Fix merging child stack into parent (#12426)

# Description
Fixes #12423 where changes to mutable variables are not properly
persisted after a REPL entry.

crates/nu-plugin-test-support/src/plugin_test.rs

@@ -337,12 +337,6 @@ impl PluginTest {
                 // All equal, and same length
                 Ok(true)
             }
-            (Value::Range { val: a_rng, .. }, Value::Range { val: b_rng, .. }) => {
-                Ok(a_rng.inclusion == b_rng.inclusion
-                    && self.value_eq(&a_rng.from, &b_rng.from)?
-                    && self.value_eq(&a_rng.to, &b_rng.to)?
-                    && self.value_eq(&a_rng.incr, &b_rng.incr)?)
-            }
             // Must collect lazy records to compare.
             (Value::LazyRecord { val: a_val, .. }, _) => self.value_eq(&a_val.collect()?, b),
             (_, Value::LazyRecord { val: b_val, .. }) => self.value_eq(a, &b_val.collect()?),

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

@@ -112,7 +112,7 @@ impl Stack {
     ///
     /// Here it is assumed that child was created with a call to Stack::with_parent
     /// with parent
-    pub fn with_changes_from_child(parent: Arc<Stack>, mut child: Stack) -> Stack {
+    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
         //
@@ -121,15 +121,18 @@ impl Stack {
         drop(child.parent_stack);
         let mut unique_stack = Stack::unwrap_unique(parent);
 
-        unique_stack.vars.append(&mut child.vars);
+        unique_stack
+            .vars
+            .retain(|(var, _)| !child.parent_deletions.contains(var));
+        for (var, value) in child.vars {
+            unique_stack.add_var(var, value);
+        }
         unique_stack.env_vars = child.env_vars;
         unique_stack.env_hidden = child.env_hidden;
         unique_stack.active_overlays = child.active_overlays;
         unique_stack
-            .vars
-            .retain(|(var, _)| !child.parent_deletions.contains(var));
-        unique_stack
     }
+
     pub fn with_env(
         &mut self,
         env_vars: &[EnvVars],

src/test_bins.rs

@@ -8,7 +8,10 @@ use nu_protocol::{
     PipelineData, Value,
 };
 use nu_std::load_standard_library;
-use std::io::{self, BufRead, Read, Write};
+use std::{
+    io::{self, BufRead, Read, Write},
+    sync::Arc,
+};
 
 /// Echo's value of env keys from args
 /// Example: nu --testbin env_echo FOO BAR
@@ -236,7 +239,7 @@ pub fn nu_repl() {
     let source_lines = args();
 
     let mut engine_state = get_engine_state();
-    let mut stack = Stack::new();
+    let mut top_stack = Arc::new(Stack::new());
 
     engine_state.add_env_var("PWD".into(), Value::test_string(cwd.to_string_lossy()));
 
@@ -245,6 +248,7 @@ pub fn nu_repl() {
     load_standard_library(&mut engine_state).expect("Could not load the standard library.");
 
     for (i, line) in source_lines.iter().enumerate() {
+        let mut stack = Stack::with_parent(top_stack.clone());
         let cwd = nu_engine::env::current_dir(&engine_state, &stack)
             .unwrap_or_else(|err| outcome_err(&engine_state, &err));
 
@@ -324,13 +328,15 @@ pub fn nu_repl() {
         let input = PipelineData::empty();
         let config = engine_state.get_config();
 
-        let stack = &mut stack.start_capture();
-        match eval_block::<WithoutDebug>(&engine_state, stack, &block, input) {
-            Ok(pipeline_data) => match pipeline_data.collect_string("", config) {
-                Ok(s) => last_output = s,
+        {
+            let stack = &mut stack.start_capture();
+            match eval_block::<WithoutDebug>(&engine_state, stack, &block, input) {
+                Ok(pipeline_data) => match pipeline_data.collect_string("", config) {
+                    Ok(s) => last_output = s,
+                    Err(err) => outcome_err(&engine_state, &err),
+                },
                 Err(err) => outcome_err(&engine_state, &err),
-            },
-            Err(err) => outcome_err(&engine_state, &err),
+            }
         }
 
         if let Some(cwd) = stack.get_env_var(&engine_state, "PWD") {
@@ -340,6 +346,7 @@ pub fn nu_repl() {
             let _ = std::env::set_current_dir(path.as_ref());
             engine_state.add_env_var("PWD".into(), cwd);
         }
+        top_stack = Arc::new(Stack::with_changes_from_child(top_stack, stack));
     }
 
     outcome_ok(last_output)

tests/shell/mod.rs

@@ -7,6 +7,7 @@ use pretty_assertions::assert_eq;
 mod environment;
 
 mod pipeline;
+mod repl;
 
 //FIXME: jt: we need to focus some fixes on wix as the plugins will differ
 #[ignore]

tests/shell/repl.rs

@@ -0,0 +1,9 @@
+use nu_test_support::{nu, nu_repl_code};
+use pretty_assertions::assert_eq;
+
+#[test]
+fn mut_variable() {
+    let lines = &["mut x = 0", "$x = 1", "$x"];
+    let actual = nu!(nu_repl_code(lines));
+    assert_eq!(actual.out, "1");
+}