Loop over references to containers instead of using explicit iteration methods.

https://github.com/rust-lang-nursery/rust-clippy/wiki#explicit_iter_loop

lib/frontend/src/frontend.rs

@@ -492,7 +492,7 @@ impl<'a, Variable> FunctionBuilder<'a, Variable>
 
     fn fill_function_args_values(&mut self, ebb: Ebb) {
         debug_assert!(self.pristine);
-        for argtyp in self.func.signature.argument_types.iter() {
+        for argtyp in &self.func.signature.argument_types {
             self.builder
                 .function_args_values
                 .push(self.func.dfg.append_ebb_arg(ebb, argtyp.value_type));

lib/frontend/src/ssa.rs

@@ -402,7 +402,7 @@ impl<Variable> SSABuilder<Variable>
         };
         // For each undef var we look up values in the predecessors and create an Ebb argument
         // only if necessary.
-        for &(var, val) in undef_vars.iter() {
+        for &(var, val) in &undef_vars {
             let (_, mut local_side_effects) =
                 self.predecessors_lookup(dfg, layout, jts, val, var, ebb, &predecessors);
             side_effects