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64 lines
1.9 KiB
Rust
64 lines
1.9 KiB
Rust
//@ known-bug: #107975
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//@ compile-flags: -Copt-level=2
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//@ run-pass
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// https://github.com/rust-lang/rust/issues/107975#issuecomment-1431758601
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use std::cell::{Ref, RefCell};
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fn main() {
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let a: usize = {
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let v = 0u8;
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&v as *const _ as usize
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};
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let b: usize = {
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let v = 0u8;
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&v as *const _ as usize
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};
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let i: usize = b - a;
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// A surprise tool that will help us later.
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let arr = [
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RefCell::new(Some(Box::new(1u8))),
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RefCell::new(None),
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RefCell::new(None),
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RefCell::new(None),
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];
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// `i` is not 0
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assert_ne!(i, 0);
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// Let's borrow the `i`-th element.
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// If `i` is out of bounds, indexing will panic.
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let r: Ref<Option<Box<u8>>> = arr[i].borrow();
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// If we got here, it means `i` was in bounds.
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// Now, two options are possible:
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// EITHER `i` is not 0 (as we have asserted above),
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// so the unwrap will panic, because only the 0-th element is `Some`
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// OR the assert lied, `i` *is* 0, and the `unwrap` will not panic.
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let r: &Box<u8> = r.as_ref().unwrap();
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// If we got here, it means `i` *was* actually 0.
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// Let's ignore the fact that the assert has lied
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// and try to take a mutable reference to the 0-th element.
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// `borrow_mut` should panic, because we are sill holding on
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// to a shared `Ref` for the same `RefCell`.
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*arr[0].borrow_mut() = None;
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// But it doesn't panic!
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// We have successfully replaced `Some(Box)` with `None`,
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// while holding a shared reference to it.
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// No unsafe involved.
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// The `Box` has been deallocated by now, so this is a dangling reference!
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let r: &u8 = &*r;
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println!("{:p}", r);
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// The following might segfault. Or it might not.
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// Depends on the platform semantics
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// and whatever happened to the pointed-to memory after deallocation.
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// let u: u8 = *r;
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// println!("{u}");
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}
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