Check usage of max_level features

With this change log will to fail to compile if two or more
(release_)max_level_* features are used.

src/lib.rs

@@ -346,6 +346,45 @@
 #![cfg_attr(rustbuild, feature(staged_api, rustc_private))]
 #![cfg_attr(rustbuild, unstable(feature = "rustc_private", issue = "27812"))]
 
+#[cfg(any(
+    all(feature = "max_level_off", feature = "max_level_error"),
+    all(feature = "max_level_off", feature = "max_level_warn"),
+    all(feature = "max_level_off", feature = "max_level_info"),
+    all(feature = "max_level_off", feature = "max_level_debug"),
+    all(feature = "max_level_off", feature = "max_level_trace"),
+    all(feature = "max_level_error", feature = "max_level_warn"),
+    all(feature = "max_level_error", feature = "max_level_info"),
+    all(feature = "max_level_error", feature = "max_level_debug"),
+    all(feature = "max_level_error", feature = "max_level_trace"),
+    all(feature = "max_level_warn", feature = "max_level_info"),
+    all(feature = "max_level_warn", feature = "max_level_debug"),
+    all(feature = "max_level_warn", feature = "max_level_trace"),
+    all(feature = "max_level_info", feature = "max_level_debug"),
+    all(feature = "max_level_info", feature = "max_level_trace"),
+    all(feature = "max_level_debug", feature = "max_level_trace"),
+))]
+compile_error!("multiple max_level_* features set");
+
+#[rustfmt::skip]
+#[cfg(any(
+    all(feature = "release_max_level_off", feature = "release_max_level_error"),
+    all(feature = "release_max_level_off", feature = "release_max_level_warn"),
+    all(feature = "release_max_level_off", feature = "release_max_level_info"),
+    all(feature = "release_max_level_off", feature = "release_max_level_debug"),
+    all(feature = "release_max_level_off", feature = "release_max_level_trace"),
+    all(feature = "release_max_level_error", feature = "release_max_level_warn"),
+    all(feature = "release_max_level_error", feature = "release_max_level_info"),
+    all(feature = "release_max_level_error", feature = "release_max_level_debug"),
+    all(feature = "release_max_level_error", feature = "release_max_level_trace"),
+    all(feature = "release_max_level_warn", feature = "release_max_level_info"),
+    all(feature = "release_max_level_warn", feature = "release_max_level_debug"),
+    all(feature = "release_max_level_warn", feature = "release_max_level_trace"),
+    all(feature = "release_max_level_info", feature = "release_max_level_debug"),
+    all(feature = "release_max_level_info", feature = "release_max_level_trace"),
+    all(feature = "release_max_level_debug", feature = "release_max_level_trace"),
+))]
+compile_error!("multiple release_max_level_* features set");
+
 #[cfg(all(not(feature = "std"), not(test)))]
 extern crate core as std;