Improvements to while_let_on_iterator

* Suggest `&mut iter` when the iterator is used after the loop.
* Suggest `&mut iter` when the iterator is a field in a struct.
* Don't lint when the iterator is a field in a struct, and the struct is
used in the loop.
* Lint when the loop is nested in another loop, but suggest `&mut iter`
unless the iterator is from a local declared inside the loop.

Author: Jarcho

clippy_lints/src/doc.rs

@@ -383,7 +383,7 @@ pub fn strip_doc_comment_decoration(doc: &str, comment_kind: CommentKind, span:
     let mut no_stars = String::with_capacity(doc.len());
     for line in doc.lines() {
         let mut chars = line.chars();
-        while let Some(c) = chars.next() {
+        for c in &mut chars {
             if c.is_whitespace() {
                 no_stars.push(c);
             } else {