++= appendAssign operator (#7346) (#7354)

# Description

Closes  https://github.com/nushell/nushell/issues/7346



# Tests + Formatting
```
> mut a = [1 2 3]
> $a ++= [4 5 6]
> $a
[1 2 3 4 5 6]
```

crates/nu-command/src/core_commands/help_operators.rs

@@ -81,6 +81,13 @@ fn generate_operator_info() -> Vec<OperatorInfo> {
             description: "Adds a value to a variable.".into(),
             precedence: 10,
         },
+        OperatorInfo {
+            op_type: "Assignment".into(),
+            operator: "++=".into(),
+            name: "AppendAssign".into(),
+            description: "Appends a list or a value to a variable.".into(),
+            precedence: 10,
+        },
         OperatorInfo {
             op_type: "Assignment".into(),
             operator: "-=".into(),

crates/nu-command/tests/commands/assignment/append_assign.rs

@@ -0,0 +1,108 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn append_assign_int() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+            mut a = [1 2];
+            $a ++= [3 4];
+            $a
+        "#
+    ));
+
+    let expected = nu!(
+        cwd: ".", pipeline(
+        r#"
+            [1 2 3 4]
+        "#
+    ));
+
+    print!("{}", actual.out);
+    print!("{}", expected.out);
+    assert_eq!(actual.out, expected.out);
+}
+
+#[test]
+fn append_assign_string() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+            mut a = [a b];
+            $a ++= [c d];
+            $a
+        "#
+    ));
+
+    let expected = nu!(
+        cwd: ".", pipeline(
+        r#"
+            [a b c d]
+        "#
+    ));
+
+    print!("{}", actual.out);
+    print!("{}", expected.out);
+    assert_eq!(actual.out, expected.out);
+}
+
+#[test]
+fn append_assign_any() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+            mut a = [1 2 a];
+            $a ++= [b 3];
+            $a
+        "#
+    ));
+
+    let expected = nu!(
+        cwd: ".", pipeline(
+        r#"
+            [1 2 a b 3]
+        "#
+    ));
+
+    print!("{}", actual.out);
+    print!("{}", expected.out);
+    assert_eq!(actual.out, expected.out);
+}
+
+#[test]
+fn append_assign_both_empty() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+            mut a = [];
+            $a ++= [];
+            $a
+        "#
+    ));
+
+    let expected = nu!(
+        cwd: ".", pipeline(
+        r#"
+            []
+        "#
+    ));
+
+    print!("{}", actual.out);
+    print!("{}", expected.out);
+    assert_eq!(actual.out, expected.out);
+}
+
+#[test]
+fn append_assign_type_mismatch() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+            mut a = [1 2];
+            $a ++= [a];
+        "#
+    ));
+
+    assert!(actual
+        .err
+        .contains("expected list<int>, found list<string>"));
+}

crates/nu-command/tests/commands/assignment/mod.rs

@@ -0,0 +1 @@
+mod append_assign;

crates/nu-command/tests/commands/mod.rs

@@ -2,6 +2,7 @@ mod alias;
 mod all;
 mod any;
 mod append;
+mod assignment;
 mod break_;
 mod cal;
 mod cd;

crates/nu-engine/src/eval.rs

@@ -474,6 +474,10 @@ pub fn eval_expression(
                             let lhs = eval_expression(engine_state, stack, lhs)?;
                             lhs.div(op_span, &rhs, op_span)?
                         }
+                        Assignment::AppendAssign => {
+                            let lhs = eval_expression(engine_state, stack, lhs)?;
+                            lhs.append(op_span, &rhs, op_span)?
+                        }
                     };
 
                     match &lhs.expr {

crates/nu-parser/src/parser.rs

@@ -4429,6 +4429,7 @@ pub fn parse_operator(
     let operator = match contents {
         b"=" => Operator::Assignment(Assignment::Assign),
         b"+=" => Operator::Assignment(Assignment::PlusAssign),
+        b"++=" => Operator::Assignment(Assignment::AppendAssign),
         b"-=" => Operator::Assignment(Assignment::MinusAssign),
         b"*=" => Operator::Assignment(Assignment::MultiplyAssign),
         b"/=" => Operator::Assignment(Assignment::DivideAssign),

crates/nu-protocol/src/ast/operator.rs

@@ -51,6 +51,7 @@ pub enum Bits {
 pub enum Assignment {
     Assign,
     PlusAssign,
+    AppendAssign,
     MinusAssign,
     MultiplyAssign,
     DivideAssign,
@@ -70,6 +71,7 @@ impl Display for Operator {
         match self {
             Operator::Assignment(Assignment::Assign) => write!(f, "="),
             Operator::Assignment(Assignment::PlusAssign) => write!(f, "+="),
+            Operator::Assignment(Assignment::AppendAssign) => write!(f, "++="),
             Operator::Assignment(Assignment::MinusAssign) => write!(f, "-="),
             Operator::Assignment(Assignment::MultiplyAssign) => write!(f, "*="),
             Operator::Assignment(Assignment::DivideAssign) => write!(f, "/="),