VecDeque::iter_mut tramples on the references it returned earlier #74029
Description
I tried running this code with Miri (on playground):
fn test_all_refs<'a, T: 'a>(dummy: &mut T, iter: impl Iterator<Item = &'a mut T>) { // Gather all those references. let mut refs: Vec<&mut T> = iter.collect(); // Use them all. Twice, to be sure we got all interleavings. for r in refs.iter_mut() { std::mem::swap(dummy, r); } for r in refs { std::mem::swap(dummy, r); } } fn main() { let mut v = std::collections::VecDeque::new(); v.push_back(1); v.push_back(2); test_all_refs(&mut 0, v.iter_mut()); }I expected to see it end gracefully.
Instead, it reports:
error: Undefined Behavior: trying to reborrow for Unique, but parent tag <4482> does not have an appropriate item in the borrow stack --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/liballoc/vec.rs:2099:53 | 2099 | ptr::write(vector.as_mut_ptr(), element); | ^^^^^^^ trying to reborrow for Unique, but parent tag <4482> does not have an appropriate item in the borrow stack | = help: this indicates a potential bug in the program: it performed an invalid operation, but the rules it violated are still experimental = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information = note: inside `<std::vec::Vec<&mut i32> as std::vec::SpecExtend<&mut i32, std::collections::vec_deque::IterMut<i32>>>::from_iter` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/liballoc/vec.rs:2099:53 = note: inside `<std::vec::Vec<&mut i32> as std::iter::FromIterator<&mut i32>>::from_iter::<std::collections::vec_deque::IterMut<i32>>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/liballoc/vec.rs:1995:9 = note: inside `<std::collections::vec_deque::IterMut<i32> as std::iter::Iterator>::collect::<std::vec::Vec<&mut i32>>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/iter/traits/iterator.rs:1671:9 note: inside `test_all_refs::<i32, std::collections::vec_deque::IterMut<i32>>` at src/main.rs:3:33 --> src/main.rs:3:33 | 3 | let mut refs: Vec<&mut T> = iter.collect(); | ^^^^^^^^^^^^^^ note: inside `main` at src/main.rs:17:5 --> src/main.rs:17:5 | 17 | test_all_refs(&mut 0, v.iter_mut()); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ = note: inside closure at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:67:34 = note: inside closure at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:52:73 = note: inside `std::sys_common::backtrace::__rust_begin_short_backtrace::<[closure@std::rt::lang_start_internal::{{closure}}#0::{{closure}}#0 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/sys_common/backtrace.rs:130:5 = note: inside closure at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:52:13 = note: inside `std::panicking::r#try::do_call::<[closure@std::rt::lang_start_internal::{{closure}}#0 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panicking.rs:342:40 = note: inside `std::panicking::r#try::<i32, [closure@std::rt::lang_start_internal::{{closure}}#0 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe]>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panicking.rs:319:15 = note: inside `std::panic::catch_unwind::<[closure@std::rt::lang_start_internal::{{closure}}#0 0:&&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panic.rs:394:14 = note: inside `std::rt::lang_start_internal` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:51:25 = note: inside `std::rt::lang_start::<()>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:67:5