debugflag to turn off nonzeroing move hint optimization.

(already thumbs-upped pre-rebase by nikomatsakis)

src/librustc/session/config.rs

@@ -594,6 +594,8 @@ 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"),
 }
 
 pub fn default_lib_output() -> CrateType {

src/librustc/session/mod.rs

@@ -272,6 +272,9 @@ impl Session {
     pub fn print_enum_sizes(&self) -> bool {
         self.opts.debugging_opts.print_enum_sizes
     }
+    pub fn nonzeroing_move_hints(&self) -> bool {
+        !self.opts.debugging_opts.disable_nonzeroing_move_hints
+    }
     pub fn sysroot<'a>(&'a self) -> &'a Path {
         match self.opts.maybe_sysroot {
             Some (ref sysroot) => sysroot,