A shared reference to static mut stored in another static can be used mutably

[ChayimFriedman2]

struct Foo(*mut i32);
unsafe impl Sync for Foo {}

static mut FOO: i32 = 0;
static PTR: Foo = Foo(unsafe { &FOO as *const i32 as *mut i32 });

fn main() {
    unsafe {
        let _r = &mut *PTR.0;
    }
}

Does not report any error, even though it definitely contains UB because a similar code does report:

static mut FOO: i32 = 0;

fn main() {
    unsafe {
        let p = &FOO as *const i32 as *mut i32;
        let _r = &mut *p;
    }
}

This may be because of #121.

[RalfJung]

This is because we don't have an aliasing model at compile time, so nothing is remembering that this pointer originates from a shared reference. We also haven't made a decision on whether aliasing rules even apply at the boundary between compiletime and runtime. They are not super useful here I think since everything is global... But that is a UCG question.

[isaacthefallenapple]

@RalfJung Does that imply this isn't UB? Or just unspecified? Chiming in because this originated from my question on SO.

[RalfJung]

It is deep in undecided/unspecified territory. So please don't do it. :)

[RalfJung]

I have opened rust-lang/unsafe-code-guidelines#424 to track this question from a specification side. Thanks for bringing this to my attention!

There's nothing to be done on the Miri side here currently, so I will close this issue.