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Option containing heap drop types in static mut acts strangley #46089

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@Ryan1729

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@Ryan1729

The following code use the recently stabilized, (but not in the current stable release yet,) feature drop_types_in_const, so it needs a recent nightly version to be compiled.

struct Foo {
    bar: String,
}

static mut FOO: Option<Foo> = None;

fn main() {
    unsafe {
        FOO = Some(std::mem::zeroed());
    }
    if let Some(_) = unsafe { FOO.as_mut() } {
        println!("Some(Foo)");
    } else {
        println!("None");
    }
}

This prints out "None", instead of the expected "Some(Foo)". However, if I change FOO = Some(std::mem::zeroed()); to FOO = Some(std::mem::uninitialized()); then it prints "Some(Foo)", (at least on my system, but that could clearly be just "luck".) The assignment also works as expected if I explicitly initialize FOO, that is, something like FOO = Some("Hello World!".to_string());, or if I use a non-drop type. If I use a different built-in drop type than String, like Vec<u32> or Box<u32> then it still prints "None". However, if I define my own stack-allocated drop type like so:

struct Baz{
    qux: u32
}

impl Drop for Baz {
    fn drop(&mut self) {}
}

and make bar an zeroed instance of that type then it prints "Some(Foo)".

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