Change explicit_counter_loop's message to add parentheses if necessary

Author: rail-rain

clippy_lints/src/loops.rs

@@ -1499,7 +1499,11 @@ fn check_for_loop_explicit_counter<'a, 'tcx>(
                                  {1}) in {2}.enumerate()` or similar iterators",
                                 name,
                                 snippet(cx, pat.span, "_"),
-                                snippet(cx, arg.span, "_")
+                                if higher::range(cx, arg).is_some() {
+                                    format!("({})", snippet(cx, arg.span, "_"))
+                                } else {
+                                    format!("{}", sugg::Sugg::hir(cx, arg, "_").maybe_par())
+                                }
                             ),
                         );
                     }

tests/ui/explicit_counter_loop.rs

@@ -113,3 +113,12 @@ mod issue_3308 {
         }
     }
 }
+
+mod issue_1670 {
+    pub fn test() {
+        let mut count = 0;
+        for _i in 3..10 {
+            count += 1;
+        }
+    }
+}

tests/ui/explicit_counter_loop.stderr

@@ -1,12 +1,12 @@
-error: the variable `_index` is used as a loop counter. Consider using `for (_index, _v) in &vec.enumerate()` or similar iterators
+error: the variable `_index` is used as a loop counter. Consider using `for (_index, _v) in (&vec).enumerate()` or similar iterators
   --> $DIR/explicit_counter_loop.rs:6:15
    |
 LL |     for _v in &vec {
    |               ^^^^
    |
    = note: `-D clippy::explicit-counter-loop` implied by `-D warnings`
 
-error: the variable `_index` is used as a loop counter. Consider using `for (_index, _v) in &vec.enumerate()` or similar iterators
+error: the variable `_index` is used as a loop counter. Consider using `for (_index, _v) in (&vec).enumerate()` or similar iterators
   --> $DIR/explicit_counter_loop.rs:12:15
    |
 LL |     for _v in &vec {
@@ -24,5 +24,11 @@ error: the variable `count` is used as a loop counter. Consider using `for (coun
 LL |         for ch in text.chars() {
    |                   ^^^^^^^^^^^^
 
-error: aborting due to 4 previous errors
+error: the variable `count` is used as a loop counter. Consider using `for (count, _i) in (3..10).enumerate()` or similar iterators
+  --> $DIR/explicit_counter_loop.rs:120:19
+   |
+LL |         for _i in 3..10 {
+   |                   ^^^^^
+
+error: aborting due to 5 previous errors