Stream lazy default output (#15955)

It was brought up in the Discord that `default { open -r foo.txt }`
results in a string instead of streaming output. This changes `default`
such that closures now stream when given simple input.

# Description
If the value isn't expected to be cached, `default` just runs the
closure without caching the value, which allows its output to be
streamed

# User-Facing Changes


# Tests + Formatting
👍 

# After Submitting

crates/nu-command/src/filters/default.rs

@@ -213,7 +213,7 @@ fn default(
         || (default_when_empty
             && matches!(input, PipelineData::Value(ref value, _) if value.is_empty()))
     {
-        default_value.pipeline_data()
+        default_value.single_run_pipeline_data()
     } else if default_when_empty && matches!(input, PipelineData::ListStream(..)) {
         let PipelineData::ListStream(ls, metadata) = input else {
             unreachable!()
@@ -221,7 +221,7 @@ fn default(
         let span = ls.span();
         let mut stream = ls.into_inner().peekable();
         if stream.peek().is_none() {
-            return default_value.pipeline_data();
+            return default_value.single_run_pipeline_data();
         }
 
         // stream's internal state already preserves the original signals config, so if this
@@ -278,8 +278,14 @@ impl DefaultValue {
         }
     }
 
-    fn pipeline_data(&mut self) -> Result<PipelineData, ShellError> {
-        self.value().map(|x| x.into_pipeline_data())
+    /// Used when we know the value won't need to be cached to allow streaming.
+    fn single_run_pipeline_data(self) -> Result<PipelineData, ShellError> {
+        match self {
+            DefaultValue::Uncalculated(mut closure) => {
+                closure.item.run_with_input(PipelineData::Empty)
+            }
+            DefaultValue::Calculated(val) => Ok(val.into_pipeline_data()),
+        }
     }
 }
 

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

@@ -244,3 +244,9 @@ fn return_closure_value() {
     let actual = nu!(r#"null | default { {||} }"#);
     assert!(actual.out.starts_with("closure"));
 }
+
+#[test]
+fn lazy_output_streams() {
+    let actual = nu!(r#"default { nu --testbin cococo 'hello' } | describe"#);
+    assert!(actual.out.contains("byte stream"));
+}