Make event_enum! error message more helpful

ruma-events-macros/src/event_enum.rs

@@ -192,11 +192,18 @@ fn to_event_content_path(name: &LitStr) -> TokenStream {
 pub(crate) fn to_camel_case(name: &LitStr) -> Ident {
     let span = name.span();
     let name = name.value();
-    assert_eq!(&name[..2], "m.");
+
+    assert_eq!(
+        &name[..2],
+        "m.",
+        "well-known matrix events have to start with `m.`"
+    );
+
     let s = name[2..]
         .split(&['.', '_'] as &[char])
         .map(|s| s.chars().next().unwrap().to_uppercase().to_string() + &s[1..])
         .collect::<String>();
+
     Ident::new(&s, span)
 }
 

tests/ui/08-enum-invalid-path.stderr

@@ -11,7 +11,7 @@ error: proc macro panicked
    |
    = help: message: assertion failed: `(left == right)`
              left: `"no"`,
-            right: `"m."`
+            right: `"m."`: well-known matrix events have to start with `m.`
 
 error[E0433]: failed to resolve: could not find `not` in `ruma_events`
  --> $DIR/08-enum-invalid-path.rs:6:9