Avoid using 'macro' items on stable.

This gets rid of the warning message on stable when building examples.

core/codegen/Cargo.toml

@@ -23,6 +23,7 @@ proc-macro2 = "1.0.27"
 devise = "0.4"
 rocket_http = { version = "=0.5.0-rc.3", path = "../http/" }
 unicode-xid = "0.2"
+version_check = "0.9"
 glob = "0.3"
 
 [dev-dependencies]

core/codegen/src/bang/export.rs

@@ -31,6 +31,23 @@ pub fn _macro(input: proc_macro::TokenStream) -> devise::Result<TokenStream> {
         })
         .collect();
 
+    // Only try using the `macro` syntax on nightly/dev or when we don't know.
+    let export = match version_check::is_feature_flaggable() {
+        Some(true) | None => quote! {
+            #(#attrs)*
+            #[cfg(all(nightly, doc))]
+            pub macro #macro_name {
+                #decl_macro_tokens
+            }
+
+            #[cfg(not(all(nightly, doc)))]
+            pub use #mod_name::#internal_name as #macro_name;
+        },
+        Some(false) => quote! {
+            pub use #mod_name::#internal_name as #macro_name;
+        }
+    };
+
     Ok(quote! {
         #[allow(non_snake_case)]
         mod #mod_name {
@@ -43,13 +60,6 @@ pub fn _macro(input: proc_macro::TokenStream) -> devise::Result<TokenStream> {
             pub use #internal_name;
         }
 
-        #(#attrs)*
+        #export
-        #[cfg(all(nightly, doc))]
-        pub macro #macro_name {
-            #decl_macro_tokens
-        }
-
-        #[cfg(not(all(nightly, doc)))]
-        pub use #mod_name::#internal_name as #macro_name;
     })
 }