WIPvariance

tools/analysis/traceevent/src/buffer.rs

@@ -2,6 +2,7 @@ use core::{
     fmt::{Debug, Formatter},
     ops::{Deref, DerefMut as _},
     marker::PhantomData,
+    cell::UnsafeCell,
 };
 use std::{
     collections::{btree_map::Entry, BTreeMap},
@@ -95,11 +96,16 @@ pub struct EventVisitor<'i, 'h, 'edm, InitDescF, T = ()> {
     pub timestamp: Timestamp,
     pub buffer_id: &'h BufferId,
 
-    // Using *mut here means EventVisitor is invariant in any lifetime contained in T.
-    // However, the only values we store in the EventDescMap are either owned by it or have a
-    // longer lifetime ('h outlives 'edm), so it's sound to be covariant in 'edm.
-    // So in practice we use 'static but then we cast back to 'h.
-    desc_map: *mut EventDescMap<'static, T, InitDescF>,
+    // Using UnsafeCell ensures that the compiler understands that anything derived from what we
+    // stored in it can change at any time, even if the EventVisitor is only manipulated via shared
+    // ref.
+    desc_map: UnsafeCell<
+        // Using *mut here means EventVisitor is invariant in any lifetime contained in T.
+        // However, the only values we store in the EventDescMap are either owned by it or have a
+        // longer lifetime ('h outlives 'edm), so it's sound to be covariant in 'edm.  So in
+        // practice we use 'static but then we cast back to 'h.
+        *mut EventDescMap<'static, T, InitDescF>
+    >,
     _phantom_desc_map: PhantomData<&'edm mut ()>,
 
     scratch: &'i ScratchAlloc,
@@ -139,7 +145,7 @@ impl<'i, 'h, 'edm, InitDescF, T> EventVisitor<'i, 'h, 'edm, InitDescF, T> {
             timestamp,
             buffer_id,
             scratch,
-            desc_map,
+            desc_map: UnsafeCell::new(desc_map),
             event_desc: OnceCell::new(),
             _phantom_desc_map: PhantomData,
         }
@@ -262,13 +268,13 @@ where
         Ok(&desc.name)
     }
 
-    fn desc_map(&self) -> &'edm mut EventDescMap<'h, T, InitDescF> {
+    fn desc_map(&'edm self) -> &'edm mut EventDescMap<'h, T, InitDescF> {
         // SAFETY: This comes from an &'edm mut reference in the first place, and since
         // EventVisitor::new() requires an &'edm mut EventDescMap, we cannot accidentally
         // borrow it mutably more than once. This makes it safe to turn it back to an &'edm
         // mut.
-        let desc_map: *mut EventDescMap<'static, T, InitDescF> = self.desc_map;
-        let desc_map: &'edm mut EventDescMap<'static, T, InitDescF> = unsafe { &mut *desc_map };
+        let desc_map: *mut *mut EventDescMap<'static, T, InitDescF> = self.desc_map.get();
+        let desc_map: &'edm mut EventDescMap<'static, T, InitDescF> = unsafe { &mut **desc_map };
         let desc_map: &'edm mut EventDescMap<'h, T, InitDescF> = unsafe{core::mem::transmute(desc_map)};
         desc_map
     }