Make to text stream ListStreams (#7577)

This PR changes `to text` so that when given a `ListStream`, it streams
the incoming values instead of collecting them all first.

The easiest way to observe/verify this PR is to convert a list to a very
slow `ListStream` with `each`:
```bash
ls | get name | each {|n| sleep 1sec; $n} | to text
```
The `to text` output will appear 1 item at a time.

crates/nu-command/src/formats/to/text.rs

@@ -3,7 +3,7 @@ use nu_protocol::{
     ast::Call,
     engine::{Command, EngineState, Stack},
     format_duration, format_filesize_from_conf, Category, Config, Example, IntoPipelineData,
-    PipelineData, ShellError, Signature, Type, Value,
+    ListStream, PipelineData, RawStream, ShellError, Signature, Type, Value,
 };
 
 #[derive(Clone)]
@@ -40,6 +40,26 @@ impl Command for ToText {
             "\n"
         };
 
+        if let PipelineData::ListStream(stream, _) = input {
+            Ok(PipelineData::ExternalStream {
+                stdout: Some(RawStream::new(
+                    Box::new(ListStreamIterator {
+                        stream,
+                        separator: line_ending.into(),
+                        config: config.clone(),
+                    }),
+                    engine_state.ctrlc.clone(),
+                    span,
+                )),
+                stderr: None,
+                exit_code: None,
+                span,
+                metadata: None,
+                trim_end_newline: false,
+            })
+        } else {
+            // FIXME: don't collect! stream the output wherever possible!
+            // Even if the data is collected when it arrives at `to text`, we should be able to stream it out
             let collected_input = local_into_string(input.into_value(span), line_ending, config);
 
             Ok(Value::String {
@@ -48,6 +68,7 @@ impl Command for ToText {
             }
             .into_pipeline_data())
         }
+    }
 
     fn examples(&self) -> Vec<Example> {
         vec![
@@ -70,6 +91,26 @@ impl Command for ToText {
     }
 }
 
+struct ListStreamIterator {
+    stream: ListStream,
+    separator: String,
+    config: Config,
+}
+
+impl Iterator for ListStreamIterator {
+    type Item = Result<Vec<u8>, ShellError>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if let Some(item) = self.stream.next() {
+            let mut string = local_into_string(item, &self.separator, &self.config);
+            string.push_str(&self.separator);
+            Some(Ok(string.as_bytes().to_vec()))
+        } else {
+            None
+        }
+    }
+}
+
 fn local_into_string(value: Value, separator: &str, config: &Config) -> String {
     match value {
         Value::Bool { val, .. } => val.to_string(),

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

@@ -90,6 +90,7 @@ mod split_row;
 mod str_;
 mod table;
 mod take;
+mod to_text;
 mod touch;
 mod transpose;
 mod try_;

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

@@ -0,0 +1,19 @@
+use nu_test_support::nu;
+
+#[test]
+fn list_to_text() {
+    let actual = nu!(r#"["foo" "bar" "baz"] | to text"#);
+
+    // these actually have newlines between them in the real world but nu! strips newlines, grr
+    assert_eq!(actual.out, "foobarbaz");
+}
+
+// the output should be the same when `to text` gets a ListStream instead of a Value::List
+#[test]
+fn list_stream_to_text() {
+    // use `each` to convert the list to a ListStream
+    let actual = nu!(r#"["foo" "bar" "baz"] | each {|i| $i} | to text"#);
+
+    // these actually have newlines between them in the real world but nu! strips newlines, grr
+    assert_eq!(actual.out, "foobarbaz");
+}