Improve error messages for faulty flag combinations

Author: TrueDoctor

src/lib.rs

@@ -481,6 +481,10 @@ compile_error!("`wasm-bindgen` cannot be used with `no-std`");
 #[cfg(feature = "stdweb")]
 compile_error!("`stdweb` cannot be used with `no-std`");
 
+#[cfg(target_family = "wasm")]
+#[cfg(not(any(feature = "no_time", feature = "wasm-bindgen", feature = "stdweb")))]
+compile_error!("WASM targets have to use either the `wasm-bindgen`, `stdweb` or `no_time` feature");
+
 #[cfg(target_family = "wasm")]
 #[cfg(feature = "no_std")]
 compile_error!("`no_std` cannot be used for WASM target");