fix(derive): suggest valid component scopes

hemx-derive/src/lib.rs

@@ -723,8 +723,14 @@ fn component_contract_errors(
     let mut errors = Vec::new();
 
     if let Some(component) = component.filter(|_| !implemented.is_empty() && generated.is_empty()) {
+        let available = syms_components(path);
+        let repair = if available.is_empty() {
+            "no generated components with handles are available; add a data-hemx-handle to the component template or check build.rs generation".to_owned()
+        } else {
+            format!("available generated components: {}", available.join(", "))
+        };
         errors.push(format!(
-            "#[hemx::component({component:?})] does not match any generated handles; check the .heml file name or component name"
+            "#[hemx::component({component:?})] does not match any generated handles; {repair}"
         ));
     }
 
@@ -860,6 +866,26 @@ fn syms_handles(path: &PathBuf, component: Option<&str>) -> Vec<String> {
         .collect()
 }
 
+fn syms_components(path: &PathBuf) -> Vec<String> {
+    let Ok(syms) = std::fs::read_to_string(path) else {
+        return Vec::new();
+    };
+    let mut components = syms
+        .lines()
+        .filter_map(|line| {
+            let mut fields = line.split('\t');
+            matches!(fields.next(), Some("handle"))
+                .then(|| fields.next())
+                .flatten()
+                .and_then(symbol_component)
+                .map(ToOwned::to_owned)
+        })
+        .collect::<Vec<_>>();
+    components.sort_unstable();
+    components.dedup();
+    components
+}
+
 fn symbol_component(symbol: &str) -> Option<&str> {
     let path = symbol.split_once("::")?.0;
     path.rsplit_once('/')

hemx-derive/tests/compile_fail.rs

@@ -223,6 +223,10 @@ mod handlers {
         stderr.contains("#[hemx::component(\"todos\")] does not match any generated handles"),
         "missing unknown component diagnostic in stderr:\n{stderr}"
     );
+    assert!(
+        stderr.contains("available generated components: admin"),
+        "unknown scope diagnostic must name the valid repair choice:\n{stderr}"
+    );
 }
 
 #[test]