Add note about possible safety issue with GLOBAL_NODE

src/env/environment_impl.rs

@@ -273,6 +273,10 @@ impl Iterator for EnvNodeIter {
 }
 
 lazy_static! {
+    ///  XXX: Possible safety issue here as EnvNodeRef is not Send/Sync and shouldn't
+    ///  be placed in a static context without some sort of Send/Sync wrapper.
+    ///  lazy_static papers over this but it triggers rust lints if you use
+    ///  once_cell::sync::Lazy or std::sync::OnceLock instead.
     static ref GLOBAL_NODE: EnvNodeRef = EnvNodeRef::new(false, None);
 }
 
@@ -699,6 +703,7 @@ pub struct EnvStackImpl {
 impl EnvStackImpl {
     /// Return a new impl representing global variables, with a single local scope.
     pub fn new() -> EnvMutex<EnvStackImpl> {
+        // XXX: Safety issue: We are accessing GLOBAL_NODE without having the global mutex locked!
         let globals = GLOBAL_NODE.clone();
         let locals = EnvNodeRef::new(false, None);
         let base = EnvScopedImpl::new(locals, globals);