WIPvariance

tools/analysis/traceevent/src/buffer.rs

@@ -128,6 +128,8 @@ pub struct EventVisitor<'i, 'h, 'edm, InitDescF, T = ()> {
     // the only value we store back in the Cell is 'static (None) so that invariance is overkill.
     // In order to stay invariant in 'h, we use *mut insteaf of &'edm mut.
     desc_map: Cell<Option<*mut EventDescMap<T, InitDescF>>>,
+    _phantom_desc_map: PhantomData<&'edm mut ()>,
+
     scratch: &'i ScratchAlloc,
 
     // Unfortunately for now OnceCell<'h> makes EventVisitor invariant in 'h:
@@ -139,14 +141,6 @@ pub struct EventVisitor<'i, 'h, 'edm, InitDescF, T = ()> {
     // stored when the EventVisitor is created.
     event_desc: OnceCell<(*const EventDesc, *const T)>,
 
-    _phantom: PhantomData<(
-        &'h(),
-        // This is a lie. In reality, we have the equivalent of an &'edm mut EventDescMap, but we
-        // can't use &'edm mut (or *mut) because otherwise we conservatively become invariant in
-        // 'edm, whereas covariance is appropriate given how it is used (set once, with no risk of
-        // storing a value with lifetime that is too restricted).
-        &'edm ()
-    )>,
 }
 
 impl<'i, 'h, 'edm, InitDescF, T> EventVisitor<'i, 'h, 'edm, InitDescF, T> {
@@ -166,7 +160,7 @@ impl<'i, 'h, 'edm, InitDescF, T> EventVisitor<'i, 'h, 'edm, InitDescF, T> {
             scratch,
             desc_map: Cell::new(Some(desc_map)),
             event_desc: OnceCell::new(),
-            _phantom: PhantomData,
+            _phantom_desc_map: PhantomData,
         }
     }