Turn off never type fallback

compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs

@@ -438,6 +438,11 @@ fn parse_never_type_options_attr(
     }
 
     let fallback = fallback.unwrap_or_else(|| {
+        if true {
+            // For a crater experiment, turn off all fallback
+            return NoFallback;
+        }
+
         if tcx.features().never_type_fallback { FallbackToNiko } else { FallbackToUnit }
     });