Change type of mutable_noalias to bool

compiler/rustc_session/src/options.rs

@@ -1437,7 +1437,7 @@ options! {
         "use line numbers relative to the function in mir pretty printing"),
     move_size_limit: Option<usize> = (None, parse_opt_number, [TRACKED],
         "the size at which the `large_assignments` lint starts to be emitted"),
-    mutable_noalias: Option<bool> = (None, parse_opt_bool, [TRACKED],
+    mutable_noalias: bool = (true, parse_bool, [TRACKED],
         "emit noalias metadata for mutable references (default: yes)"),
     nll_facts: bool = (false, parse_bool, [UNTRACKED],
         "dump facts from NLL analysis into side files (default: no)"),