Avoid clone in `Signature::get_positional()` (#13338)

# Description
`Signature::get_positional()` was returning an owned `PositionalArg`,
which contains a bunch of strings. `ClosureEval` uses this in
`try_add_arg`, making all of that unnecessary cloning a little bit hot.

# User-Facing Changes
Slightly better performance

2 files changed, 6 insertions, 7 deletions

diff --git a/crates/nu-protocol/src/signature.rs b/crates/nu-protocol/src/signature.rs
index 5928ce0c0..3241b0df2 100644
--- a/crates/nu-protocol/src/signature.rs
+++ b/crates/nu-protocol/src/signature.rs
@@ -485,15 +485,14 @@ impl Signature {
         (name, s)
     }
 
-    pub fn get_positional(&self, position: usize) -> Option<PositionalArg> {
+    pub fn get_positional(&self, position: usize) -> Option<&PositionalArg> {
         if position < self.required_positional.len() {
-            self.required_positional.get(position).cloned()
+            self.required_positional.get(position)
         } else if position < (self.required_positional.len() + self.optional_positional.len()) {
             self.optional_positional
                 .get(position - self.required_positional.len())
-                .cloned()
         } else {
-            self.rest_positional.clone()
+            self.rest_positional.as_ref()
         }
     }
 
diff --git a/crates/nu-protocol/tests/test_signature.rs b/crates/nu-protocol/tests/test_signature.rs
index e22029bab..8faf772c3 100644
--- a/crates/nu-protocol/tests/test_signature.rs
+++ b/crates/nu-protocol/tests/test_signature.rs
@@ -39,7 +39,7 @@ fn test_signature_chained() {
 
     assert_eq!(
         signature.get_positional(0),
-        Some(PositionalArg {
+        Some(&PositionalArg {
             name: "required".to_string(),
             desc: "required description".to_string(),
             shape: SyntaxShape::String,
@@ -49,7 +49,7 @@ fn test_signature_chained() {
     );
     assert_eq!(
         signature.get_positional(1),
-        Some(PositionalArg {
+        Some(&PositionalArg {
             name: "optional".to_string(),
             desc: "optional description".to_string(),
             shape: SyntaxShape::String,
@@ -59,7 +59,7 @@ fn test_signature_chained() {
     );
     assert_eq!(
         signature.get_positional(2),
-        Some(PositionalArg {
+        Some(&PositionalArg {
             name: "rest".to_string(),
             desc: "rest description".to_string(),
             shape: SyntaxShape::String,