Improve empty pipelines (#7383)

# Description

This fix changes pipelines to allow them to actually be empty. Mapping
over empty pipelines gives empty pipelines. Empty pipelines immediately
return `None` when iterated.

This removes a some of where `Span::new(0, 0)` was coming from, though
there are other cases where we still use it.

# User-Facing Changes

None

# 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-cli/src/completions/completer.rs

@@ -92,7 +92,7 @@ impl NuCompleter {
             &self.engine_state,
             &mut callee_stack,
             block,
-            PipelineData::new(span),
+            PipelineData::empty(),
             true,
             true,
         );

crates/nu-cli/src/completions/custom_completions.rs

@@ -67,7 +67,7 @@ impl Completer for CustomCompletion {
                 redirect_stdout: true,
                 redirect_stderr: true,
             },
-            PipelineData::new(span),
+            PipelineData::empty(),
         );
 
         let mut custom_completion_options = None;

crates/nu-cli/src/config_files.rs

@@ -8,7 +8,7 @@ use nu_path::canonicalize_with;
 use nu_protocol::engine::{EngineState, Stack, StateWorkingSet};
 #[cfg(feature = "plugin")]
 use nu_protocol::Spanned;
-use nu_protocol::{HistoryFileFormat, PipelineData, Span};
+use nu_protocol::{HistoryFileFormat, PipelineData};
 use std::path::PathBuf;
 
 #[cfg(feature = "plugin")]
@@ -38,7 +38,7 @@ pub fn read_plugin_file(
                 stack,
                 &contents,
                 &plugin_filename,
-                PipelineData::new(Span::new(0, 0)),
+                PipelineData::empty(),
             );
         }
     }
@@ -85,7 +85,7 @@ pub fn eval_config_contents(
                 stack,
                 &contents,
                 &config_filename,
-                PipelineData::new(Span::new(0, 0)),
+                PipelineData::empty(),
             );
 
             // Merge the environment in case env vars changed in the config

crates/nu-cli/src/eval_file.rs

@@ -39,13 +39,7 @@ pub fn evaluate_file(
     if working_set.find_decl(b"main", &Type::Any).is_some() {
         let args = format!("main {}", args.join(" "));
 
-        if !eval_source(
-            engine_state,
-            stack,
-            &file,
-            &path,
-            PipelineData::new(Span::new(0, 0)),
-        ) {
+        if !eval_source(engine_state, stack, &file, &path, PipelineData::empty()) {
             std::process::exit(1);
         }
         if !eval_source(engine_state, stack, args.as_bytes(), "<commandline>", input) {

crates/nu-cli/src/print.rs

@@ -50,14 +50,13 @@ Since this command has no output, there is no point in piping it with other comm
         let args: Vec<Value> = call.rest(engine_state, stack, 0)?;
         let no_newline = call.has_flag("no-newline");
         let to_stderr = call.has_flag("stderr");
-        let head = call.head;
 
         for arg in args {
             arg.into_pipeline_data()
                 .print(engine_state, stack, no_newline, to_stderr)?;
         }
 
-        Ok(PipelineData::new(head))
+        Ok(PipelineData::empty())
     }
 
     fn examples(&self) -> Vec<Example> {

crates/nu-cli/src/prompt_update.rs

@@ -4,7 +4,7 @@ use log::info;
 use nu_engine::eval_subexpression;
 use nu_protocol::{
     engine::{EngineState, Stack, StateWorkingSet},
-    Config, PipelineData, Span, Value,
+    Config, PipelineData, Value,
 };
 use reedline::Prompt;
 
@@ -37,12 +37,8 @@ fn get_prompt_string(
                 let block = engine_state.get_block(block_id);
                 let mut stack = stack.captures_to_stack(&captures);
                 // Use eval_subexpression to force a redirection of output, so we can use everything in prompt
-                let ret_val = eval_subexpression(
-                    engine_state,
-                    &mut stack,
-                    block,
-                    PipelineData::new(Span::new(0, 0)), // Don't try this at home, 0 span is ignored
-                );
+                let ret_val =
+                    eval_subexpression(engine_state, &mut stack, block, PipelineData::empty());
                 info!(
                     "get_prompt_string (block) {}:{}:{}",
                     file!(),
@@ -62,12 +58,7 @@ fn get_prompt_string(
             Value::Block { val: block_id, .. } => {
                 let block = engine_state.get_block(block_id);
                 // Use eval_subexpression to force a redirection of output, so we can use everything in prompt
-                let ret_val = eval_subexpression(
-                    engine_state,
-                    stack,
-                    block,
-                    PipelineData::new(Span::new(0, 0)), // Don't try this at home, 0 span is ignored
-                );
+                let ret_val = eval_subexpression(engine_state, stack, block, PipelineData::empty());
                 info!(
                     "get_prompt_string (block) {}:{}:{}",
                     file!(),

crates/nu-cli/src/repl.rs

@@ -149,7 +149,7 @@ pub fn evaluate_repl(
             stack,
             s.item.as_bytes(),
             &format!("entry #{}", entry_num),
-            PipelineData::new(Span::new(0, 0)),
+            PipelineData::empty(),
         );
         engine_state.merge_env(stack, get_guaranteed_cwd(engine_state, stack))?;
     }
@@ -431,7 +431,7 @@ pub fn evaluate_repl(
                         stack,
                         s.as_bytes(),
                         &format!("entry #{}", entry_num),
-                        PipelineData::new(Span::new(0, 0)),
+                        PipelineData::empty(),
                     );
                 }
                 let cmd_duration = start_time.elapsed();
@@ -637,7 +637,7 @@ pub fn eval_string_with_input(
 
     let input_as_pipeline_data = match input {
         Some(input) => PipelineData::Value(input, None),
-        None => PipelineData::new(Span::test_data()),
+        None => PipelineData::empty(),
     };
 
     eval_block(
@@ -722,7 +722,7 @@ pub fn eval_hook(
         val: "condition".to_string(),
         span: value_span,
     };
-    let mut output = PipelineData::new(Span::new(0, 0));
+    let mut output = PipelineData::empty();
 
     let code_path = PathMember::String {
         val: "code".to_string(),
@@ -823,7 +823,7 @@ pub fn eval_hook(
                         };
 
                         engine_state.merge_delta(delta)?;
-                        let input = PipelineData::new(value_span);
+                        let input = PipelineData::empty();
 
                         let var_ids: Vec<VarId> = vars
                             .into_iter()
@@ -943,7 +943,7 @@ pub fn run_hook_block(
 ) -> Result<Value, ShellError> {
     let block = engine_state.get_block(block_id);
 
-    let input = optional_input.unwrap_or_else(|| PipelineData::new(span));
+    let input = optional_input.unwrap_or_else(PipelineData::empty);
 
     let mut callee_stack = stack.gather_captures(&block.captures);