feat(derive): reject empty handlers

req: derive_handler/001

slhx-derive/src/lib.rs

@@ -1,7 +1,7 @@
 use proc_macro::TokenStream;
 use quote::quote;
 use std::path::PathBuf;
-use syn::{parse_macro_input, ItemFn};
+use syn::{parse_macro_input, FnArg, ItemFn, ReturnType, Type};
 
 #[proc_macro_attribute]
 pub fn handler(_attr: TokenStream, item: TokenStream) -> TokenStream {
@@ -9,15 +9,27 @@ pub fn handler(_attr: TokenStream, item: TokenStream) -> TokenStream {
     let name = function.sig.ident.to_string();
 
     if let Some(syms_path) = syms_path() {
-        if syms_path.exists() && !syms_contains_handle(&syms_path, &name) {
-            let message = format!(
-                "unknown slhx handle `{name}`; add `data-slhx-handle=\"{name}\"` to a template or rename this handler"
-            );
-            return quote!(
-                #function
-                compile_error!(#message);
-            )
-            .into();
+        if syms_path.exists() {
+            if !syms_contains_handle(&syms_path, &name) {
+                let message = format!(
+                    "unknown slhx handle `{name}`; add `data-slhx-handle=\"{name}\"` to a template or rename this handler"
+                );
+                return quote!(
+                    #function
+                    compile_error!(#message);
+                )
+                .into();
+            }
+            if !has_form_param(&function) && !has_non_unit_return(&function) {
+                let message = format!(
+                    "slhx handler `{name}` must accept a form/context parameter or return a value implementing IntoEffect"
+                );
+                return quote!(
+                    #function
+                    compile_error!(#message);
+                )
+                .into();
+            }
         }
     }
 
@@ -72,6 +84,27 @@ fn generated_path(file: &str) -> Option<PathBuf> {
     std::env::var_os("OUT_DIR").map(|out_dir| PathBuf::from(out_dir).join(file))
 }
 
+fn has_form_param(function: &ItemFn) -> bool {
+    function.sig.inputs.iter().any(|arg| match arg {
+        FnArg::Typed(arg) => type_ends_with(&arg.ty, "Form"),
+        FnArg::Receiver(_) => false,
+    })
+}
+
+fn has_non_unit_return(function: &ItemFn) -> bool {
+    match &function.sig.output {
+        ReturnType::Default => false,
+        ReturnType::Type(_, ty) => !matches!(ty.as_ref(), Type::Tuple(tuple) if tuple.elems.is_empty()),
+    }
+}
+
+fn type_ends_with(ty: &Type, ident: &str) -> bool {
+    match ty {
+        Type::Path(path) => path.path.segments.last().is_some_and(|segment| segment.ident == ident),
+        _ => false,
+    }
+}
+
 fn syms_contains_handle(path: &PathBuf, ident: &str) -> bool {
     let Ok(syms) = std::fs::read_to_string(path) else {
         return true;
@@ -91,7 +124,8 @@ fn compile_error(message: &str) -> TokenStream {
 
 #[cfg(test)]
 mod tests {
-    use super::syms_contains_handle;
+    use super::{has_form_param, has_non_unit_return, syms_contains_handle};
+    use syn::parse_quote;
 
     #[test]
     fn syms_lookup_matches_handle_ident() {
@@ -107,4 +141,17 @@ mod tests {
 
         let _ = std::fs::remove_file(path);
     }
+
+    #[test]
+    fn handler_shape_accepts_form_or_effect_return() {
+        // req: derive_handler/001
+        let with_form = parse_quote!(fn save(form: slhx::Form<String>) {});
+        let with_return = parse_quote!(fn ping() -> impl slhx::IntoEffect { slhx::EffectBatch::default() });
+        let empty = parse_quote!(fn noop() {});
+
+        assert!(has_form_param(&with_form));
+        assert!(has_non_unit_return(&with_return));
+        assert!(!has_form_param(&empty));
+        assert!(!has_non_unit_return(&empty));
+    }
 }