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In Rust 1.29.0 one of my tests has started failing. I managed to get the strange bug down to this example:
#[derive(Clone, Debug)]
struct CountDrop<'a>(&'a std::cell::RefCell<usize>);
struct MayContainValue<T> {
value: std::mem::ManuallyDrop<T>,
has_value: u32,
}
impl<T: Clone> Clone for MayContainValue<T> {
fn clone(&self) -> Self {
Self {
value: if self.has_value > 0 {
self.value.clone()
} else {
unsafe { std::mem::uninitialized() }
},
has_value: self.has_value,
}
}
}
impl<T> Drop for MayContainValue<T> {
fn drop(&mut self) {
if self.has_value > 0 {
unsafe {
std::mem::ManuallyDrop::drop(&mut self.value);
}
}
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn check_drops() {
let n = 2000;
let drops = std::cell::RefCell::new(0usize);
let mut slots = Vec::new();
for _ in 0..n {
slots.push(MayContainValue {
value: std::mem::ManuallyDrop::new(CountDrop(&drops)),
has_value: 1,
});
}
unsafe { std::mem::ManuallyDrop::drop(&mut slots[0].value); }
slots[0].has_value = 0;
assert_eq!(slots.len(), slots.clone().len());
}
}
I know the code looks strange; it is all ripped out of context. I reproduced this problem with cargo test on 64-bit Ubuntu on Rust 1.29.0. A friend could not reproduce on Windows with the same Rust version.
Other things that stop reproduction:
n below ~900.cargo test.CountDrop's member with u64.What's going on here? Yes, MayContainValue can have an uninitialized member, but this is never used in any way.
I also managed to reproduce this on play.rust-lang.org.
I'm not interested in 'solutions' that involve re-engineering MayContainValue in some safe way with Option or enum, I'm using manual storage and occupied/vacant discrimination for a good reason.
668
TL;DR: Yes, creating an uninitialized reference is always undefined behavior. You cannot use mem::uninitialized safely with generics. There is not currently a good workaround for your specific case.
Running your code in valgrind reports 3 errors, each with the same stack trace:
==741== Conditional jump or move depends on uninitialised value(s)
==741== at 0x11907F: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::spec_extend (vec.rs:1892)
==741== by 0x11861C: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<&'a T, I>>::spec_extend (vec.rs:1942)
==741== by 0x11895C: <alloc::vec::Vec<T>>::extend_from_slice (vec.rs:1396)
==741== by 0x11C1A2: alloc::slice::hack::to_vec (slice.rs:168)
==741== by 0x11C643: alloc::slice::<impl [T]>::to_vec (slice.rs:369)
==741== by 0x118C1E: <alloc::vec::Vec<T> as core::clone::Clone>::clone (vec.rs:1676)
==741== by 0x11AF89: md::tests::check_drops (main.rs:51)
==741== by 0x119D39: md::__test::TESTS::{{closure}} (main.rs:36)
==741== by 0x11935D: core::ops::function::FnOnce::call_once (function.rs:223)
==741== by 0x11F09E: {{closure}} (lib.rs:1451)
==741== by 0x11F09E: call_once<closure,()> (function.rs:223)
==741== by 0x11F09E: <F as alloc::boxed::FnBox<A>>::call_box (boxed.rs:642)
==741== by 0x17B469: __rust_maybe_catch_panic (lib.rs:105)
==741== by 0x14044F: try<(),std::panic::AssertUnwindSafe<alloc::boxed::Box<FnBox<()>>>> (panicking.rs:289)
==741== by 0x14044F: catch_unwind<std::panic::AssertUnwindSafe<alloc::boxed::Box<FnBox<()>>>,()> (panic.rs:392)
==741== by 0x14044F: {{closure}} (lib.rs:1406)
==741== by 0x14044F: std::sys_common::backtrace::__rust_begin_short_backtrace (backtrace.rs:136)
Reducing while keeping the Valgrind error (or one extremely similar) leads to
use std::{iter, mem};
fn main() {
let a = unsafe { mem::uninitialized::<&()>() };
let mut b = iter::once(a);
let c = b.next();
let _d = match c {
Some(_) => 1,
None => 2,
};
}
Running this smaller reproduction in Miri in the playground leads to this error:
error[E0080]: constant evaluation error: attempted to read undefined bytes
--> src/main.rs:7:20
|
7 | let _d = match c {
| ^ attempted to read undefined bytes
|
note: inside call to `main`
--> /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:74:34
|
74| lang_start_internal(&move || main().report(), argc, argv)
| ^^^^^^
note: inside call to `closure`
--> /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:59:75
|
59| ::sys_common::backtrace::__rust_begin_short_backtrace(move || main())
| ^^^^^^
note: inside call to `closure`
--> /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/sys_common/backtrace.rs:136:5
|
13| f()
| ^^^
note: inside call to `std::sys_common::backtrace::__rust_begin_short_backtrace::<[closure@DefId(1/1:1823 ~ std[82ff]::rt[0]::lang_start_internal[0]::{{closure}}[0]::{{closure}}[0]) 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>`
--> /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:59:13
|
59| ::sys_common::backtrace::__rust_begin_short_backtrace(move || main())
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside call to `closure`
--> /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panicking.rs:310:40
|
31| ptr::write(&mut (*data).r, f());
| ^^^
note: inside call to `std::panicking::try::do_call::<[closure@DefId(1/1:1822 ~ std[82ff]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>`
--> /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panicking.rs:306:5
|
30| / fn do_call<F: FnOnce() -> R, R>(data: *mut u8) {
30| | unsafe {
30| | let data = data as *mut Data<F, R>;
30| | let f = ptr::read(&mut (*data).f);
31| | ptr::write(&mut (*data).r, f());
31| | }
31| | }
| |_____^
note: inside call to `std::panicking::try::<i32, [closure@DefId(1/1:1822 ~ std[82ff]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe]>`
--> /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panic.rs:392:9
|
39| panicking::try(f)
| ^^^^^^^^^^^^^^^^^
note: inside call to `std::panic::catch_unwind::<[closure@DefId(1/1:1822 ~ std[82ff]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>`
--> /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:58:25
|
58| let exit_code = panic::catch_unwind(|| {
| _________________________^
59| | ::sys_common::backtrace::__rust_begin_short_backtrace(move || main())
60| | });
| |__________^
note: inside call to `std::rt::lang_start_internal`
--> /root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:74:5
|
74| lang_start_internal(&move || main().report(), argc, argv)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The short version is that mem::uninitialized creates a null pointer, which is being treated as a reference. That's the undefined behavior.
In your original code, the Vec::clone is implemented by iterating over an iterator. Iterator::next returns an Option<T>, so you have an option of a reference, which causes the null pointer optimization to kick in. This counts as a None, which terminates the iteration early, resulting in your empty second vector.
It turns out that having mem::uninitialized, a piece of code that gives you C-like semantics, is a giant footgun and is frequently misused (surprise!), so you aren't alone here. The main things you should follow as replacements are:
MaybeUninit
187
Rust 1.29.0 changed the definition of ManuallyDrop. It used to be a union (with a single member), but now it's a struct and a lang item. The role of the lang item in the compiler is to force the type to not have a destructor, even if it wraps a type that has once.
I tried copying the old definition of ManuallyDrop (which requires nightly, unless a T: Copy bound is added) and using that instead of the one from std, and it avoids the issue (at least on the Playground). I also tried dropping the second slot (slots[1]) instead of the first (slots[0]) and that also happens to work.
Although I haven't been able to reproduce the problem natively on my system (running Arch Linux x86_64), I found something interesting by using miri:
francis@francis-arch /data/git/miri master
$ MIRI_SYSROOT=~/.xargo/HOST cargo run -- /data/src/rust/so-manually-drop-1_29/src/main.rs
Finished dev [unoptimized + debuginfo] target(s) in 0.03s
Running `target/debug/miri /data/src/rust/so-manually-drop-1_29/src/main.rs`
error[E0080]: constant evaluation error: attempted to read undefined bytes
--> /home/francis/.rustup/toolchains/nightly-2018-09-15-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/liballoc/vec.rs:1903:32
|
1903 | for element in iterator {
| ^^^^^^^^ attempted to read undefined bytes
|
note: inside call to `<std::vec::Vec<T> as std::vec::SpecExtend<T, I>><MayContainValue<CountDrop>, std::iter::Cloned<std::slice::Iter<MayContainValue<CountDrop>>>>::spec_extend`
--> /home/francis/.rustup/toolchains/nightly-2018-09-15-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/liballoc/vec.rs:1953:9
|
1953 | self.spec_extend(iterator.cloned())
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside call to `<std::vec::Vec<T> as std::vec::SpecExtend<&'a T, I>><MayContainValue<CountDrop>, std::slice::Iter<MayContainValue<CountDrop>>>::spec_extend`
--> /home/francis/.rustup/toolchains/nightly-2018-09-15-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/liballoc/vec.rs:1402:9
|
1402 | self.spec_extend(other.iter())
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside call to `<std::vec::Vec<T>><MayContainValue<CountDrop>>::extend_from_slice`
--> /home/francis/.rustup/toolchains/nightly-2018-09-15-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/liballoc/slice.rs:168:9
|
168 | vector.extend_from_slice(s);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside call to `std::slice::hack::to_vec::<MayContainValue<CountDrop>>`
--> /home/francis/.rustup/toolchains/nightly-2018-09-15-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/liballoc/slice.rs:369:9
|
369 | hack::to_vec(self)
| ^^^^^^^^^^^^^^^^^^
note: inside call to `std::slice::<impl [T]><MayContainValue<CountDrop>>::to_vec`
--> /home/francis/.rustup/toolchains/nightly-2018-09-15-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/liballoc/vec.rs:1687:9
|
1687 | <[T]>::to_vec(&**self)
| ^^^^^^^^^^^^^^^^^^^^^^
note: inside call to `<std::vec::Vec<T> as std::clone::Clone><MayContainValue<CountDrop>>::clone`
--> /data/src/rust/so-manually-drop-1_29/src/main.rs:54:33
|
54 | assert_eq!(slots.len(), slots.clone().len());
| ^^^^^^^^^^^^^
note: inside call to `tests::check_drops`
--> /data/src/rust/so-manually-drop-1_29/src/main.rs:33:5
|
33 | tests::check_drops();
| ^^^^^^^^^^^^^^^^^^^^
error: aborting due to previous error
For more information about this error, try `rustc --explain E0080`.
(Note: I can get the same error without using Xargo, but then miri doesn't show the source code for the stack frames in std.)
If I do this again with the original definition of ManuallyDrop, then miri doesn't report any issue. This confirms that the new definition of ManuallyDrop causes your program to have undefined behavior.
When I change std::mem::uninitialized() to std::mem::zeroed(), I can reliably reproduce the issue. When running natively, if it happens that the uninitialized memory is all zeroes, then you'll get the issue, otherwise you won't.
By calling std::mem::zeroed(), I've made the program generate null references, which are documented as undefined behavior in Rust. When the vector is cloned, an iterator is used (as shown in miri's output above). Iterator::next returns an Option<T>; that T here has a reference in it (coming from CountDrops), which causes Option's memory layout to be optimized: instead of having a discrete discriminant, it uses a null reference to represent its None value. Since I am generating null references, the iterator returns None on the first item and thus the vector ends up empty.
What's interesting is that when ManuallyDrop was defined as a union, Option's memory layout was not optimized.
println!("{}", std::mem::size_of::<Option<std::mem::ManuallyDrop<CountDrop<'static>>>>());
// prints 16 in Rust 1.28, but 8 in Rust 1.29
There is a discussion about this situation in #52898.
5
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