Turn nonzeroing move hints back off by default.

This is a temporary workaround for the bugs that have been found in
the implementation of PR rust-lang#26173.

 * pnkfelix is unavailable in the short-term (i.e. for the next week) to fix them.

 * When the bugs are fixed, we will turn this back on by default.

(If you want to play with the known-to-be-buggy optimization in the
meantime, you can opt-back in via the debugging option that this
commit is toggling.)

src/librustc/session/config.rs

@@ -596,8 +596,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
           "Force drop flag checks on or off"),
     trace_macros: bool = (false, parse_bool,
           "For every macro invocation, print its name and arguments"),
-    disable_nonzeroing_move_hints: bool = (false, parse_bool,
-          "Force nonzeroing move optimization off"),
+    enable_nonzeroing_move_hints: bool = (false, parse_bool,
+          "Force nonzeroing move optimization on"),
 }
 
 pub fn default_lib_output() -> CrateType {

src/librustc/session/mod.rs

@@ -273,7 +273,7 @@ impl Session {
         self.opts.debugging_opts.print_enum_sizes
     }
     pub fn nonzeroing_move_hints(&self) -> bool {
-        !self.opts.debugging_opts.disable_nonzeroing_move_hints
+        self.opts.debugging_opts.enable_nonzeroing_move_hints
     }
     pub fn sysroot<'a>(&'a self) -> &'a Path {
         match self.opts.maybe_sysroot {