Display aliases and custom commands in which; fix #2810 (#2834)

* Display aliases and custom commands in which; Fix #2810

Example output of nu after the commit is applied:

```shell
/home/leo/repos/nushell(feature/which_inspect_alias)> def docker-ps [] { docker ps --format '{{json .}}' | from json -o }
/home/leo/repos/nushell(feature/which_inspect_alias)> which docker-ps
───┬───────────┬────────────────────────┬─────────
 # │    arg    │          path          │ builtin
───┼───────────┼────────────────────────┼─────────
 0 │ docker-ps │ nushell custom command │ No
───┴───────────┴────────────────────────┴─────────
/home/leo/repos/nushell(feature/which_inspect_alias)> alias d = gid pd
/home/leo/repos/nushell(feature/which_inspect_alias)> which d
───┬─────┬───────────────┬─────────
 # │ arg │     path      │ builtin
───┼─────┼───────────────┼─────────
 0 │ d   │ nushell alias │ No
───┴─────┴───────────────┴─────────
```

* Update documentation

crates/nu-cli/src/commands/which_.rs

@@ -20,7 +20,7 @@ impl WholeStreamCommand for Which {
     }
 
     fn usage(&self) -> &str {
-        "Finds a program file."
+        "Finds a program file, alias or custom command."
     }
 
     async fn run(&self, args: CommandArgs) -> Result<OutputStream, ShellError> {
@@ -44,13 +44,12 @@ fn entry(arg: impl Into<String>, path: Value, builtin: bool, tag: Tag) -> Value
     UntaggedValue::row(map).into_value(tag)
 }
 
-macro_rules! entry_builtin {
-    ($arg:expr, $tag:expr) => {
+macro_rules! create_entry {
+    ($arg:expr, $path:expr, $tag:expr, $is_builtin:expr) => {
         entry(
             $arg.clone(),
-            UntaggedValue::Primitive(Primitive::String("nushell built-in command".to_string()))
-                .into_value($tag.clone()),
-            true,
+            UntaggedValue::Primitive(Primitive::String($path.to_string())).into_value($tag.clone()),
+            $is_builtin,
             $tag,
         )
     };
@@ -86,12 +85,8 @@ async fn which(args: CommandArgs) -> Result<OutputStream, ShellError> {
         application.item.clone()
     };
     if !external {
-        let builtin = scope.has_command(&item);
-        if builtin {
-            output.push(ReturnSuccess::value(entry_builtin!(
-                item,
-                application.tag.clone()
-            )));
+        if let Some(entry) = entry_for(&scope, &item, application.tag.clone()) {
+            output.push(ReturnSuccess::value(entry));
         }
     }
 
@@ -115,6 +110,18 @@ async fn which(args: CommandArgs) -> Result<OutputStream, ShellError> {
     }
 }
 
+fn entry_for(scope: &Scope, name: &str, tag: Tag) -> Option<Value> {
+    if scope.has_custom_command(name) {
+        Some(create_entry!(name, "Nushell custom command", tag, false))
+    } else if scope.has_command(name) {
+        Some(create_entry!(name, "Nushell built-in command", tag, true))
+    } else if scope.has_alias(name) {
+        Some(create_entry!(name, "Nushell alias", tag, false))
+    } else {
+        None
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::ShellError;

crates/nu-cli/src/evaluate/scope.rs

@@ -52,14 +52,20 @@ impl Scope {
         names
     }
 
-    pub fn has_command(&self, name: &str) -> bool {
-        for frame in self.frames.lock().iter() {
-            if frame.has_command(name) {
-                return true;
-            }
-        }
+    fn has_cmd_helper(&self, name: &str, f: fn(&ScopeFrame, &str) -> bool) -> bool {
+        self.frames.lock().iter().any(|frame| f(frame, name))
+    }
 
-        false
+    pub fn has_command(&self, name: &str) -> bool {
+        self.has_cmd_helper(name, ScopeFrame::has_command)
+    }
+
+    pub fn has_custom_command(&self, name: &str) -> bool {
+        self.has_cmd_helper(name, ScopeFrame::has_custom_command)
+    }
+
+    pub fn has_alias(&self, name: &str) -> bool {
+        self.has_cmd_helper(name, ScopeFrame::has_alias)
     }
 
     pub fn expect_command(&self, name: &str) -> Result<Command, ShellError> {
@@ -203,6 +209,14 @@ impl ScopeFrame {
         self.commands.contains_key(name)
     }
 
+    pub fn has_custom_command(&self, name: &str) -> bool {
+        self.custom_commands.contains_key(name)
+    }
+
+    pub fn has_alias(&self, name: &str) -> bool {
+        self.aliases.contains_key(name)
+    }
+
     pub fn get_command_names(&self) -> Vec<String> {
         self.commands.keys().map(|x| x.to_string()).collect()
     }

docs/commands/which.md

@@ -79,3 +79,27 @@ Passing the `all` flag identifies all instances of a command or binary
  builtin │ No
 ─────────┴────────────────────────────────
 ```
+
+`which` also identifies aliases
+
+```shell
+> alias e = echo
+> which e
+───┬─────┬───────────────┬─────────
+ # │ arg │     path      │ builtin
+───┼─────┼───────────────┼─────────
+ 0 │ e   │ Nushell alias │ No
+───┴─────┴───────────────┴─────────
+```
+
+and custom commands
+
+```shell
+> def my_cool_echo [arg] { echo $arg }
+> which my_cool_echo
+───┬──────────────┬────────────────────────┬─────────
+ # │     arg      │          path          │ builtin
+───┼──────────────┼────────────────────────┼─────────
+ 0 │ my_cool_echo │ Nushell custom command │ No
+───┴──────────────┴────────────────────────┴─────────
+```