error, try `rustc --explain E0793`.