fix(derive): fail missing surface generation

Make #[slhx::surface] report a compile-time error when slhx.generated.rs is absent instead of silently expanding to an empty module. Add compile-fail coverage for the missing generated include path.

req: build/004

req: test/003

slhx-derive/build.rs

@@ -0,0 +1,4 @@
+fn main() {
+    let out = std::path::PathBuf::from(std::env::var_os("OUT_DIR").unwrap());
+    std::fs::write(out.join("slhx.generated.rs"), "").unwrap();
+}

slhx-derive/src/lib.rs

@@ -66,13 +66,22 @@ fn inject_surface_include(item: TokenStream) -> TokenStream {
 
 fn surface_include() -> String {
     let Some(path) = generated_path("slhx.generated.rs") else {
-        return String::new();
+        return format!(
+            " compile_error!({:?}); ",
+            "#[slhx::surface] requires OUT_DIR; run inside a Cargo crate with slhx_build::app() in build.rs"
+        );
     };
 
     if path.exists() {
         format!(" include!({:?}); ", path.display().to_string())
     } else {
-        String::new()
+        format!(
+            " compile_error!({:?}); ",
+            format!(
+                "#[slhx::surface] could not find {}; add slhx_build::app().run()? to build.rs or check template generation",
+                path.display()
+            )
+        )
     }
 }
 

slhx-derive/tests/compile_fail.rs

@@ -46,14 +46,7 @@ fn known() {}
 "#,
     );
 
-    let output = Command::new("cargo")
+    let output = check_fixture(&fixture);
-        .arg("check")
-        .arg("--quiet")
-        .arg("--manifest-path")
-        .arg(fixture.path.join("Cargo.toml"))
-        .env("CARGO_TARGET_DIR", fixture.path.join("target"))
-        .output()
-        .expect("cargo check fixture runs");
 
     assert!(!output.status.success(), "fixture unexpectedly compiled");
     let stderr = String::from_utf8_lossy(&output.stderr);
@@ -67,6 +60,55 @@ fn known() {}
     );
 }
 
+#[test]
+fn surface_macro_reports_missing_generated_include() {
+    // req: build/004 req: test/003
+    let fixture = Fixture::new("slhx-derive-surface-include-fail");
+    fixture.write(
+        "Cargo.toml",
+        &format!(
+            r#"[package]
+name = "slhx-derive-surface-include-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::surface]
+pub mod ui {}
+"#,
+    );
+
+    let output = check_fixture(&fixture);
+
+    assert!(!output.status.success(), "fixture unexpectedly compiled");
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        stderr.contains("#[slhx::surface] could not find"),
+        "missing surface include 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 generated_resource_references_fail_when_name_is_absent() {
     // req: style/002 req: codegen/001 req: test/003
@@ -110,14 +152,7 @@ pub const MISSING: slhx::CssClass = ui::classes::missing;
 "#,
     );
 
-    let output = Command::new("cargo")
+    let output = check_fixture(&fixture);
-        .arg("check")
-        .arg("--quiet")
-        .arg("--manifest-path")
-        .arg(fixture.path.join("Cargo.toml"))
-        .env("CARGO_TARGET_DIR", fixture.path.join("target"))
-        .output()
-        .expect("cargo check fixture runs");
 
     assert!(!output.status.success(), "fixture unexpectedly compiled");
     let stderr = String::from_utf8_lossy(&output.stderr);
@@ -127,6 +162,17 @@ pub const MISSING: slhx::CssClass = ui::classes::missing;
     );
 }
 
+fn check_fixture(fixture: &Fixture) -> std::process::Output {
+    Command::new("cargo")
+        .arg("check")
+        .arg("--quiet")
+        .arg("--manifest-path")
+        .arg(fixture.path.join("Cargo.toml"))
+        .env("CARGO_TARGET_DIR", fixture.path.join("target"))
+        .output()
+        .expect("cargo check fixture runs")
+}
+
 fn repo_path(crate_name: &str) -> PathBuf {
     Path::new(env!("CARGO_MANIFEST_DIR"))
         .parent()

slhx-derive/tests/surface.rs

@@ -6,6 +6,6 @@ mod ui {
 }
 
 #[test]
-fn surface_macro_preserves_inline_module_without_generated_file() {
+fn surface_macro_preserves_inline_module_with_generated_file() {
     assert_eq!(ui::EXISTING, 1);
 }