src/test/ui/print_type_sizes/packed.rs

// compile-flags: -Z print-type-sizes
// build-pass (FIXME(62277): could be check-pass?)
// ignore-pass
// ^-- needed because `--pass check` does not emit the output needed.
//     FIXME: consider using an attribute instead of side-effects.

// This file illustrates how packing is handled; it should cause
// the elimination of padding that would normally be introduced
// to satisfy alignment desirata.
//
// It avoids using u64/i64 because on some targets that is only 4-byte
// aligned (while on most it is 8-byte aligned) and so the resulting
// padding and overall computed sizes can be quite different.

#![allow(dead_code)]
#![feature(start)]

#[derive(Default)]
#[repr(packed)]
struct Packed1 {
    a: u8,
    b: u8,
    g: i32,
    c: u8,
    h: i16,
    d: u8,
}

#[derive(Default)]
#[repr(packed(2))]
struct Packed2 {
    a: u8,
    b: u8,
    g: i32,
    c: u8,
    h: i16,
    d: u8,
}

#[derive(Default)]
#[repr(packed(2))]
#[repr(C)]
struct Packed2C {
    a: u8,
    b: u8,
    g: i32,
    c: u8,
    h: i16,
    d: u8,
}

#[derive(Default)]
struct Padded {
    a: u8,
    b: u8,
    g: i32,
    c: u8,
    h: i16,
    d: u8,
}

#[start]
fn start(_: isize, _: *const *const u8) -> isize {
    let _c: Packed1 = Default::default();
    let _d: Packed2 = Default::default();
    let _e: Packed2C = Default::default();
    let _f: Padded = Default::default();
    0
}