rust/tests/ui/issues/issue-49298.rs

// run-pass
#![feature(test)]
#![allow(unused_mut)] // under NLL we get warning about `x` below: rust-lang/rust#54499

// This test is bogus (i.e., should be check-fail) during the period
// where #54986 is implemented and #54987 is *not* implemented. For
// now: just ignore it
//
// ignore-test

// This test is checking that the space allocated for `x.1` does not
// overlap with `y`. (The reason why such a thing happened at one
// point was because `x.0: Void` and thus the whole type of `x` was
// uninhabited, and so the compiler thought it was safe to use the
// space of `x.1` to hold `y`.)
//
// That's a fine thing to test when this code is accepted by the
// compiler, and this code is being transcribed accordingly into
// the ui test issue-21232-partial-init-and-use.rs

extern crate test;

enum Void {}

fn main() {
    let mut x: (Void, usize);
    let mut y = 42;
    x.1 = 13;

    // Make sure `y` stays on the stack.
    test::black_box(&mut y);

    // Check that the write to `x.1` did not overwrite `y`.
    // Note that this doesn't fail with optimizations enabled,
    // because we can't keep `x.1` on the stack, like we can `y`,
    // as we can't borrow partially initialized variables.
    assert_eq!(y.to_string(), "42");

    // Check that `(Void, usize)` has space for the `usize` field.
    assert_eq!(std::mem::size_of::<(Void, usize)>(),
               std::mem::size_of::<usize>());
}