2 Displays a warning when a union is declared with the default representation (without a `#[repr(C)]` attribute).
5 Unions in Rust have unspecified layout by default, despite many people thinking that they
6 lay out each field at the start of the union (like C does). That is, there are no guarantees
7 about the offset of the fields for unions with multiple non-ZST fields without an explicitly
8 specified layout. These cases may lead to undefined behavior in unsafe blocks.
18 let _x: u32 = unsafe {
19 Foo { a: 0_i32 }.b // Undefined behavior: `b` is allowed to be padding
32 let _x: u32 = unsafe {
33 Foo { a: 0_i32 }.b // Now defined behavior, this is just an i32 -> u32 transmute