perf(build): preserve no-op codegen artifacts

hemx-build/src/lib.rs

@@ -223,15 +223,26 @@ impl AppBuilder {
             resources.add_stylesheet(&self.template_dir, &path, &source)?;
         }
 
-        std::fs::write(
-            out_dir.join("hemx.generated.rs"),
-            resources.generated_rs(self.global_exports),
+        write_if_changed(
+            &out_dir.join("hemx.generated.rs"),
+            resources.generated_rs(self.global_exports).as_bytes(),
         )?;
-        std::fs::write(out_dir.join("hemx.syms"), resources.syms())?;
+        write_if_changed(&out_dir.join("hemx.syms"), resources.syms().as_bytes())?;
         Ok(())
     }
 }
 
+fn write_if_changed(path: &Path, contents: &[u8]) -> io::Result<bool> {
+    match std::fs::read(path) {
+        Ok(existing) if existing == contents => return Ok(false),
+        Ok(_) => {}
+        Err(error) if error.kind() == io::ErrorKind::NotFound => {}
+        Err(error) => return Err(error),
+    }
+    std::fs::write(path, contents)?;
+    Ok(true)
+}
+
 pub fn app() -> AppBuilder {
     AppBuilder {
         out_dir: None,
@@ -2162,6 +2173,42 @@ fn parse_error(path: &Path, err: impl std::fmt::Display) -> io::Error {
 mod tests {
     use super::*;
 
+    #[test]
+    fn no_op_build_preserves_generated_artifact_timestamps() {
+        // req: build/009
+        let base = test_dir("hemx-build-no-op");
+        let templates = base.join("templates");
+        let out = base.join("out");
+        let _ = std::fs::remove_dir_all(&base);
+        std::fs::create_dir_all(&templates).unwrap();
+        std::fs::write(
+            templates.join("panel.heml"),
+            r#"<button data-hemx-handle="save">Save</button>"#,
+        )
+        .unwrap();
+
+        app().template_dir(&templates).out_dir(&out).run().unwrap();
+        let generated = out.join("hemx.generated.rs");
+        let symbols = out.join("hemx.syms");
+        let generated_modified = std::fs::metadata(&generated).unwrap().modified().unwrap();
+        let symbols_modified = std::fs::metadata(&symbols).unwrap().modified().unwrap();
+
+        std::thread::sleep(std::time::Duration::from_millis(20));
+        app().template_dir(&templates).out_dir(&out).run().unwrap();
+
+        assert_eq!(
+            std::fs::metadata(generated).unwrap().modified().unwrap(),
+            generated_modified,
+            "no-op codegen must not invalidate downstream Rust compilation"
+        );
+        assert_eq!(
+            std::fs::metadata(symbols).unwrap().modified().unwrap(),
+            symbols_modified,
+            "no-op symbol generation must preserve its artifact timestamp"
+        );
+        let _ = std::fs::remove_dir_all(&base);
+    }
+
     #[test]
     fn emits_generated_resources_from_heml() {
         // req: codegen/003