don't lint while_let_on_iterator on nested loops

The problem is with a nested loop, the iterator may well be reused. This
changeset introduces a false negative, when the iterator is initialized
within the outer loop. A further PR could get rid of this false negative
by checking if the iterator is indeed initialized within the outer loop.

clippy_lints/src/lib.rs

@@ -250,7 +250,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
     reg.register_late_lint_pass(box shadow::Pass);
     reg.register_late_lint_pass(box types::LetPass);
     reg.register_late_lint_pass(box types::UnitCmp);
-    reg.register_late_lint_pass(box loops::Pass);
+    reg.register_late_lint_pass(box loops::Pass::default());
     reg.register_late_lint_pass(box lifetimes::LifetimePass);
     reg.register_late_lint_pass(box entry::HashMapLint);
     reg.register_late_lint_pass(box ranges::StepByZero);