auto merge of #19899 : japaric/rust/unops-by-value, r=nikomatsakis

- The following operator traits now take their argument by value: `Neg`, `Not`. This breaks all existing implementations of these traits.

- The unary operation `OP a` now "desugars" to `OpTrait::op_method(a)` and consumes its argument.

[breaking-change]

---

r? @nikomatsakis This PR is very similar to the binops-by-value PR
cc @aturon

src/librustc/middle/expr_use_visitor.rs

@@ -576,8 +576,14 @@ impl<'d,'t,'tcx,TYPER:mc::Typer<'tcx>> ExprUseVisitor<'d,'t,'tcx,TYPER> {
                 self.walk_block(&**blk);
             }
 
-            ast::ExprUnary(_, ref lhs) => {
-                if !self.walk_overloaded_operator(expr, &**lhs, Vec::new(), PassArgs::ByRef) {
+            ast::ExprUnary(op, ref lhs) => {
+                let pass_args = if ast_util::is_by_value_unop(op) {
+                    PassArgs::ByValue
+                } else {
+                    PassArgs::ByRef
+                };
+
+                if !self.walk_overloaded_operator(expr, &**lhs, Vec::new(), pass_args) {
                     self.consume_expr(&**lhs);
                 }
             }
@@ -937,7 +943,9 @@ impl<'d,'t,'tcx,TYPER:mc::Typer<'tcx>> ExprUseVisitor<'d,'t,'tcx,TYPER> {
         match pass_args {
             PassArgs::ByValue => {
                 self.consume_expr(receiver);
-                self.consume_expr(rhs[0]);
+                for &arg in rhs.iter() {
+                    self.consume_expr(arg);
+                }
 
                 return true;
             },