Auto merge of rust-lang#12409 - cookie-s:fix-identityop-duplicate-errors, r=Alexendoo

[`identity_op`]: Fix duplicate diagnostics

Relates to rust-lang#12379

In the `identity_op` lint, the following diagnostic was emitted two times

```
  --> tests/ui/identity_op.rs:156:5
   |
LL |     1 * 1;
   |     ^^^^^ help: consider reducing it to: `1`
   |
```

because both of the left operand and the right operand are the identity element of the multiplication.

This PR fixes the issue so that if a diagnostic is created for an operand, the check of the other operand will be skipped. It's fine because the result is always the same in the affected operators.

---

changelog: [`identity_op`]: Fix duplicate diagnostics

Author: bors

clippy_lints/src/operators/identity_op.rs

@@ -42,16 +42,15 @@ pub(crate) fn check<'tcx>(
 
     match op {
         BinOpKind::Add | BinOpKind::BitOr | BinOpKind::BitXor => {
-            check_op(
+            let _ = check_op(
                 cx,
                 left,
                 0,
                 expr.span,
                 peeled_right_span,
                 needs_parenthesis(cx, expr, right),
                 right_is_coerced_to_value,
-            );
-            check_op(
+            ) || check_op(
                 cx,
                 right,
                 0,
@@ -62,7 +61,7 @@ pub(crate) fn check<'tcx>(
             );
         },
         BinOpKind::Shl | BinOpKind::Shr | BinOpKind::Sub => {
-            check_op(
+            let _ = check_op(
                 cx,
                 right,
                 0,
@@ -73,16 +72,26 @@ pub(crate) fn check<'tcx>(
             );
         },
         BinOpKind::Mul => {
-            check_op(
+            let _ = check_op(
                 cx,
                 left,
                 1,
                 expr.span,
                 peeled_right_span,
                 needs_parenthesis(cx, expr, right),
                 right_is_coerced_to_value,
+            ) || check_op(
+                cx,
+                right,
+                1,
+                expr.span,
+                peeled_left_span,
+                Parens::Unneeded,
+                left_is_coerced_to_value,
             );
-            check_op(
+        },
+        BinOpKind::Div => {
+            let _ = check_op(
                 cx,
                 right,
                 1,
@@ -92,26 +101,16 @@ pub(crate) fn check<'tcx>(
                 left_is_coerced_to_value,
             );
         },
-        BinOpKind::Div => check_op(
-            cx,
-            right,
-            1,
-            expr.span,
-            peeled_left_span,
-            Parens::Unneeded,
-            left_is_coerced_to_value,
-        ),
         BinOpKind::BitAnd => {
-            check_op(
+            let _ = check_op(
                 cx,
                 left,
                 -1,
                 expr.span,
                 peeled_right_span,
                 needs_parenthesis(cx, expr, right),
                 right_is_coerced_to_value,
-            );
-            check_op(
+            ) || check_op(
                 cx,
                 right,
                 -1,
@@ -201,12 +200,12 @@ fn check_remainder(cx: &LateContext<'_>, left: &Expr<'_>, right: &Expr<'_>, span
     }
 }
 
-fn check_op(cx: &LateContext<'_>, e: &Expr<'_>, m: i8, span: Span, arg: Span, parens: Parens, is_erased: bool) {
+fn check_op(cx: &LateContext<'_>, e: &Expr<'_>, m: i8, span: Span, arg: Span, parens: Parens, is_erased: bool) -> bool {
     if let Some(Constant::Int(v)) = constant_simple(cx, cx.typeck_results(), e).map(Constant::peel_refs) {
         let check = match *cx.typeck_results().expr_ty(e).peel_refs().kind() {
             ty::Int(ity) => unsext(cx.tcx, -1_i128, ity),
             ty::Uint(uty) => clip(cx.tcx, !0, uty),
-            _ => return,
+            _ => return false,
         };
         if match m {
             0 => v == 0,
@@ -215,8 +214,10 @@ fn check_op(cx: &LateContext<'_>, e: &Expr<'_>, m: i8, span: Span, arg: Span, pa
             _ => unreachable!(),
         } {
             span_ineffective_operation(cx, span, arg, parens, is_erased);
+            return true;
         }
     }
+    false
 }
 
 fn span_ineffective_operation(

tests/ui/identity_op.fixed

@@ -1,5 +1,3 @@
-//@compile-flags: -Zdeduplicate-diagnostics=yes
-
 #![warn(clippy::identity_op)]
 #![allow(unused)]
 #![allow(

tests/ui/identity_op.rs

@@ -1,5 +1,3 @@
-//@compile-flags: -Zdeduplicate-diagnostics=yes
-
 #![warn(clippy::identity_op)]
 #![allow(unused)]
 #![allow(