Fix Scope lifetime variance

Author: Zoxc

rayon-core/src/scope/mod.rs

@@ -33,6 +33,9 @@ impl<'scope> ScopeBuilder<'scope> {
         }
     }
 
+    // FIXME: &'scope Scope<'scope> means that we can spawn scopes during `scope` and after it.
+    // and before it.
+    // FIXME: Scopes spawned outside this function must not have access to `tlv`
     pub fn scope<OP, R>(&'scope mut self, op: OP) -> R
     where
         OP: FnOnce(&'scope Scope<'scope>) -> R + 'scope,
@@ -81,7 +84,9 @@ pub struct Scope<'scope> {
     /// all of which outlive `'scope`.  They're not actually required to be
     /// `Sync`, but it's still safe to let the `Scope` implement `Sync` because
     /// the closures are only *moved* across threads to be executed.
-    marker: PhantomData<&'scope ()>,
+    /// This also ensures 'scope is invariant, which is required for safety,
+    /// since otherwise you could shrink it as spawn closures which reference shorter data.
+    marker: PhantomData<Box<FnOnce(&Scope<'scope>) + Send + Sync + 'scope>>,
 
     /// The TLV at the scope's creation. Used to set the TLV for spawned jobs.
     tlv: usize,