document uniqueness requirement for app names

...and provide a friendlier diagnostic when there's a conflict.

Fixes #131

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

src/command.rs

@@ -79,7 +79,11 @@ pub enum Command {
 
 #[derive(clap::Args, Debug)]
 pub struct Componentize {
-    /// The name of a Python module containing the app to wrap
+    /// The name of a Python module containing the app to wrap.
+    ///
+    /// Note that this should not match (any of) the world name(s) you are targeting since `componentize-py` will
+    /// generate code using those name(s), and Python doesn't know how to load two top-level modules with the same
+    /// name.
     pub app_name: String,
 
     /// Specify a directory containing the app and/or its dependencies.  May be specified more than once.

src/lib.rs

@@ -312,6 +312,13 @@ pub async fn componentize(
         .chain(main_world)
         .collect::<IndexSet<_>>();
 
+    if worlds
+        .iter()
+        .any(|&id| app_name == resolve.worlds[id].name.to_snake_case().escape())
+    {
+        bail!("App name `{app_name}` conflicts with world name; please rename your application module.");
+    }
+
     let summary = Summary::try_new(
         &resolve,
         &worlds,