Add Span merging functions (#12511)

# Description
This PR adds a few functions to `Span` for merging spans together:
- `Span::append`: merges two spans that are known to be in order.
- `Span::concat`: returns a span that encompasses all the spans in a
slice. The spans must be in order.
- `Span::merge`: merges two spans (no order necessary).
- `Span::merge_many`: merges an iterator of spans into a single span (no
order necessary).

These are meant to replace the free-standing `nu_protocol::span`
function.

The spans in a `LiteCommand` (the `parts`) should always be in order
based on the lite parser and lexer. So, the parser code sees the most
usage of `Span::append` and `Span::concat` where the order is known. In
other code areas, `Span::merge` and `Span::merge_many` are used since
the order between spans is often not known.

crates/nu-command/src/help/help_.rs

@@ -2,7 +2,6 @@ use crate::help::{help_aliases, help_commands, help_modules};
 use fancy_regex::Regex;
 use nu_ansi_term::Style;
 use nu_engine::command_prelude::*;
-use nu_protocol::span;
 use nu_utils::IgnoreCaseExt;
 
 #[derive(Clone)]
@@ -97,9 +96,8 @@ You can also learn more at https://www.nushell.sh/book/"#;
                 span: _,
             }) = result
             {
-                let rest_spans: Vec<Span> = rest.iter().map(|arg| arg.span).collect();
                 Err(ShellError::NotFound {
-                    span: span(&rest_spans),
+                    span: Span::merge_many(rest.iter().map(|s| s.span)),
                 })
             } else {
                 result

crates/nu-command/src/help/help_aliases.rs

@@ -1,7 +1,6 @@
 use crate::help::highlight_search_in_table;
 use nu_color_config::StyleComputer;
 use nu_engine::{command_prelude::*, scope::ScopeData};
-use nu_protocol::span;
 
 #[derive(Clone)]
 pub struct HelpAliases;
@@ -110,13 +109,13 @@ pub fn help_aliases(
 
         let Some(alias) = engine_state.find_decl(name.as_bytes(), &[]) else {
             return Err(ShellError::AliasNotFound {
-                span: span(&rest.iter().map(|r| r.span).collect::<Vec<Span>>()),
+                span: Span::merge_many(rest.iter().map(|s| s.span)),
             });
         };
 
         let Some(alias) = engine_state.get_decl(alias).as_alias() else {
             return Err(ShellError::AliasNotFound {
-                span: span(&rest.iter().map(|r| r.span).collect::<Vec<Span>>()),
+                span: Span::merge_many(rest.iter().map(|s| s.span)),
             });
         };
 

crates/nu-command/src/help/help_commands.rs

@@ -1,7 +1,6 @@
 use crate::help::highlight_search_in_table;
 use nu_color_config::StyleComputer;
 use nu_engine::{command_prelude::*, get_full_help};
-use nu_protocol::span;
 
 #[derive(Clone)]
 pub struct HelpCommands;
@@ -104,7 +103,7 @@ pub fn help_commands(
             )
         } else {
             Err(ShellError::CommandNotFound {
-                span: span(&[rest[0].span, rest[rest.len() - 1].span]),
+                span: Span::merge_many(rest.iter().map(|s| s.span)),
             })
         }
     }

crates/nu-command/src/help/help_externs.rs

@@ -1,7 +1,6 @@
 use crate::help::highlight_search_in_table;
 use nu_color_config::StyleComputer;
 use nu_engine::{command_prelude::*, get_full_help, scope::ScopeData};
-use nu_protocol::span;
 
 #[derive(Clone)]
 pub struct HelpExterns;
@@ -124,7 +123,7 @@ pub fn help_externs(
             )
         } else {
             Err(ShellError::CommandNotFound {
-                span: span(&[rest[0].span, rest[rest.len() - 1].span]),
+                span: Span::merge_many(rest.iter().map(|s| s.span)),
             })
         }
     }

crates/nu-command/src/help/help_modules.rs

@@ -1,7 +1,7 @@
 use crate::help::highlight_search_in_table;
 use nu_color_config::StyleComputer;
 use nu_engine::{command_prelude::*, scope::ScopeData};
-use nu_protocol::{span, DeclId};
+use nu_protocol::DeclId;
 
 #[derive(Clone)]
 pub struct HelpModules;
@@ -117,7 +117,7 @@ pub fn help_modules(
         let Some(module_id) = engine_state.find_module(name.as_bytes(), &[]) else {
             return Err(ShellError::ModuleNotFoundAtRuntime {
                 mod_name: name,
-                span: span(&rest.iter().map(|r| r.span).collect::<Vec<Span>>()),
+                span: Span::merge_many(rest.iter().map(|s| s.span)),
             });
         };
 

crates/nu-command/src/platform/is_terminal.rs

@@ -1,5 +1,4 @@
 use nu_engine::command_prelude::*;
-use nu_protocol::span;
 use std::io::IsTerminal as _;
 
 #[derive(Clone)]
@@ -57,12 +56,9 @@ impl Command for IsTerminal {
                 });
             }
             _ => {
-                let spans: Vec<_> = call.arguments.iter().map(|arg| arg.span()).collect();
-                let span = span(&spans);
-
                 return Err(ShellError::IncompatibleParametersSingle {
                     msg: "Only one stream may be checked".into(),
-                    span,
+                    span: Span::merge_many(call.arguments.iter().map(|arg| arg.span())),
                 });
             }
         };

crates/nu-command/src/platform/kill.rs

@@ -1,5 +1,4 @@
 use nu_engine::command_prelude::*;
-use nu_protocol::span;
 use std::process::{Command as CommandSys, Stdio};
 
 #[derive(Clone)]
@@ -96,7 +95,7 @@ impl Command for Kill {
                             })?
                             .span,
                         right_message: "signal".to_string(),
-                        right_span: span(&[
+                        right_span: Span::merge(
                             call.get_named_arg("signal")
                                 .ok_or_else(|| ShellError::GenericError {
                                     error: "Flag error".into(),
@@ -107,7 +106,7 @@ impl Command for Kill {
                                 })?
                                 .span,
                             signal_span,
-                        ]),
+                        ),
                     });
                 }
                 cmd.arg("-9");