Force runtime selection (#435)

ntex-rt/build.rs

@@ -0,0 +1,24 @@
+use std::{collections::HashSet, env};
+
+fn main() {
+    let mut features = HashSet::<&'static str>::default();
+
+    for (key, _val) in env::vars() {
+        let _ = match key.as_ref() {
+            "CARGO_FEATURE_COMPIO" => features.insert("compio"),
+            "CARGO_FEATURE_TOKIO" => features.insert("tokio"),
+            "CARGO_FEATURE_GLOMMIO" => features.insert("glommio"),
+            "CARGO_FEATURE_ASYNC_STD" => features.insert("async-std"),
+            _ => false,
+        };
+    }
+
+    if features.is_empty() {
+        panic!("Runtime must be selected '--feature=ntex\\$runtime', available options are \"compio\", \"tokio\", \"async-std\", \"glommio\"");
+    } else if features.len() > 1 {
+        panic!(
+            "Only one runtime feature could be selected, current selection {:?}",
+            features
+        );
+    }
+}