fix: Generalize mut_arg, like mut_subcommand

This makes us accept `str` and not do any allocations at the cost of
panicing if unsupported which I think fits our overall story in trying
to catch development-time errors.

CHANGELOG.md

@@ -239,6 +239,7 @@ Easier to catch changes:
 - *(assert)* Ensure `overrides_with` IDs are valid
 - *(assert)* Ensure no self-`overrides_with` now that Actions replace it
 - *(assert)* Ensure subcommand names are not duplicated
+- *(assert)* Assert on `mut_arg` receiving an invalid arg ID or `mut_subcommand` receiving an invalid command name
 
 ### Compatibility
 

src/builder/command.rs

@@ -205,6 +205,10 @@ impl Command {
     ///
     /// This can be useful for modifying the auto-generated help or version arguments.
     ///
+    /// # Panics
+    ///
+    /// If the argument is undefined
+    ///
     /// # Examples
     ///
     /// ```rust
@@ -227,15 +231,16 @@ impl Command {
     /// assert!(res.is_ok());
     /// ```
     #[must_use]
-    pub fn mut_arg<F>(mut self, arg_id: impl Into<Id>, f: F) -> Self
+    #[cfg_attr(debug_assertions, track_caller)]
+    pub fn mut_arg<F>(mut self, arg_id: impl AsRef<str>, f: F) -> Self
     where
         F: FnOnce(Arg) -> Arg,
     {
-        let id = arg_id.into();
+        let id = arg_id.as_ref();
         let a = self
             .args
-            .remove_by_name(id.as_str())
-            .unwrap_or_else(|| Arg::new(id));
+            .remove_by_name(id)
+            .unwrap_or_else(|| panic!("Argument `{}` is undefined", id));
 
         self.args.push(f(a));
         self
@@ -246,6 +251,10 @@ impl Command {
     /// This can be useful for modifying auto-generated arguments of nested subcommands with
     /// [`Command::mut_arg`].
     ///
+    /// # Panics
+    ///
+    /// If the subcommand is undefined
+    ///
     /// # Examples
     ///
     /// ```rust
@@ -275,7 +284,7 @@ impl Command {
         let subcmd = if let Some(idx) = pos {
             self.subcommands.remove(idx)
         } else {
-            Self::new(Str::from(name.to_owned()))
+            panic!("Command `{}` is undefined", name)
         };
 
         self.subcommands.push(f(subcmd));

tests/builder/version.rs

@@ -291,13 +291,9 @@ fn version_required() {
 }
 
 #[test]
+#[should_panic = "Argument `version` is undefined"]
 fn mut_arg_version_no_auto_version() {
-    let res = common()
-        .mut_arg("version", |v| v.short('z').action(ArgAction::SetTrue))
-        .try_get_matches_from("foo -z".split(' '));
-
-    assert!(res.is_ok(), "{}", res.unwrap_err());
-    assert_eq!(res.unwrap().get_one::<bool>("version").copied(), Some(true));
+    let _ = common().mut_arg("version", |v| v.short('z').action(ArgAction::SetTrue));
 }
 
 #[cfg(debug_assertions)]