Allow different names for ...rest (#3954)

* Allow different names for ...rest

* Resolves #3945

* This change requires an explicit name for the rest argument in `WholeStreamCommand`,
  which is why there are so many changed files.

* Remove redundant clone

* Add tests

crates/nu-engine/src/documentation.rs

@@ -205,7 +205,7 @@ pub fn get_documentation(
         }
 
         if let Some(rest_positional) = &signature.rest_positional {
-            long_desc.push_str(&format!("  ...args: {}\n", rest_positional.1));
+            long_desc.push_str(&format!("  ...args: {}\n", rest_positional.2));
         }
     }
     if !signature.named.is_empty() {

crates/nu-engine/src/evaluate/lang.rs

@@ -125,7 +125,7 @@ fn get_signature(sig: &mut Signature, tag: Tag) -> Vec<Value> {
     }
 
     match r {
-        Some((shape, desc)) => {
+        Some((rest_name, shape, desc)) => {
             let mut indexmap = IndexMap::new();
             // let output = format!("Rest|{}|{}|{}\n", name, shape.syntax_shape_name(), desc);
             // eprintln!("{}", output);
@@ -136,7 +136,7 @@ fn get_signature(sig: &mut Signature, tag: Tag) -> Vec<Value> {
             );
             indexmap.insert(
                 "parameter_name".to_string(),
-                UntaggedValue::string("".to_string()).into_value(&tag),
+                UntaggedValue::string(rest_name.to_string()).into_value(&tag),
             );
             indexmap.insert(
                 "parameter_type".to_string(),

crates/nu-engine/src/types/deduction.rs

@@ -468,7 +468,10 @@ impl VarSyntaxShapeDeductor {
             if let Expression::Variable(var_name, _) = &positional.expr {
                 let deduced_shape = {
                     if pos_idx >= signature.positional.len() {
-                        signature.rest_positional.as_ref().map(|(shape, _)| shape)
+                        signature
+                            .rest_positional
+                            .as_ref()
+                            .map(|(_, shape, _)| shape)
                     } else {
                         match &signature.positional[pos_idx].0 {
                             PositionalType::Mandatory(_, shape)

crates/nu-engine/src/whole_stream_command.rs

@@ -139,7 +139,7 @@ impl WholeStreamCommand for Arc<Block> {
                     _ => break,
                 }
             }
-            if block.params.rest_positional.is_some() {
+            if let Some(rest_pos) = &block.params.rest_positional {
                 let elements: Vec<_> = args_iter.collect();
                 let start = if let Some(first) = elements.first() {
                     first.tag.span.start()
@@ -153,15 +153,15 @@ impl WholeStreamCommand for Arc<Block> {
                 };
 
                 ctx.scope.add_var(
-                    "$rest",
+                    format!("${}", rest_pos.0),
                     UntaggedValue::Table(elements).into_value(Span::new(start, end)),
                 );
             }
-        } else if block.params.rest_positional.is_some() {
+        } else if let Some(rest_pos) = &block.params.rest_positional {
             //If there is a rest arg, but no args were provided,
             //we have to set $rest to an empty table
             ctx.scope.add_var(
-                "$rest",
+                format!("${}", rest_pos.0),
                 UntaggedValue::Table(Vec::new()).into_value(Span::new(0, 0)),
             );
         }