running tests under miri fails

[matklad]

Hi! I've tried running the test suite with miri and got the following result:

running 62 tests
error[E0080]: constant evaluation error: stopping looking for borrow being accessed (Alias(None)) because of barrier (88940)
    --> /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/slice/mod.rs:5108:5
     |
5108 |     Repr { raw: FatPtr { data, len } }.rust_mut
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ stopping looking for borrow being accessed (Alias(None)) because of barrier (88940)
     |

error[E0080]: constant evaluation error: stopping looking for borrow being accessed (Alias(None)) because of barrier (38549)
    --> /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/slice/mod.rs:5108:5
     |
5108 |     Repr { raw: FatPtr { data, len } }.rust_mut
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ stopping looking for borrow being accessed (Alias(None)) because of barrier (38549)
     |
     = note: inside call to `std::slice::from_raw_parts_mut::<test::Node>` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/liballoc/vec.rs:1712:13
     = note: inside call to `<std::vec::Vec<test::Node> as std::ops::DerefMut>::deref_mut` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/liballoc/vec.rs:1026:23
note: inside call to `std::vec::Vec::<test::Node>::push` at src/lib.rs:169:13
    --> src/lib.rs:169:13
     |
169  |             chunks.current.push(value);
     |             ^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside call to `Arena::<test::Node>::alloc_fast_path` at src/lib.rs:161:9
    --> src/lib.rs:161:9
     |
161  |         self.alloc_fast_path(value)
     |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside call to `Arena::<test::Node>::alloc` at src/test.rs:24:16
    --> src/test.rs:24:16
     |
24   |         node = arena.alloc(Node(Some(node), 2, DropTracker(&drop_counter)));
     |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside call to `test::arena_as_intended` at src/test.rs:16:1
    --> src/test.rs:16:1
     |
16   | / fn arena_as_intended() {
17   | |     let drop_counter = Cell::new(0);
18   | |     {
19   | |         let arena = Arena::with_capacity(2);
...    |
60   | |     assert_eq!(drop_counter.get(), 7);
61   | | }
     | |_^
     = note: inside call to closure at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/ops/function.rs:231:5
     = note: inside call to `<[closure@src/test.rs:16:1: 61:2] as std::ops::FnOnce<()>>::call_once - shim` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/ops/function.rs:231:5
     = note: inside call to `<fn() as std::ops::FnOnce<()>>::call_once - shim(fn())` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libtest/lib.rs:1522:5
     = note: inside call to `test::test::__rust_begin_short_backtrace::<fn()>` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libtest/lib.rs:1513:30
     = note: inside call to closure at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/ops/function.rs:231:5
     = note: inside call to `<[closure@DefId(9/1:109 ~ test[6977]::run_test[0]::{{closure}}[3]) 0:fn()] as std::ops::FnOnce<()>>::call_once - shim` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/ops/function.rs:231:5
     = note: inside call to `<[closure@DefId(9/1:109 ~ test[6977]::run_test[0]::{{closure}}[3]) 0:fn()] as std::ops::FnOnce<()>>::call_once - shim(vtable)` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/liballoc/boxed.rs:704:9
     = note: inside call to `<std::boxed::Box<dyn std::boxed::FnBox<(), Output = ()> + std::marker::Send> as std::ops::FnOnce<()>>::call_once` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panic.rs:309:9
     = note: inside call to `<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::boxed::FnBox<(), Output = ()> + std::marker::Send>> as std::ops::FnOnce<()>>::call_once` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panicking.rs:293:40
     = note: inside call to `std::panicking::try::do_call::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::boxed::FnBox<(), Output = ()> + std::marker::Send>>, ()>` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panicking.rs:289:5
     = note: inside call to `std::panicking::try::<(), std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::boxed::FnBox<(), Output = ()> + std::marker::Send>>>` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panic.rs:388:9
     = note: inside call to `std::panic::catch_unwind::<std::panic::AssertUnwindSafe<std::boxed::Box<dyn std::boxed::FnBox<(), Output = ()> + std::marker::Send>>, ()>` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libtest/lib.rs:1468:26
     = note: inside call to closure at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libtest/lib.rs:1490:13
     = note: inside call to `test::test::run_test::run_test_inner` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libtest/lib.rs:1509:28
     = note: inside call to `test::test::run_test` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libtest/lib.rs:1167:13
     = note: inside call to `test::test::run_tests::<[closure@DefId(9/1:80 ~ test[6977]::run_tests_console[0]::{{closure}}[2]) 0:&mut test::test::ConsoleTestState, 1:&mut std::boxed::Box<dyn test::test::formatters::OutputFormatter>]>` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libtest/lib.rs:972:5
     = note: inside call to `test::test::run_tests_console` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libtest/lib.rs:293:15
     = note: inside call to `test::test::test_main` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libtest/lib.rs:327:5
     = note: inside call to `test::test::test_main_static`
     = note: inside call to `main` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:64:34
     = note: inside call to closure at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:52:53
     = note: inside call to closure at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panicking.rs:293:40
     = note: inside call to `std::panicking::try::do_call::<[closure@DefId(1/1:1833 ~ std[c18b]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panicking.rs:289:5
     = note: inside call to `std::panicking::try::<i32, [closure@DefId(1/1:1833 ~ std[c18b]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe]>` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/panic.rs:388:9
     = note: inside call to `std::panic::catch_unwind::<[closure@DefId(1/1:1833 ~ std[c18b]::rt[0]::lang_start_internal[0]::{{closure}}[0]) 0:&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe], i32>` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:52:25
     = note: inside call to `std::rt::lang_start_internal` at /home/matklad/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:64:5
     = note: inside call to `std::rt::lang_start::<()>`

@RalfJung says that this error means that code invokes UB ^^

EDIT: pasted the wrgon detail initially, updated

[RalfJung]

that this error means that code invokes UB ^^

Well, that or the model isn't what we think it should be.

[RalfJung]

I looked into this. The problem is as follows:

• The first arena.alloc returns a shared reference into one of the Vec<T> that hold the actual data.
• The second arena.alloc pushes to that same Vec after ensuring that there is enough capacity.
• As part of Vec::push, a mutable reference to all elements in the Vec (as a slice) gets created. This mutable reference aliases with the shared reference that was handed out by the first arena.alloc -> UB.

The mutable reference gets created by self.as_mut_ptr(), which implicitly is self.deref_mut().as_mut_ptr(), and the deref_mut() creates and returns said reference. The irony is that it gets turned into a raw pointer immediately, but at that point the damage is already done.

This crate here cannot really do anything about that. If this is a pattern that Vec wishes to support, it's push implementation has to be adjusted accordingly.

[RalfJung]

Reported upstream as rust-lang/rust#60847

[RalfJung]

This should be fixed by rust-lang/rust#61114.

[matklad]

confirm that this is not the issue anymore!

[RalfJung]

We also have Miri on CI now for this project: #27