add shape ExternalResolved to show found externals via syntax highlighting in the repl (#11135)

# Description

This PR enables a new feature that shows which externals are found in
your path via the syntax highlighter as you type.

![external_resolved](https://github.com/nushell/nushell/assets/343840/e5fa91f0-6fac-485c-8afc-5711fc0ed9bc)

This idea could use some improvement where it caches the items in your
path and on some trigger, expires that cache and creates a new on. Right
now, all it does is call the `which` crate on every character you type.
This could be problematic if you have hundreds of paths in your PATH or
if some of your paths in your Path point to extraordinarily slow file
systems. WSL pointing to Windows comes to mind. Either way, I've thrown
it up here for people to try and provide feedback. I think the novelty
of showing what is valid and what isn't is pretty cool. I believe
fish-shell also does this, IIRC.

# User-Facing Changes
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# Tests + Formatting
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> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
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> toolkit check pr
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Cargo.lock

@@ -2723,6 +2723,7 @@ dependencies = [
  "sysinfo",
  "unicode-segmentation",
  "uuid",
+ "which 5.0.0",
 ]
 
 [[package]]

crates/nu-cli/Cargo.toml

@@ -40,6 +40,7 @@ pathdiff = "0.2"
 sysinfo = "0.29"
 unicode-segmentation = "1.10"
 uuid = { version = "1.6.0", features = ["v4"] }
+which = "5.0.0"
 
 [features]
 plugin = []

crates/nu-cli/src/syntax_highlight.rs

@@ -17,10 +17,27 @@ impl Highlighter for NuHighlighter {
     fn highlight(&self, line: &str, _cursor: usize) -> StyledText {
         trace!("highlighting: {}", line);
 
+        let highlight_resolved_externals =
+            self.engine_state.get_config().highlight_resolved_externals;
         let mut working_set = StateWorkingSet::new(&self.engine_state);
         let block = parse(&mut working_set, None, line.as_bytes(), false);
         let (shapes, global_span_offset) = {
-            let shapes = flatten_block(&working_set, &block);
+            let mut shapes = flatten_block(&working_set, &block);
+            // Highlighting externals has a config point because of concerns that using which to resolve
+            // externals may slow down things too much.
+            if highlight_resolved_externals {
+                for (span, shape) in shapes.iter_mut() {
+                    if *shape == FlatShape::External {
+                        let str_contents =
+                            working_set.get_span_contents(Span::new(span.start, span.end));
+
+                        let str_word = String::from_utf8_lossy(str_contents).to_string();
+                        if which::which(str_word).ok().is_some() {
+                            *shape = FlatShape::ExternalResolved;
+                        }
+                    }
+                }
+            }
             (shapes, self.engine_state.next_span_start())
         };
 
@@ -91,6 +108,7 @@ impl Highlighter for NuHighlighter {
                 FlatShape::InternalCall(_) => add_colored_token(&shape.1, next_token),
                 FlatShape::External => add_colored_token(&shape.1, next_token),
                 FlatShape::ExternalArg => add_colored_token(&shape.1, next_token),
+                FlatShape::ExternalResolved => add_colored_token(&shape.1, next_token),
                 FlatShape::Keyword => add_colored_token(&shape.1, next_token),
                 FlatShape::Literal => add_colored_token(&shape.1, next_token),
                 FlatShape::Operator => add_colored_token(&shape.1, next_token),

crates/nu-color-config/src/shape_color.rs

@@ -15,6 +15,7 @@ pub fn default_shape_color(shape: String) -> Style {
         "shape_directory" => Style::new().fg(Color::Cyan),
         "shape_external" => Style::new().fg(Color::Cyan),
         "shape_externalarg" => Style::new().fg(Color::Green).bold(),
+        "shape_external_resolved" => Style::new().fg(Color::LightYellow).bold(),
         "shape_filepath" => Style::new().fg(Color::Cyan),
         "shape_flag" => Style::new().fg(Color::Blue).bold(),
         "shape_float" => Style::new().fg(Color::Purple).bold(),

crates/nu-parser/src/flatten.rs

@@ -18,6 +18,7 @@ pub enum FlatShape {
     Directory,
     External,
     ExternalArg,
+    ExternalResolved,
     Filepath,
     Flag,
     Float,
@@ -57,6 +58,7 @@ impl Display for FlatShape {
             FlatShape::Directory => write!(f, "shape_directory"),
             FlatShape::External => write!(f, "shape_external"),
             FlatShape::ExternalArg => write!(f, "shape_externalarg"),
+            FlatShape::ExternalResolved => write!(f, "shape_external_resolved"),
             FlatShape::Filepath => write!(f, "shape_filepath"),
             FlatShape::Flag => write!(f, "shape_flag"),
             FlatShape::Float => write!(f, "shape_float"),

crates/nu-protocol/src/config/mod.rs

@@ -72,6 +72,7 @@ pub struct Config {
     pub datetime_table_format: Option<String>,
     pub error_style: ErrorStyle,
     pub use_kitty_protocol: bool,
+    pub highlight_resolved_externals: bool,
 }
 
 impl Default for Config {
@@ -137,6 +138,7 @@ impl Default for Config {
             error_style: ErrorStyle::Fancy,
 
             use_kitty_protocol: false,
+            highlight_resolved_externals: false,
         }
     }
 }
@@ -622,6 +624,9 @@ impl Value {
                     "use_kitty_protocol" => {
                         process_bool_config(value, &mut errors, &mut config.use_kitty_protocol);
                     }
+                    "highlight_resolved_externals" => {
+                        process_bool_config(value, &mut errors, &mut config.highlight_resolved_externals);
+                    }
                     // Menus
                     "menus" => match create_menus(value) {
                         Ok(map) => config.menus = map,

crates/nu-utils/src/sample_config/default_config.nu

@@ -42,6 +42,7 @@ let dark_theme = {
     shape_directory: cyan
     shape_external: cyan
     shape_externalarg: green_bold
+    shape_external_resolved: light_yellow_bold
     shape_filepath: cyan
     shape_flag: blue_bold
     shape_float: purple_bold
@@ -106,6 +107,7 @@ let light_theme = {
     shape_directory: cyan
     shape_external: cyan
     shape_externalarg: green_bold
+    shape_external_resolved: light_purple_bold
     shape_filepath: cyan
     shape_flag: blue_bold
     shape_float: purple_bold
@@ -233,7 +235,8 @@ $env.config = {
     edit_mode: emacs # emacs, vi
     shell_integration: false # enables terminal shell integration. Off by default, as some terminals have issues with this.
     render_right_prompt_on_last_line: false # true or false to enable or disable right prompt to be rendered on last line of the prompt.
-    use_kitty_protocol: false # enables keyboard enhancement protocol implemented by kitty console, only if your terminal support this
+    use_kitty_protocol: false # enables keyboard enhancement protocol implemented by kitty console, only if your terminal support this.
+    highlight_resolved_externals: false # true enables highlighting of external commands in the repl resolved by which.
 
     hooks: {
         pre_prompt: [{ null }] # run before the prompt is shown