loops: fix false positives with explicit_iter_loop and references (fixes #261)

src/loops.rs

@@ -4,7 +4,7 @@
 use rustc::middle::ty;
 use std::collections::HashSet;
 
-use utils::{snippet, span_lint, get_parent_expr, match_trait_method, match_type, walk_ptrs_ty,
+use utils::{snippet, span_lint, get_parent_expr, match_trait_method, match_type,
             in_external_macro, expr_block, span_help_and_lint};
 use utils::{VEC_PATH, LL_PATH};
 
@@ -191,7 +191,9 @@ fn visit_expr(&mut self, expr: &'v Expr) {
 /// Return true if the type of expr is one that provides IntoIterator impls
 /// for &T and &mut T, such as Vec.
 fn is_ref_iterable_type(cx: &Context, e: &Expr) -> bool {
-    let ty = walk_ptrs_ty(cx.tcx.expr_ty(e));
+    // no walk_ptrs_ty: calling iter() on a reference can make sense because it
+    // will allow further borrows afterwards
+    let ty = cx.tcx.expr_ty(e);
     is_array(ty) ||
         match_type(cx, ty, &VEC_PATH) ||
         match_type(cx, ty, &LL_PATH) ||

tests/compile-fail/for_loop.rs

@@ -37,6 +37,7 @@ fn main() {
     for _v in &mut vec { } // these are fine
 
     for _v in [1, 2, 3].iter() { } //~ERROR it is more idiomatic to loop over `&[
+    for _v in (&mut [1, 2, 3]).iter() { } // no error
     let ll: LinkedList<()> = LinkedList::new();
     for _v in ll.iter() { } //~ERROR it is more idiomatic to loop over `&ll`
     let vd: VecDeque<()> = VecDeque::new();