Add user-friendly error when no alloc or std feature are enabled

src/features_check/error.rs

@@ -0,0 +1 @@
+"serde_json doesn't work without `alloc` or default `std` feature yet"

src/features_check/mod.rs

@@ -0,0 +1,13 @@
+//! Shows a user-friendly compiler error on incompatible selected features.
+
+#[allow(unused_macros)]
+macro_rules! hide_from_rustfmt {
+    ($mod:item) => {
+        $mod
+    };
+}
+
+#[cfg(all(not(feature = "std"), not(feature = "alloc")))]
+hide_from_rustfmt! {
+    mod error;
+}

src/lib.rs

@@ -432,6 +432,8 @@ pub mod map;
 pub mod ser;
 pub mod value;
 
+mod features_check;
+
 mod io;
 mod iter;
 mod number;