Auto merge of #359 - Nilstrieb:retag-me-im-a-field, r=Amanieu

Fix issue with field retagging in scopeguard

With field retagging in SB, this causes UB because the read out value is invalidated by the move into `mem::forget`. Using `ManuallyDrop<T>` avoids the problem. With that issue fixed, we can now enable field retagging in CI. Field retagging is required to justify some of the `noalias` optimizations that rustc
performs currently.

<details>
<summary>Miri error</summary>

```rust
Undefined Behavior: trying to retag from <565074> for Unique permission at alloc212719[0x20], but that tag does not exist in the borrow stack for this location
    --> src/scopeguard.rs:40:13
     |
40   |             value
     |             ^^^^^
     |             |
     |             trying to retag from <565074> for Unique permission at alloc212719[0x20], but that tag does not exist in the borrow stack for this location
     |             this error occurs as part of retag at alloc212719[0x20..0x40]
     |
     = 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: <565074> was created by a Unique retag at offsets [0x20..0x40]
    --> src/scopeguard.rs:33:25
     |
33   |             let value = ptr::read(&guard.value);
     |                         ^^^^^^^^^^^^^^^^^^^^^^^
help: <565074> was later invalidated at offsets [0x20..0x40] by a Unique retag
    --> src/scopeguard.rs:39:25
     |
39   |             mem::forget(guard);
     |                         ^^^^^
     = note: backtrace:
     = note: inside `scopeguard::ScopeGuard::<&mut raw::RawTable<(i32, i32)>, [closure@src/raw/mod.rs:1637:45: 1637:52]>::into_inner` at src/scopeguard.rs:40:13
note: inside `<raw::RawTable<(i32, i32)> as core::clone::Clone>::clone_from` at src/raw/mod.rs:1671:17
    --> src/raw/mod.rs:1671:17
     |
1671 |                 ScopeGuard::into_inner(self_);
     |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `<map::HashMap<i32, i32> as core::clone::Clone>::clone_from` at src/map.rs:202:9
    --> src/map.rs:202:9
     |
202  |         self.table.clone_from(&source.table);
     |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `map::test_map::test_clone_from` at src/map.rs:6692:9
    --> src/map.rs:6692:9
     |
6692 |         m2.clone_from(&m);
     |         ^^^^^^^^^^^^^^^^^
note: inside closure at src/map.rs:6684:5
    --> src/map.rs:6684:5
     |
6683 |       #[test]
     |       ------- in this procedural macro expansion
6684 | /     fn test_clone_from() {
6685 | |         let mut m = HashMap::new();
6686 | |         let mut m2 = HashMap::new();
6687 | |         assert_eq!(m.len(), 0);
...    |
6695 | |         assert_eq!(m2.len(), 2);
6696 | |     }
     | |_____^
     = 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
```

</details>

ci/miri.sh

@@ -9,4 +9,4 @@ rustup toolchain install nightly --component miri
 rustup override set nightly
 cargo miri setup
 
-cargo miri test
+MIRIFLAGS='-Zmiri-retag-fields' cargo miri test

src/scopeguard.rs

@@ -1,6 +1,6 @@
 // Extracted from the scopeguard crate
 use core::{
-    mem,
+    mem::ManuallyDrop,
     ops::{Deref, DerefMut},
     ptr,
 };
@@ -28,15 +28,13 @@ where
     #[inline]
     pub fn into_inner(guard: Self) -> T {
         // Cannot move out of Drop-implementing types, so
-        // ptr::read the value and forget the guard.
+        // ptr::read the value out of a ManuallyDrop<Self>
+        // Don't use mem::forget as that might invalidate value
+        let guard = ManuallyDrop::new(guard);
         unsafe {
             let value = ptr::read(&guard.value);
-            // read the closure so that it is dropped, and assign it to a local
-            // variable to ensure that it is only dropped after the guard has
-            // been forgotten. (In case the Drop impl of the closure, or that
-            // of any consumed captured variable, panics).
-            let _dropfn = ptr::read(&guard.dropfn);
-            mem::forget(guard);
+            // read the closure so that it is dropped
+            let _ = ptr::read(&guard.dropfn);
             value
         }
     }