Add rust target configuration for feature #1554

Will panic if the feature is being used outside nightly.
See #1575 (comment)

Author: agodnic

src/codegen/mod.rs

@@ -2580,8 +2580,10 @@ impl CodeGenerator for Enum {
         match variation {
             EnumVariation::Rust { non_exhaustive: nh } => {
                 attrs.push(attributes::repr(repr_name));
-                if nh {
+                if nh && ctx.options().rust_features().non_exhaustive {
                     attrs.push(attributes::non_exhaustive());
+                } else if nh && !ctx.options().rust_features().non_exhaustive {
+                    panic!("The rust target you're using doesn't seem to support non_exhaustive enums");
                 }
             },
             EnumVariation::Bitfield => {

src/features.rs

@@ -206,6 +206,8 @@ rust_feature_def!(
     Nightly {
         /// `thiscall` calling convention ([Tracking issue](https://github.com/rust-lang/rust/issues/42202))
         => thiscall_abi;
+        /// `non_exhaustive` enums/structs ([Tracking issue](https://github.com/rust-lang/rust/issues/44109))
+        => non_exhaustive;
     }
 );