Ease the transition to requiring features by just warning if there's no feature list

while we could make this change (it's all unstable after all), there are crates.io crates that use the feature and that the compiler depends upon. We can instead roll out this feature while still supporting the old way.

src/libsyntax_pos/lib.rs

@@ -387,7 +387,10 @@ impl Span {
     /// `#[allow_internal_unstable]`).
     pub fn allows_unstable(&self, feature: &str) -> bool {
         match self.ctxt().outer().expn_info() {
-            Some(info) => info.allow_internal_unstable.iter().any(|&f| f == feature),
+            Some(info) => info
+                .allow_internal_unstable
+                .iter()
+                .any(|&f| f == feature || f == "allow_internal_unstable_backcompat_hack"),
             None => false,
         }
     }