Fix while_let_on_iterator

Reborrow mutable references rather then take a reference to them.

clippy_lints/src/loops/while_let_on_iterator.rs

@@ -7,7 +7,7 @@ use clippy_utils::{
 use if_chain::if_chain;
 use rustc_errors::Applicability;
 use rustc_hir::intravisit::{walk_expr, ErasedMap, NestedVisitorMap, Visitor};
-use rustc_hir::{def::Res, Expr, ExprKind, HirId, Local, MatchSource, Node, PatKind, QPath, UnOp};
+use rustc_hir::{def::Res, Expr, ExprKind, HirId, Local, MatchSource, Mutability, Node, PatKind, QPath, UnOp};
 use rustc_lint::LateContext;
 use rustc_span::{symbol::sym, Span, Symbol};
 
@@ -48,7 +48,12 @@ pub(super) fn check(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
     // borrowed mutably. TODO: If the struct can be partially moved from and the struct isn't used
     // afterwards a mutable borrow of a field isn't necessary.
     let ref_mut = if !iter_expr.fields.is_empty() || needs_mutable_borrow(cx, &iter_expr, loop_expr) {
-        "&mut "
+        if cx.typeck_results().node_type(iter_expr.hir_id).ref_mutability() == Some(Mutability::Mut) {
+            // Reborrow for mutable references. It may not be possible to get a mutable reference here.
+            "&mut *"
+        } else {
+            "&mut "
+        }
     } else {
         ""
     };
@@ -69,6 +74,8 @@ pub(super) fn check(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
 struct IterExpr {
     /// The span of the whole expression, not just the path and fields stored here.
     span: Span,
+    /// The HIR id of the whole expression, not just the path and fields stored here.
+    hir_id: HirId,
     /// The fields used, in order of child to parent.
     fields: Vec<Symbol>,
     /// The path being used.
@@ -78,12 +85,14 @@ struct IterExpr {
 /// the expression might have side effects.
 fn try_parse_iter_expr(cx: &LateContext<'_>, mut e: &Expr<'_>) -> Option<IterExpr> {
     let span = e.span;
+    let hir_id = e.hir_id;
     let mut fields = Vec::new();
     loop {
         match e.kind {
             ExprKind::Path(ref path) => {
                 break Some(IterExpr {
                     span,
+                    hir_id,
                     fields,
                     path: cx.qpath_res(path, e.hir_id),
                 });