fix: fix commandline when called with no arguments (#8207)

# Description

This fixes the `commandline` command when it's run with no arguments, so
it outputs the command being run. New line characters are included.

This allows for:

- [A way to get current command inside pre_execution hook · Issue #6264
· nushell/nushell](https://github.com/nushell/nushell/issues/6264)
- The possibility of *Atuin* to work work *Nushell*. *Atuin* hooks need
to know the current repl input before it is run.

# User-Facing Changes

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-cmd-lang/Cargo.toml

@@ -24,6 +24,7 @@ fancy-regex = "0.11.0"
 itertools = "0.10.0"
 log = "0.4.14"
 shadow-rs = { version = "0.21.0", default-features = false }
+unicode-segmentation = "1.10.0"
 
 [build-dependencies]
 shadow-rs = { version = "0.21.0", default-features = false }

crates/nu-cmd-lang/src/core_commands/commandline.rs

@@ -1,10 +1,10 @@
 use nu_engine::CallExt;
 use nu_protocol::ast::Call;
-use nu_protocol::engine::ReplOperation;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::Category;
 use nu_protocol::IntoPipelineData;
 use nu_protocol::{PipelineData, ShellError, Signature, SyntaxShape, Type, Value};
+use unicode_segmentation::UnicodeSegmentation;
 
 #[derive(Clone)]
 pub struct Commandline;
@@ -17,6 +17,11 @@ impl Command for Commandline {
     fn signature(&self) -> Signature {
         Signature::build("commandline")
             .input_output_types(vec![(Type::Nothing, Type::Nothing)])
+            .switch(
+                "cursor",
+                "Set or get the current cursor position",
+                Some('c'),
+            )
             .switch(
                 "append",
                 "appends the string to the end of the buffer",
@@ -56,30 +61,77 @@ impl Command for Commandline {
         _input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
         if let Some(cmd) = call.opt::<Value>(engine_state, stack, 0)? {
-            let mut ops = engine_state
-                .repl_operation_queue
+            let mut buffer = engine_state
+                .repl_buffer_state
                 .lock()
-                .expect("repl op queue mutex");
-            ops.push_back(if call.has_flag("append") {
-                ReplOperation::Append(cmd.as_string()?)
+                .expect("repl buffer state mutex");
+            let mut cursor_pos = engine_state
+                .repl_cursor_pos
+                .lock()
+                .expect("repl cursor pos mutex");
+
+            if call.has_flag("cursor") {
+                let cmd_str = cmd.as_string()?;
+                match cmd_str.parse::<i64>() {
+                    Ok(n) => {
+                        *cursor_pos = if n <= 0 {
+                            0usize
+                        } else {
+                            buffer
+                                .grapheme_indices(true)
+                                .map(|(i, _c)| i)
+                                .nth(n as usize)
+                                .unwrap_or(buffer.len())
+                        }
+                    }
+                    Err(_) => {
+                        return Err(ShellError::CantConvert {
+                            to_type: "int".to_string(),
+                            from_type: "string".to_string(),
+                            span: cmd.span()?,
+                            help: Some(format!(
+                                r#"string "{cmd_str}" does not represent a valid integer"#
+                            )),
+                        })
+                    }
+                }
+            } else if call.has_flag("append") {
+                buffer.push_str(&cmd.as_string()?);
             } else if call.has_flag("insert") {
-                ReplOperation::Insert(cmd.as_string()?)
+                let cmd_str = cmd.as_string()?;
+                buffer.insert_str(*cursor_pos, &cmd_str);
+                *cursor_pos += cmd_str.len();
             } else {
-                ReplOperation::Replace(cmd.as_string()?)
-            });
-            Ok(Value::Nothing { span: call.head }.into_pipeline_data())
-        } else if let Some(ref cmd) = *engine_state
-            .repl_buffer_state
-            .lock()
-            .expect("repl buffer state mutex")
-        {
-            Ok(Value::String {
-                val: cmd.clone(),
-                span: call.head,
+                *buffer = cmd.as_string()?;
+                *cursor_pos = buffer.len();
             }
-            .into_pipeline_data())
-        } else {
             Ok(Value::Nothing { span: call.head }.into_pipeline_data())
+        } else {
+            let buffer = engine_state
+                .repl_buffer_state
+                .lock()
+                .expect("repl buffer state mutex");
+            if call.has_flag("cursor") {
+                let cursor_pos = engine_state
+                    .repl_cursor_pos
+                    .lock()
+                    .expect("repl cursor pos mutex");
+                let char_pos = buffer
+                    .grapheme_indices(true)
+                    .position(|(i, _c)| i == *cursor_pos)
+                    .unwrap_or(buffer.len());
+                Ok(Value::String {
+                    val: char_pos.to_string(),
+                    span: call.head,
+                }
+                .into_pipeline_data())
+            } else {
+                Ok(Value::String {
+                    val: buffer.to_string(),
+                    span: call.head,
+                }
+                .into_pipeline_data())
+            }
         }
     }
 }