Use iterator chain instead of string concat. (#2655)

* Use iterator chain instead of string concat

* Add regression test for multi-value lines

crates/nu-cli/src/commands/lines.rs

@@ -75,9 +75,9 @@ async fn lines(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputSt
                 } => {
                     let mut leftover_string = leftover_string.lock();
 
-                    let st = (&*leftover_string).clone() + &st;
+                    let lo_lines = (&*leftover_string).lines().map(|x| x.to_string());
-
+                    let st_lines = st.lines().map(|x| x.to_string());
-                    let mut lines: Vec<String> = st.lines().map(|x| x.to_string()).collect();
+                    let mut lines: Vec<String> = lo_lines.chain(st_lines).collect();
 
                     leftover_string.clear();
 

crates/nu-cli/tests/commands/lines.rs

@@ -34,3 +34,18 @@ fn lines_proper_buffering() {
 
     assert_eq!(actual.out, "[8194,4]");
 }
+
+#[test]
+fn lines_multi_value_split() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            open sample-simple.json
+            | get first second
+            | lines
+            | count
+        "#
+    ));
+
+    assert_eq!(actual.out, "6");
+}

tests/fixtures/formats/sample-simple.json

@@ -0,0 +1,4 @@
+{
+    "first": "first",
+    "second": "this\nshould\nbe\nseparate\nlines"
+}