Rollup merge of #66705 - pitdicker:atomic_mut_ptr, r=KodrAus

Atomic as_mut_ptr

I encountered the following pattern a few times: In Rust we use some atomic type like `AtomicI32`, and an FFI interface exposes this as `*mut i32` (or some similar `libc` type).

It was not obvious to me if a just transmuting a pointer to the atomic was acceptable, or if this should use a cast that goes through an `UnsafeCell`. See https://github.com/rust-lang/rust/issues/66136#issuecomment-557802477

Transmuting the pointer directly:
```rust
let atomic = AtomicI32::new(1);
let ptr = &atomic as *const AtomicI32 as *mut i32;
unsafe {
    ffi(ptr);
}
```

A dance with `UnsafeCell`:
```rust
let atomic = AtomicI32::new(1);
unsafe {
    let ptr = (&*(&atomic as *const AtomicI32 as *const UnsafeCell<i32>)).get();
    ffi(ptr);
}
```

Maybe in the end both ways could be valid. But why not expose a direct method to get a pointer from the standard library?

An `as_mut_ptr` method on atomics can be safe, because only the use of the resulting pointer is where things can get unsafe. I documented its use for FFI, and "Doing non-atomic reads and writes on the resulting integer can be a data race."

The standard library could make use this method in a few places in the WASM module.

cc @RalfJung as you answered my original question.

3 files changed, 5 insertions, 5 deletions

diff --git a/src/libstd/sys/wasm/alloc.rs b/src/libstd/sys/wasm/alloc.rs
index c1af6ec1262..05e55334ac0 100644
--- a/src/libstd/sys/wasm/alloc.rs
+++ b/src/libstd/sys/wasm/alloc.rs
@@ -67,7 +67,7 @@ mod lock {
             //
             //     unsafe {
             //         let r = core::arch::wasm32::i32_atomic_wait(
-            //             &LOCKED as *const AtomicI32 as *mut i32,
+            //             LOCKED.as_mut_ptr(),
             //             1,  //     expected value
             //             -1, //     timeout
             //         );
@@ -143,7 +143,7 @@ mod lock {
             //
             //     unsafe {
             //         core::arch::wasm32::atomic_notify(
-            //             &LOCKED as *const AtomicI32 as *mut i32,
+            //             LOCKED.as_mut_ptr(),
             //             1, //     only one thread
             //         );
             //     }
diff --git a/src/libstd/sys/wasm/condvar_atomics.rs b/src/libstd/sys/wasm/condvar_atomics.rs
index 580d2121844..f452bbd3487 100644
--- a/src/libstd/sys/wasm/condvar_atomics.rs
+++ b/src/libstd/sys/wasm/condvar_atomics.rs
@@ -89,6 +89,6 @@ impl Condvar {
     #[inline]
     fn ptr(&self) -> *mut i32 {
         assert_eq!(mem::size_of::<usize>(), mem::size_of::<i32>());
-        &self.cnt as *const AtomicUsize as *mut i32
+        self.cnt.as_mut_ptr() as *mut i32
     }
 }
diff --git a/src/libstd/sys/wasm/mutex_atomics.rs b/src/libstd/sys/wasm/mutex_atomics.rs
index 0e4f3d80aa9..cddd584dd22 100644
--- a/src/libstd/sys/wasm/mutex_atomics.rs
+++ b/src/libstd/sys/wasm/mutex_atomics.rs
@@ -56,7 +56,7 @@ impl Mutex {
     #[inline]
     fn ptr(&self) -> *mut i32 {
         assert_eq!(mem::size_of::<usize>(), mem::size_of::<i32>());
-        &self.locked as *const AtomicUsize as *mut isize as *mut i32
+        self.locked.as_mut_ptr() as *mut i32
     }
 }
 
@@ -145,6 +145,6 @@ impl ReentrantMutex {
 
     #[inline]
     fn ptr(&self) -> *mut i32 {
-        &self.owner as *const AtomicU32 as *mut i32
+        self.owner.as_mut_ptr() as *mut i32
     }
 }