Merge rayon-rs#1010

1010: Fix ownership invalidation of saved scope panics r=cuviper a=cuviper

Miri complained when we tried to reconstruct a panic `Box` from an
`AtomicPtr`, because a later call to `mem::forget` invalidated that
pointer, even though we just meant not to drop it. `ManuallyDrop` is a
way to accomplish that *before* we take the pointer.

```
test scope::test::panic_propagate_nested_scope_spawn - should panic ... error: Undefined Behavior: trying to retag from <547680> for Unique permission at alloc207942[0x0], but that tag does not exist in the borrow stack for this location
    --> ~/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1014:9
     |
1014 |         Box(unsafe { Unique::new_unchecked(raw) }, alloc)
     |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |         |
     |         trying to retag from <547680> for Unique permission at alloc207942[0x0], but that tag does not exist in the borrow stack for this location
     |         this error occurs as part of retag at alloc207942[0x0..0x10]
     |
     = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows 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: <547680> was created by a SharedReadWrite retag at offsets [0x0..0x10]
    --> rayon-core/src/scope/mod.rs:732:36
     |
732  |             .compare_exchange(nil, &mut *err, Ordering::Release, Ordering::Relaxed)
     |                                    ^^^^^^^^^
help: <547680> was later invalidated at offsets [0x0..0x10] by a Unique retag
    --> rayon-core/src/scope/mod.rs:735:25
     |
735  |             mem::forget(err); // ownership now transferred into self.panic
     |                         ^^^
```


Co-authored-by: Josh Stone <cuviper@gmail.com>

Author: bors[bot]

rayon-core/src/scope/mod.rs

@@ -13,7 +13,7 @@ use crate::unwind;
 use std::any::Any;
 use std::fmt;
 use std::marker::PhantomData;
-use std::mem;
+use std::mem::ManuallyDrop;
 use std::ptr;
 use std::sync::atomic::{AtomicPtr, Ordering};
 use std::sync::Arc;
@@ -725,14 +725,20 @@ impl<'scope> ScopeBase<'scope> {
 
     fn job_panicked(&self, err: Box<dyn Any + Send + 'static>) {
         // capture the first error we see, free the rest
-        let nil = ptr::null_mut();
-        let mut err = Box::new(err); // box up the fat ptr
-        if self
-            .panic
-            .compare_exchange(nil, &mut *err, Ordering::Release, Ordering::Relaxed)
-            .is_ok()
-        {
-            mem::forget(err); // ownership now transferred into self.panic
+        if self.panic.load(Ordering::Relaxed).is_null() {
+            let nil = ptr::null_mut();
+            let mut err = ManuallyDrop::new(Box::new(err)); // box up the fat ptr
+            let err_ptr: *mut Box<dyn Any + Send + 'static> = &mut **err;
+            if self
+                .panic
+                .compare_exchange(nil, err_ptr, Ordering::Release, Ordering::Relaxed)
+                .is_ok()
+            {
+                // ownership now transferred into self.panic
+            } else {
+                // another panic raced in ahead of us, so drop ours
+                let _: Box<Box<_>> = ManuallyDrop::into_inner(err);
+            }
         }
     }