Truncate constants to target type in comparison (#16782)

changelog: [`bad_bit_mask`]: truncate constant to target type before
warning about impossible equality

Fixes rust-lang/rust-clippy#16781

clippy_lints/src/operators/bit_mask.rs

@@ -36,12 +36,19 @@ fn invert_cmp(cmp: BinOpKind) -> BinOpKind {
     }
 }
 
-fn check_compare<'a>(cx: &LateContext<'a>, bit_op: &Expr<'a>, cmp_op: BinOpKind, cmp_value: u128, span: Span) {
+fn check_compare<'a>(cx: &LateContext<'a>, bit_op: &Expr<'a>, cmp_op: BinOpKind, mut cmp_value: u128, span: Span) {
     if let ExprKind::Binary(op, left, right) = &bit_op.kind {
         if op.node != BinOpKind::BitAnd && op.node != BinOpKind::BitOr || is_from_proc_macro(cx, bit_op) {
             return;
         }
         if let Some(mask) = fetch_int_literal(cx, right).or_else(|| fetch_int_literal(cx, left)) {
+            let ty = cx.typeck_results().expr_ty(bit_op);
+            if !ty.is_ptr_sized_integral()
+                && let bits = ty.primitive_size(cx.tcx)
+            {
+                // Strip high bits that don't fit into the result type as they won't be used in the comparison
+                cmp_value &= bits.unsigned_int_max();
+            }
             check_bit_mask(cx, op.node, cmp_op, mask, cmp_value, span);
         }
     }

tests/ui/bit_masks.rs

@@ -89,3 +89,13 @@ fn ineffective() {
     x | 3 > 4; // not an error (yet), better written as x >= 4
     x | 4 <= 19;
 }
+
+mod issue16781 {
+    fn unsigned(x: u8) -> bool {
+        x & 0xf0 == 0x11 << 4
+    }
+
+    fn signed(x: i8) -> bool {
+        x & 0x70 == 0x11 << 4
+    }
+}