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This commit is a fix for #48493 where calling `Weak::new` where `T` is an uninhabited type would segfault. The cause for this issue was pretty subtle and the fix here is mostly a holdover until #47650 is implemented. The `Weak<!>` struct internally contains a `NonNull<RcBox<!>>`. The `RcBox<!>` type is uninhabited, however, as it directly embeds the `!` type. Consequently the size of `RcBox<!>` is zero, which means that `NonNull<RcBox<!>>` always contains a pointer with a value of 1. Currently all boxes of zero-sized-types are actually pointers to the address 1 (as they shouldn't be read anyway). The problem comes about when later on we have a method called `Weak::inner` which previously returned `&RcBox<T>`. This was actually invalid because the instance of `&RcBox<T> ` never existed (only the uninitialized part). This means that when we try to actually modify `&RcBox`'s weak count in the destructor for `Weak::new` we're modifying the address 1! This ends up causing a segfault. This commit takes the strategy of modifying the `Weak::inner` method to return an `Option<&RcBox<T>>` that is `None` whenever the size of `RcBox<T>` is 0 (so it couldn't actually exist). This does unfortunately add more dispatch code to operations like `Weak<Any>::clone`. Eventually the "correct" fix for this is to have `RcBox<T>` store a union of `T` and a ZST like `()`, and that way the size of `RcBox<T>` will never be 0 and we won't need this branch. Until #47650 is implemented, though, we can't use `union`s and unsized types together. Closes #48493
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| // Copyright 2018 The Rust Project Developers. See the COPYRIGHT | ||
| // file at the top-level directory of this distribution and at | ||
| // http://rust-lang.org/COPYRIGHT. | ||
| // | ||
| // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or | ||
| // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license | ||
| // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your | ||
| // option. This file may not be copied, modified, or distributed | ||
| // except according to those terms. | ||
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| use std::collections::HashMap; | ||
| use std::collections::BTreeMap; | ||
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| #[derive(Eq, PartialEq, Hash, PartialOrd, Ord)] | ||
| enum Void {} | ||
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| trait Foo {} | ||
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| impl<T> Foo for T {} | ||
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| fn main() { | ||
| std::rc::Weak::<Void>::new(); | ||
| std::rc::Weak::<Void>::new().clone(); | ||
| (std::rc::Weak::<Void>::new() as std::rc::Weak<Foo>); | ||
| (std::rc::Weak::<Void>::new() as std::rc::Weak<Foo>).clone(); | ||
| std::sync::Weak::<Void>::new(); | ||
| (std::sync::Weak::<Void>::new() as std::sync::Weak<Foo>); | ||
| (std::sync::Weak::<Void>::new() as std::sync::Weak<Foo>).clone(); | ||
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| let mut h: HashMap<Void, Void> = HashMap::new(); | ||
| assert_eq!(h.len(), 0); | ||
| assert_eq!(h.iter().count(), 0); | ||
| assert_eq!(h.iter_mut().count(), 0); | ||
| assert_eq!(h.into_iter().count(), 0); | ||
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| let mut h: BTreeMap<Void, Void> = BTreeMap::new(); | ||
| assert_eq!(h.len(), 0); | ||
| assert_eq!(h.iter().count(), 0); | ||
| assert_eq!(h.iter_mut().count(), 0); | ||
| assert_eq!(h.into_iter().count(), 0); | ||
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| let mut h: Vec<Void> = Vec::new(); | ||
| assert_eq!(h.len(), 0); | ||
| assert_eq!(h.iter().count(), 0); | ||
| assert_eq!(h.iter_mut().count(), 0); | ||
| assert_eq!(h.into_iter().count(), 0); | ||
| } |