Enforce that nobody can push/pop from the global environment stack

This is just a precaution.

src/env/environment.rs

@@ -173,12 +173,14 @@ fn lock(&self) -> EnvMutexGuard<EnvScopedImpl> {
 /// This backs the parser's "vars".
 pub struct EnvStack {
     inner: EnvMutex<EnvStackImpl>,
+    can_push_pop: bool, // If false, panic on push/pop. Used for the global stack.
 }
 
 impl EnvStack {
     pub fn new() -> EnvStack {
         EnvStack {
             inner: EnvStackImpl::new(),
+            can_push_pop: true,
         }
     }
 
@@ -291,6 +293,7 @@ pub fn remove(&self, key: &wstr, mode: EnvMode) -> EnvStackSetResult {
 
     /// Push the variable stack. Used for implementing local variables for functions and for-loops.
     pub fn push(&self, new_scope: bool) {
+        assert!(self.can_push_pop, "push/pop not allowed on global stack");
         let mut imp = self.lock();
         if new_scope {
             imp.push_shadowing();
@@ -301,6 +304,7 @@ pub fn push(&self, new_scope: bool) {
 
     /// Pop the variable stack. Used for implementing local variables for functions and for-loops.
     pub fn pop(&self) {
+        assert!(self.can_push_pop, "push/pop not allowed on global stack");
         let popped = self.lock().pop();
         // Only dispatch variable changes if we are the principal environment.
         if self.is_principal() {
@@ -363,7 +367,10 @@ pub fn universal_sync(&self, always: bool) -> Vec<Event> {
     pub fn globals() -> &'static EnvStack {
         use std::sync::OnceLock;
         static GLOBALS: OnceLock<EnvStack> = OnceLock::new();
-        GLOBALS.get_or_init(EnvStack::new)
+        GLOBALS.get_or_init(|| EnvStack {
+            inner: EnvStackImpl::new(),
+            can_push_pop: false,
+        })
     }
 
     /// Access the principal variable stack, associated with the principal parser.

src/tests/env.rs

@@ -1,4 +1,4 @@
-use crate::env::{EnvMode, EnvVar, EnvVarFlags, Environment};
+use crate::env::{EnvMode, EnvStack, EnvVar, EnvVarFlags, Environment};
 use crate::parser::Parser;
 use crate::tests::prelude::*;
 use crate::wchar::prelude::*;
@@ -177,3 +177,16 @@ fn test_env_snapshot() {
     vars.pop();
     popd();
 }
+
+// Can't push/pop from globals.
+#[test]
+#[should_panic]
+fn test_no_global_push() {
+    EnvStack::globals().push(true);
+}
+
+#[test]
+#[should_panic]
+fn test_no_global_pop() {
+    EnvStack::globals().pop();
+}