Fix argument indices in MIR for closures.

Previously, all references to closure arguments went to the argument before the
one they should (e.g. to arg1 when it was supposed to be arg2). This was because
the MIR builder did not account for the implicit arguments that come before the
explicit arguments, and closures have one implicit argument - the struct
containing the captures.

src/librustc_mir/build/mod.rs

@@ -139,6 +139,7 @@ impl<'a,'tcx> Builder<'a,'tcx> {
     {
         self.in_scope(argument_extent, block, |this| {
             let arg_decls = {
+                let num_implicit_args = implicit_arguments.len();
                 let implicit_arg_decls = implicit_arguments.into_iter()
                                                            .map(|ty| ArgDecl { ty: ty });
 
@@ -149,7 +150,7 @@ impl<'a,'tcx> Builder<'a,'tcx> {
                     .into_iter()
                     .enumerate()
                     .map(|(index, (ty, pattern))| {
-                        let lvalue = Lvalue::Arg(index as u32);
+                        let lvalue = Lvalue::Arg((num_implicit_args + index) as u32);
                         let pattern = this.hir.irrefutable_pat(pattern);
                         unpack!(block = this.lvalue_into_pattern(block,
                                                                  argument_extent,