fix(derive): require generated handler symbols

Make #[slhx::handler] fail when slhx.syms is missing instead of silently skipping generated handle checks. Add compile-fail coverage with an actionable build.rs hint.

req: build/003

req: test/003

slhx-derive/src/lib.rs

@@ -8,39 +8,54 @@ pub fn handler(_attr: TokenStream, item: TokenStream) -> TokenStream {
     let function = parse_macro_input!(item as ItemFn);
     let name = function.sig.ident.to_string();
 
-    if let Some(syms_path) = syms_path() {
-        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();
-            }
-            if handle_requires_form(&syms_path, &name) && !has_form_param(&function) {
-                let message = format!(
-                    "slhx handler `{name}` handles a generated form and must accept slhx::Form<_>"
-                );
-                return quote!(
-                    #function
-                    compile_error!(#message);
-                )
-                .into();
-            }
-        }
+    let Some(syms_path) = syms_path() else {
+        let message = "#[slhx::handler] requires OUT_DIR; run inside a Cargo crate with slhx_build::app() in build.rs";
+        return quote!(
+            #function
+            compile_error!(#message);
+        )
+        .into();
+    };
+    if !syms_path.exists() {
+        let message = format!(
+            "#[slhx::handler] could not find {}; add slhx_build::app().run()? to build.rs or check template generation",
+            syms_path.display()
+        );
+        return quote!(
+            #function
+            compile_error!(#message);
+        )
+        .into();
+    }
+    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();
+    }
+    if handle_requires_form(&syms_path, &name) && !has_form_param(&function) {
+        let message = format!(
+            "slhx handler `{name}` handles a generated form and must accept slhx::Form<_>"
+        );
+        return quote!(
+            #function
+            compile_error!(#message);
+        )
+        .into();
     }
 
     quote!(#function).into()

slhx-derive/tests/compile_fail.rs

@@ -60,6 +60,57 @@ fn known() {}
     );
 }
 
+#[test]
+fn handler_macro_reports_missing_syms() {
+    // req: build/003 req: test/003
+    let fixture = Fixture::new("slhx-derive-handler-syms-fail");
+    fixture.write(
+        "Cargo.toml",
+        &format!(
+            r#"[package]
+name = "slhx-derive-handler-syms-fail"
+version = "0.0.0"
+edition = "2021"
+
+[lib]
+path = "src/lib.rs"
+
+[dependencies]
+slhx = {{ path = {:?} }}
+"#,
+            repo_path("slhx")
+        ),
+    );
+    fixture.write(
+        "build.rs",
+        r#"fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+}
+"#,
+    );
+    fixture.write(
+        "src/lib.rs",
+        r#"#[slhx::handler]
+fn create() -> impl slhx::IntoEffect {
+    slhx::EffectBatch::default()
+}
+"#,
+    );
+
+    let output = check_fixture(&fixture);
+
+    assert!(!output.status.success(), "fixture unexpectedly compiled");
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        stderr.contains("#[slhx::handler] could not find"),
+        "missing handler syms diagnostic in stderr:\n{stderr}"
+    );
+    assert!(
+        stderr.contains("add slhx_build::app().run()? to build.rs"),
+        "missing build.rs hint in stderr:\n{stderr}"
+    );
+}
+
 #[test]
 fn form_handle_requires_form_parameter() {
     // req: form/004 req: form/006 req: test/003