E0793: Clarify that it applies to unions as well

compiler/rustc_error_codes/src/error_codes/E0793.md

@@ -1,4 +1,9 @@
-An unaligned reference to a field of a [packed] struct got created.
+An unaligned reference to a field of a [packed] `struct` or `union` was created.
+
+The `#[repr(packed)]` attribute removes padding between fields, which can
+cause fields to be stored at unaligned memory addresses. Creating references
+to such fields violates Rust's memory safety guarantees and can lead to
+undefined behavior in optimized code.
 
 Erroneous code example:
 
@@ -45,9 +50,36 @@ unsafe {
     // For formatting, we can create a copy to avoid the direct reference.
     let copy = foo.field1;
     println!("{}", copy);
+
     // Creating a copy can be written in a single line with curly braces.
     // (This is equivalent to the two lines above.)
     println!("{}", { foo.field1 });
+
+    // A reference to a field that will always be sufficiently aligned is safe:
+    println!("{}", foo.field2);
+}
+```
+
+### Unions
+
+Although creating a reference to a `union` field is `unsafe`, this error
+will still be triggered if the referenced field is not sufficiently
+aligned. Use `addr_of!` and raw pointers in the same way as for struct fields.
+
+```compile_fail,E0793
+#[repr(packed)]
+pub union Foo {
+    field1: u64,
+    field2: u8,
+}
+
+unsafe {
+    let foo = Foo { field1: 0 };
+    // Accessing the field directly is fine.
+    let val = foo.field1;
+
+    // A reference to a packed union field causes an error.
+    let val = &foo.field1; // ERROR
 }
 ```