remove panic from lpad and rpad, change truncation behaviour for lpad (#6495)

* condense `lpad` and `rpad` into `pad`

* change description

* back to original names, add change

crates/nu-command/src/strings/str_/lpad.rs

@@ -74,10 +74,10 @@ impl Command for SubCommand {
                 }),
             },
             Example {
-                description: "Use lpad to truncate a string",
+                description: "Use lpad to truncate a string to its last three characters",
                 example: "'123456789' | str lpad -l 3 -c '0'",
                 result: Some(Value::String {
-                    val: "123".to_string(),
+                    val: "789".to_string(),
                     span: Span::test_data(),
                 }),
             },
@@ -105,6 +105,13 @@ fn operate(
         column_paths: call.rest(engine_state, stack, 0)?,
     });
 
+    if options.length.expect("this exists") < 0 {
+        return Err(ShellError::UnsupportedInput(
+            String::from("The length of the string cannot be negative"),
+            call.head,
+        ));
+    }
+
     let head = call.head;
     input.map(
         move |v| {
@@ -142,7 +149,14 @@ fn action(
                 let s = *x as usize;
                 if s < val.len() {
                     Value::String {
-                        val: val.chars().take(s).collect::<String>(),
+                        val: val
+                            .chars()
+                            .rev()
+                            .take(s)
+                            .collect::<String>()
+                            .chars()
+                            .rev()
+                            .collect::<String>(),
                         span: head,
                     }
                 } else {

crates/nu-command/src/strings/str_/rpad.rs

@@ -74,7 +74,7 @@ impl Command for SubCommand {
                 }),
             },
             Example {
-                description: "Use rpad to truncate a string",
+                description: "Use rpad to truncate a string to its first three characters",
                 example: "'123456789' | str rpad -l 3 -c '0'",
                 result: Some(Value::String {
                     val: "123".to_string(),
@@ -105,6 +105,13 @@ fn operate(
         column_paths: call.rest(engine_state, stack, 0)?,
     });
 
+    if options.length.expect("this exists") < 0 {
+        return Err(ShellError::UnsupportedInput(
+            String::from("The length of the string cannot be negative"),
+            call.head,
+        ));
+    }
+
     let head = call.head;
     input.map(
         move |v| {