Remove stack debug assert (#12861)

# Description
In order for `Stack::unwrap_unique` to work as intended, we currently
manually track all references to the parent stack and ensure that they
are cleared before calling `Stack::unwrap_unique` in the REPL. We also
only call `Stack::unwrap_unique` after all code from the current REPL
entry has finished executing. Since `Value`s cannot store `Stack`
references, then this should have worked in theory. However, we forgot
to account for threads. `run-external` (and maybe the plugin writers)
can spawn threads that clone the `Stack`, holding on to references of
the parent stack. These threads are not waited/joined upon, and so may
finish after the eval has already returned. This PR removes the
`Stack::unwrap_unique` function and associated debug assert that was
[causing
panics](https://gist.github.com/cablehead/f3d2608a1629e607c2d75290829354f7)
like @cablehead found.

# After Submitting
Make values cheaper to clone as a more robust solution to the
performance issues with cloning the stack.

---------

Co-authored-by: Wind <WindSoilder@outlook.com>

crates/nu-cli/src/repl.rs

@@ -542,7 +542,9 @@ fn loop_iteration(ctx: LoopContext) -> (bool, Stack, Reedline) {
     let shell_integration_osc633 = config.shell_integration_osc633;
     let shell_integration_reset_application_mode = config.shell_integration_reset_application_mode;
 
-    let mut stack = Stack::unwrap_unique(stack_arc);
+    // TODO: we may clone the stack, this can lead to major performance issues
+    // so we should avoid it or making stack cheaper to clone.
+    let mut stack = Arc::unwrap_or_clone(stack_arc);
 
     perf(
         "line_editor setup",

crates/nu-protocol/src/engine/stack.rs

@@ -75,26 +75,10 @@ impl Stack {
         }
     }
 
-    /// Unwrap a uniquely-owned stack.
-    ///
-    /// In debug mode, this panics if there are multiple references.
-    /// In production this will instead clone the underlying stack.
-    pub fn unwrap_unique(stack_arc: Arc<Stack>) -> Stack {
-        // If you hit an error here, it's likely that you created an extra
-        // Arc pointing to the stack somewhere. Make sure that it gets dropped before
-        // getting here!
-        Arc::try_unwrap(stack_arc).unwrap_or_else(|arc| {
-            // in release mode, we clone the stack, but this can lead to
-            // major performance issues, so we should avoid it
-            debug_assert!(false, "More than one stack reference remaining!");
-            (*arc).clone()
-        })
-    }
-
     /// Create a new child stack from a parent.
     ///
     /// Changes from this child can be merged back into the parent with
-    /// Stack::with_changes_from_child
+    /// [`Stack::with_changes_from_child`]
     pub fn with_parent(parent: Arc<Stack>) -> Stack {
         Stack {
             // here we are still cloning environment variable-related information
@@ -109,19 +93,17 @@ impl Stack {
         }
     }
 
-    /// Take an Arc of a parent (assumed to be unique), and a child, and apply
+    /// Take an [`Arc`] parent, and a child, and apply all the changes from a child back to the parent.
-    /// all the changes from a child back to the parent.
     ///
-    /// Here it is assumed that child was created with a call to Stack::with_parent
+    /// Here it is assumed that `child` was created by a call to [`Stack::with_parent`] with `parent`.
-    /// with parent
+    ///
+    /// For this to be performant and not clone `parent`, `child` should be the only other
+    /// referencer of `parent`.
     pub fn with_changes_from_child(parent: Arc<Stack>, child: Stack) -> Stack {
         // we're going to drop the link to the parent stack on our new stack
         // so that we can unwrap the Arc as a unique reference
-        //
-        // This makes the new_stack be in a bit of a weird state, so we shouldn't call
-        // any structs
         drop(child.parent_stack);
-        let mut unique_stack = Stack::unwrap_unique(parent);
+        let mut unique_stack = Arc::unwrap_or_clone(parent);
 
         unique_stack
             .vars