Auto merge of rust-lang#10585 - c410-f3r:aaaaaaaaaaaa, r=Jarcho

[arithmetic_side_effects] Fix rust-lang#10583

Fixes rust-lang#10583

```
changelog: [`arithmetic_side_effects`]: Correctly handle division and module when the right-hand-side is unknown
```

Author: bors

clippy_lints/src/operators/arithmetic_side_effects.rs

@@ -154,10 +154,18 @@ impl ArithmeticSideEffects {
                 Self::literal_integer(cx, actual_rhs),
             ) {
                 (None, None) => false,
-                (None, Some(n)) | (Some(n), None) => match (&op.node, n) {
+                (None, Some(n)) => match (&op.node, n) {
                     // Division and module are always valid if applied to non-zero integers
                     (hir::BinOpKind::Div | hir::BinOpKind::Rem, local_n) if local_n != 0 => true,
-                    // Addition or subtracting zeros is always a no-op
+                    // Adding or subtracting zeros is always a no-op
+                    (hir::BinOpKind::Add | hir::BinOpKind::Sub, 0)
+                    // Multiplication by 1 or 0 will never overflow
+                    | (hir::BinOpKind::Mul, 0 | 1)
+                    => true,
+                    _ => false,
+                },
+                (Some(n), None) => match (&op.node, n) {
+                    // Adding or subtracting zeros is always a no-op
                     (hir::BinOpKind::Add | hir::BinOpKind::Sub, 0)
                     // Multiplication by 1 or 0 will never overflow
                     | (hir::BinOpKind::Mul, 0 | 1)

tests/ui/arithmetic_side_effects.rs

@@ -425,4 +425,8 @@ pub fn integer_arithmetic() {
     i ^= i;
 }
 
+pub fn issue_10583(a: u16) -> u16 {
+    10 / a
+}
+
 fn main() {}

tests/ui/arithmetic_side_effects.stderr

@@ -576,6 +576,12 @@ error: arithmetic operation that can potentially result in unexpected side-effec
 LL |     i * 2;
    |     ^^^^^
 
+error: arithmetic operation that can potentially result in unexpected side-effects
+  --> $DIR/arithmetic_side_effects.rs:394:5
+   |
+LL |     1 % i / 2;
+   |     ^^^^^
+
 error: arithmetic operation that can potentially result in unexpected side-effects
   --> $DIR/arithmetic_side_effects.rs:395:5
    |
@@ -642,5 +648,11 @@ error: arithmetic operation that can potentially result in unexpected side-effec
 LL |     i %= var2;
    |     ^^^^^^^^^
 
-error: aborting due to 107 previous errors
+error: arithmetic operation that can potentially result in unexpected side-effects
+  --> $DIR/arithmetic_side_effects.rs:429:5
+   |
+LL |     10 / a
+   |     ^^^^^^
+
+error: aborting due to 109 previous errors