add a -Zchalk command-line option

src/librustc/session/config.rs

@@ -1289,6 +1289,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "tell the linker to strip debuginfo when building without debuginfo enabled."),
     share_generics: Option<bool> = (None, parse_opt_bool, [TRACKED],
           "make the current crate share its generic instantiations"),
+    chalk: bool = (false, parse_bool, [TRACKED],
+          "enable the experimental Chalk-based trait solving engine"),
 }
 
 pub fn default_lib_output() -> CrateType {