channel: stacked borrows violation with -Zmiri-tag-raw-pointers

[taiki-e]

MIRIFLAGS="-Zmiri-tag-raw-pointers -Zmiri-disable-isolation -Zmiri-ignore-leaks" \
    cargo miri test -p crossbeam-channel --test mpsc -- select_tests::sync2

output

error: Undefined Behavior: trying to reborrow for SharedReadWrite at alloc509444, but parent tag <untagged> does not have an appropriate item in the borrow stack
    --> /Users/taiki/projects/crossbeam/crossbeam-channel/src/flavors/zero.rs:137:22
     |
137  |         let packet = &*(token.zero as *const Packet<T>);
     |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ trying to reborrow for SharedReadWrite at alloc509444, but parent tag <untagged> 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 `crossbeam_channel::flavors::zero::Channel::<i32>::write` at /Users/taiki/projects/crossbeam/crossbeam-channel/src/flavors/zero.rs:137:22
     = note: inside `crossbeam_channel::flavors::zero::Channel::<i32>::send` at /Users/taiki/projects/crossbeam/crossbeam-channel/src/flavors/zero.rs:219:17
     = note: inside `crossbeam_channel::Sender::<i32>::send` at /Users/taiki/projects/crossbeam/crossbeam-channel/src/channel.rs:428:41
note: inside `SyncSender::<i32>::send` at crossbeam-channel/tests/mpsc.rs:54:9
    --> crossbeam-channel/tests/mpsc.rs:54:9
     |
54   |         self.inner.send(t).map_err(|cc::SendError(m)| SendError(m))
     |         ^^^^^^^^^^^^^^^^^^
note: inside closure at crossbeam-channel/tests/mpsc.rs:2099:13
    --> crossbeam-channel/tests/mpsc.rs:2099:13
     |
2099 |             tx.send(1).unwrap();
     |             ^^^^^^^^^^
     = note: inside `std::sys_common::backtrace::__rust_begin_short_backtrace::<[closure@crossbeam-channel/tests/mpsc.rs:2095:31: 2100:10], ()>` at /Users/taiki/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/sys_common/backtrace.rs:123:18
     = note: inside closure at /Users/taiki/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/thread/mod.rs:477:17
     = note: inside `<std::panic::AssertUnwindSafe<[closure@std::thread::Builder::spawn_unchecked<[closure@crossbeam-channel/tests/mpsc.rs:2095:31: 2100:10], ()>::{closure#1}::{closure#0}]> as std::ops::FnOnce<()>>::call_once` at /Users/taiki/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/panic/unwind_safe.rs:271:9
     = note: inside `std::panicking::r#try::do_call::<std::panic::AssertUnwindSafe<[closure@std::thread::Builder::spawn_unchecked<[closure@crossbeam-channel/tests/mpsc.rs:2095:31: 2100:10], ()>::{closure#1}::{closure#0}]>, ()>` at /Users/taiki/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/panicking.rs:406:40
     = note: inside `std::panicking::r#try::<(), std::panic::AssertUnwindSafe<[closure@std::thread::Builder::spawn_unchecked<[closure@crossbeam-channel/tests/mpsc.rs:2095:31: 2100:10], ()>::{closure#1}::{closure#0}]>>` at /Users/taiki/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/panicking.rs:370:19
     = note: inside `std::panic::catch_unwind::<std::panic::AssertUnwindSafe<[closure@std::thread::Builder::spawn_unchecked<[closure@crossbeam-channel/tests/mpsc.rs:2095:31: 2100:10], ()>::{closure#1}::{closure#0}]>, ()>` at /Users/taiki/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/panic.rs:133:14
     = note: inside closure at /Users/taiki/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/thread/mod.rs:476:30
     = note: inside `<[closure@std::thread::Builder::spawn_unchecked<[closure@crossbeam-channel/tests/mpsc.rs:2095:31: 2100:10], ()>::{closure#1}] as std::ops::FnOnce<()>>::call_once - shim(vtable)` at /Users/taiki/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
     = note: inside `<std::boxed::Box<dyn std::ops::FnOnce()> as std::ops::FnOnce<()>>::call_once` at /Users/taiki/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1854:9
     = note: inside `<std::boxed::Box<std::boxed::Box<dyn std::ops::FnOnce()>> as std::ops::FnOnce<()>>::call_once` at /Users/taiki/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1854:9
     = note: inside `std::sys::unix::thread::Thread::new::thread_start` at /Users/taiki/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/std/src/sys/unix/thread.rs:108:17

If run without -Zmiri-tag-raw-pointers, the test succeeds.

cc #578

[RalfJung]

"parent tag <untagged>" indicates a ptr created by a cast from usize, which is not supported with -Zmiri-tag-raw-pointers.

[taiki-e]

@RalfJung Thanks. I filed #764 to remove ptr-to-int casts (and fix another stacked borrows violation in channel).