Make stream info visible to users in describe (#7589)

Closes #7581.

After this PR, `describe` shows `(stream)` next to input that arrived at
`describe` as a `ListStream`:
```bash
〉ls | describe
table<name: string, type: string, size: filesize, modified: date> (stream)
〉[1 2 3] | each {|i| $i} | describe
list<int> (stream)
```

`describe` must collect all items of the stream to display type
information for lists and tables. If users need to avoid collecting
input, they can use the `-n`/`--no-collect` flag:

```bash
〉[1 2 3] | each {|i| $i} | describe --no-collect
stream
```

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

@@ -19,6 +19,11 @@ impl Command for Describe {
     fn signature(&self) -> Signature {
         Signature::build("describe")
             .input_output_types(vec![(Type::Any, Type::String)])
+            .switch(
+                "no-collect",
+                "do not collect streams of structured data",
+                Some('n'),
+            )
             .category(Category::Core)
     }
 
@@ -30,32 +35,58 @@ impl Command for Describe {
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
         let head = call.head;
-        if matches!(input, PipelineData::ExternalStream { .. }) {
-            Ok(PipelineData::Value(
-                Value::string("raw input", call.head),
-                None,
-            ))
-        } else {
-            let value = input.into_value(call.head);
-            let description = match value {
-                Value::CustomValue { val, .. } => val.value_string(),
-                _ => value.get_type().to_string(),
-            };
 
-            Ok(Value::String {
-                val: description,
-                span: head,
+        let no_collect: bool = call.has_flag("no-collect");
+
+        let description = match input {
+            PipelineData::ExternalStream { .. } => "raw input".into(),
+            PipelineData::ListStream(_, _) => {
+                if no_collect {
+                    "stream".into()
+                } else {
+                    let value = input.into_value(head);
+                    let base_description = match value {
+                        Value::CustomValue { val, .. } => val.value_string(),
+                        _ => value.get_type().to_string(),
+                    };
+
+                    format!("{base_description} (stream)")
+                }
             }
-            .into_pipeline_data())
+            _ => {
+                let value = input.into_value(head);
+                match value {
+                    Value::CustomValue { val, .. } => val.value_string(),
+                    _ => value.get_type().to_string(),
+                }
+            }
+        };
+
+        Ok(Value::String {
+            val: description,
+            span: head,
         }
+        .into_pipeline_data())
     }
 
     fn examples(&self) -> Vec<Example> {
-        vec![Example {
-            description: "Describe the type of a string",
-            example: "'hello' | describe",
-            result: Some(Value::test_string("string")),
-        }]
+        vec![
+            Example {
+                description: "Describe the type of a string",
+                example: "'hello' | describe",
+                result: Some(Value::test_string("string")),
+            },
+            Example {
+                description: "Describe a stream of data, collecting it first",
+                example: "[1 2 3] | each {|i| $i} | describe",
+                result: Some(Value::test_string("list<int> (stream)")),
+            },
+            Example {
+                description: "Describe the input but do not collect streams",
+                example: "[1 2 3] | each {|i| $i} | describe --no-collect",
+                result: Some(Value::test_string("stream")),
+            },
+        ]
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/example_test.rs

@@ -13,7 +13,7 @@ mod test_examples {
         MathRound, Path, Random, Split, SplitColumn, SplitRow, Str, StrJoin, StrLength, StrReplace,
         Url, Values, Wrap,
     };
-    use crate::{Break, Mut, To};
+    use crate::{Break, Each, Mut, To};
     use itertools::Itertools;
     use nu_protocol::{
         ast::Block,
@@ -61,6 +61,7 @@ mod test_examples {
             // Base functions that are needed for testing
             // Try to keep this working set small to keep tests running as fast as possible
             let mut working_set = StateWorkingSet::new(&engine_state);
+            working_set.add_decl(Box::new(Each));
             working_set.add_decl(Box::new(Let));
             working_set.add_decl(Box::new(Str));
             working_set.add_decl(Box::new(StrJoin));

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

@@ -375,7 +375,7 @@ mod test {
             r#"[7,8,9,10] | par-each {|el ind| $ind } | describe"#
         ));
 
-        assert_eq!(actual.out, "list<int>");
+        assert_eq!(actual.out, "list<int> (stream)");
     }
 
     #[test]

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

@@ -77,7 +77,7 @@ fn gets_first_row_as_list_when_amount_given() {
             "#
     ));
 
-    assert_eq!(actual.out, "list<int>");
+    assert_eq!(actual.out, "list<int> (stream)");
 }
 
 #[test]

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

@@ -76,7 +76,7 @@ fn gets_last_row_as_list_when_amount_given() {
             "#
     ));
 
-    assert_eq!(actual.out, "list<int>");
+    assert_eq!(actual.out, "list<int> (stream)");
 }
 
 #[test]

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

@@ -9,7 +9,7 @@ fn float_in_seq_leads_to_lists_of_floats() {
         "#
     ));
 
-    assert_eq!(actual.out, "list<float>");
+    assert_eq!(actual.out, "list<float> (stream)");
 }
 
 #[test]
@@ -21,5 +21,5 @@ fn ints_in_seq_leads_to_lists_of_ints() {
         "#
     ));
 
-    assert_eq!(actual.out, "list<int>");
+    assert_eq!(actual.out, "list<int> (stream)");
 }