Fix a FP in explicit_counter_loop

Fix a false positive in `explicit_counter_loop` where the loop counter is used after incremented,
adjust the test so that counters are incremented at the end of the loop
and add the test for this false positive.

clippy_lints/src/loops.rs

@@ -2134,7 +2134,7 @@ enum VarState {
     DontWarn,
 }
 
-/// Scan a for loop for variables that are incremented exactly once.
+/// Scan a for loop for variables that are incremented exactly once and not used after that.
 struct IncrementVisitor<'a, 'tcx> {
     cx: &'a LateContext<'tcx>,          // context reference
     states: FxHashMap<HirId, VarState>, // incremented variables
@@ -2154,6 +2154,10 @@ impl<'a, 'tcx> Visitor<'tcx> for IncrementVisitor<'a, 'tcx> {
         if let Some(def_id) = var_def_id(self.cx, expr) {
             if let Some(parent) = get_parent_expr(self.cx, expr) {
                 let state = self.states.entry(def_id).or_insert(VarState::Initial);
+                if *state == VarState::IncrOnce {
+                    *state = VarState::DontWarn;
+                    return;
+                }
 
                 match parent.kind {
                     ExprKind::AssignOp(op, ref lhs, ref rhs) => {

tests/ui/explicit_counter_loop.rs

@@ -38,54 +38,54 @@ mod issue_1219 {
         let text = "banana";
         let mut count = 0;
         for ch in text.chars() {
+            println!("{}", count);
             if ch == 'a' {
                 continue;
             }
             count += 1;
-            println!("{}", count);
         }
 
         // should not trigger the lint because the count is conditional
         let text = "banana";
         let mut count = 0;
         for ch in text.chars() {
+            println!("{}", count);
             if ch == 'a' {
                 count += 1;
             }
-            println!("{}", count);
         }
 
         // should trigger the lint because the count is not conditional
         let text = "banana";
         let mut count = 0;
         for ch in text.chars() {
+            println!("{}", count);
             count += 1;
             if ch == 'a' {
                 continue;
             }
-            println!("{}", count);
         }
 
         // should trigger the lint because the count is not conditional
         let text = "banana";
         let mut count = 0;
         for ch in text.chars() {
+            println!("{}", count);
             count += 1;
             for i in 0..2 {
                 let _ = 123;
             }
-            println!("{}", count);
         }
 
         // should not trigger the lint because the count is incremented multiple times
         let text = "banana";
         let mut count = 0;
         for ch in text.chars() {
+            println!("{}", count);
             count += 1;
             for i in 0..2 {
                 count += 1;
             }
-            println!("{}", count);
         }
     }
 }
@@ -96,30 +96,30 @@ mod issue_3308 {
         let mut skips = 0;
         let erasures = vec![];
         for i in 0..10 {
+            println!("{}", skips);
             while erasures.contains(&(i + skips)) {
                 skips += 1;
             }
-            println!("{}", skips);
         }
 
         // should not trigger the lint because the count is incremented multiple times
         let mut skips = 0;
         for i in 0..10 {
+            println!("{}", skips);
             let mut j = 0;
             while j < 5 {
                 skips += 1;
                 j += 1;
             }
-            println!("{}", skips);
         }
 
         // should not trigger the lint because the count is incremented multiple times
         let mut skips = 0;
         for i in 0..10 {
+            println!("{}", skips);
             for j in 0..5 {
                 skips += 1;
             }
-            println!("{}", skips);
         }
     }
 }
@@ -145,3 +145,16 @@ mod issue_4732 {
         let _closure = || println!("index: {}", index);
     }
 }
+
+mod issue_4677 {
+    pub fn test() {
+        let slice = &[1, 2, 3];
+
+        // should not trigger the lint because the count is used after incremented
+        let mut count = 0;
+        for _i in slice {
+            count += 1;
+            println!("{}", count);
+        }
+    }
+}