miri shows undefined behaviour when run with -Zmiri-tag-raw-pointers

[jrmuizel]

test tests::extra_free ... error: Undefined Behavior: trying to reborrow <248731> for SharedReadWrite permission at alloc96889[0x22], but that tag only grants SharedReadOnly permission for this location
    --> /Users/jrmuizel/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:487:1
     |
487  | pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     | |
     | trying to reborrow <248731> for SharedReadWrite permission at alloc96889[0x22], but that tag only grants SharedReadOnly permission for this location
     | this error occurs as part of a reborrow at alloc96889[0x0..0x28]
     |
     = 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
help: <248731> was created by a retag at offsets [0x0..0x28]
    --> src/collect.rs:311:24
     |
311  |             white.push(s.into());
     |                        ^^^^^^^^
     = note: inside `std::ptr::drop_in_place::<dyn cc_box_ptr::CcBoxPtr> - shim(Some(dyn cc_box_ptr::CcBoxPtr))` at /Users/jrmuizel/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:487:1
note: inside `drop_value` at src/lib.rs:922:5
    --> src/lib.rs:922:5
     |
922  |     ptr::drop_in_place(ptr.as_ptr());
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `collect::collect_roots` at src/collect.rs:338:18
    --> src/collect.rs:338:18
     |
338  |         unsafe { crate::drop_value(*i) };
     |                  ^^^^^^^^^^^^^^^^^^^^^
note: inside `collect::collect_cycles` at src/collect.rs:192:5
    --> src/collect.rs:192:5
     |
192  |     collect_roots();
     |     ^^^^^^^^^^^^^^^
note: inside `tests::extra_free` at src/lib.rs:1367:13
    --> src/lib.rs:1367:13
     |
1367 |             collect_cycles(); // <- incorrectly? frees env_a.
     |             ^^^^^^^^^^^^^^^^
note: inside closure at src/lib.rs:1314:5
    --> src/lib.rs:1314:5
     |
1313 |       #[test]
     |       ------- in this procedural macro expansion
1314 | /     fn extra_free() {
1315 | |         struct Env {
1316 | |             pub closures: Vec<Cc<RefCell<Clos>>>,
1317 | |             pub next: Option<Cc<Env>>,
...    |
1381 | |         collect_cycles();
1382 | |     }
     | |_____^
     = note: this error originates in the attribute macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

[jrmuizel]

I put a fix here: https://github.com/fitzgen/bacon-rajan-cc/tree/fix-track-raw-ptr