Auto merge of #6076 - rail-rain:fix_fp_explicit_counter_loop, r=matthiaskrgr

Fix a FP in `explicit_counter_loop`

Fixes #4677 and #6074

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.

---

changelog: Fix a false positive in `explicit_counter_loop` where the loop counter is used after incremented

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 @@ fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
         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 @@ pub fn test() {
         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 @@ pub fn test() {
         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 @@ pub fn test() {
         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);
+        }
+    }
+}