Rewrite bit mask lints (#15587)

The helper functions/types could be pulled into `clippy_utils`. I'm
going to go through and sort that crate out in a separate PR first
rather than add to it now.

changelog:  [`ineffective_bit_masks`]: Lint xor expressions

Author: llogiq

clippy_lints/src/operators/bit_mask.rs

@@ -979,6 +979,7 @@ impl<'tcx> ConstEvalCtxt<'tcx> {
         }
     }
 
+    #[expect(clippy::too_many_lines)]
     fn binop(&self, op: BinOpKind, left: &Expr<'_>, right: &Expr<'_>) -> Option<Constant> {
         let l = self.expr(left)?;
         let r = self.expr(right);
@@ -1027,15 +1028,20 @@ impl<'tcx> ConstEvalCtxt<'tcx> {
                 },
                 ty::Uint(ity) => {
                     let bits = ity.bits();
+                    let mask = !0u128 >> (128 - bits);
 
                     match op {
                         BinOpKind::Add => l.checked_add(r).and_then(|n| ity.ensure_fits(n)).map(Constant::Int),
                         BinOpKind::Sub => l.checked_sub(r).and_then(|n| ity.ensure_fits(n)).map(Constant::Int),
                         BinOpKind::Mul => l.checked_mul(r).and_then(|n| ity.ensure_fits(n)).map(Constant::Int),
                         BinOpKind::Div => l.checked_div(r).map(Constant::Int),
                         BinOpKind::Rem => l.checked_rem(r).map(Constant::Int),
-                        BinOpKind::Shr if r < bits => l.checked_shr(r.try_into().ok()?).map(Constant::Int),
-                        BinOpKind::Shl if r < bits => l.checked_shl(r.try_into().ok()?).map(Constant::Int),
+                        BinOpKind::Shr if r < bits => {
+                            l.checked_shr(r.try_into().ok()?).map(|x| Constant::Int(x & mask))
+                        },
+                        BinOpKind::Shl if r < bits => {
+                            l.checked_shl(r.try_into().ok()?).map(|x| Constant::Int(x & mask))
+                        },
                         BinOpKind::BitXor => Some(Constant::Int(l ^ r)),
                         BinOpKind::BitOr => Some(Constant::Int(l | r)),
                         BinOpKind::BitAnd => Some(Constant::Int(l & r)),

clippy_utils/src/consts.rs

@@ -0,0 +1,125 @@
+//@aux-build:proc_macros.rs
+
+#![warn(clippy::bad_bit_mask)]
+#![expect(clippy::erasing_op, clippy::identity_op)]
+
+use core::hint::black_box;
+use core::ops::BitAnd;
+use proc_macros::{external, with_span};
+
+fn main() {
+    let x = black_box(5u32);
+
+    let _ = x & 0b0000 == 0b0000; //~ bad_bit_mask
+    let _ = x & 0b0000 == 0b0001; //~ bad_bit_mask
+    let _ = x & 0b0001 == 0b0000;
+    let _ = x & 0b0001 == 0b0001;
+    let _ = x & 0b0010 == 0b0001; //~ bad_bit_mask
+    let _ = x & 0b0001 == 0b0010; //~ bad_bit_mask
+    let _ = x & 0b0011 == 0b0001;
+    let _ = x & 0b0011 == 0b0010;
+    let _ = x & 0b0011 == 0b0011;
+    let _ = x & 0b0011 == 0b0100; //~ bad_bit_mask
+    let _ = x & 0b0100 == 0b0100;
+    let _ = x & 0b0110 == 0b0111; //~ bad_bit_mask
+    let _ = x & 0b0011 == 0b0101; //~ bad_bit_mask
+    let _ = x & 0b1111 == 0b1110;
+    let _ = x & 0b1111 == 0b1010;
+
+    let _ = x | 0b0000 == 0b0000;
+    let _ = x | 0b0000 == 0b0001;
+    let _ = x | 0b0001 == 0b0000; //~ bad_bit_mask
+    let _ = x | 0b0001 == 0b0001;
+    let _ = x | 0b0010 == 0b0001; //~ bad_bit_mask
+    let _ = x | 0b0001 == 0b0010; //~ bad_bit_mask
+    let _ = x | 0b0011 == 0b0001; //~ bad_bit_mask
+    let _ = x | 0b0011 == 0b0010; //~ bad_bit_mask
+    let _ = x | 0b0011 == 0b0011;
+    let _ = x | 0b0011 == 0b0100; //~ bad_bit_mask
+    let _ = x | 0b0100 == 0b0100;
+    let _ = x | 0b0110 == 0b0111;
+    let _ = x | 0b0011 == 0b0101; //~ bad_bit_mask
+    let _ = x | 0b1111 == 0b1110; //~ bad_bit_mask
+    let _ = x | 0b1111 == 0b1010; //~ bad_bit_mask
+
+    let _ = x & 0b0000 != 0b0000; //~ bad_bit_mask
+    let _ = x & 0b0000 != 0b0001; //~ bad_bit_mask
+    let _ = x & 0b0001 != 0b0000;
+    let _ = x & 0b0001 != 0b0001;
+    let _ = x & 0b0010 != 0b0001; //~ bad_bit_mask
+    let _ = x & 0b0001 != 0b0010; //~ bad_bit_mask
+    let _ = x & 0b0011 != 0b0001;
+    let _ = x & 0b0011 != 0b0010;
+    let _ = x & 0b0011 != 0b0011;
+    let _ = x & 0b0011 != 0b0100; //~ bad_bit_mask
+    let _ = x & 0b0100 != 0b0100;
+    let _ = x & 0b0110 != 0b0111; //~ bad_bit_mask
+    let _ = x & 0b0011 != 0b0101; //~ bad_bit_mask
+    let _ = x & 0b1111 != 0b1110;
+    let _ = x & 0b1111 != 0b1010;
+
+    let _ = x | 0b0000 != 0b0000;
+    let _ = x | 0b0000 != 0b0001;
+    let _ = x | 0b0001 != 0b0000; //~ bad_bit_mask
+    let _ = x | 0b0001 != 0b0001;
+    let _ = x | 0b0010 != 0b0001; //~ bad_bit_mask
+    let _ = x | 0b0001 != 0b0010; //~ bad_bit_mask
+    let _ = x | 0b0011 != 0b0001; //~ bad_bit_mask
+    let _ = x | 0b0011 != 0b0010; //~ bad_bit_mask
+    let _ = x | 0b0011 != 0b0011;
+    let _ = x | 0b0011 != 0b0100; //~ bad_bit_mask
+    let _ = x | 0b0100 != 0b0100;
+    let _ = x | 0b0110 != 0b0111;
+    let _ = x | 0b0011 != 0b0101; //~ bad_bit_mask
+    let _ = x | 0b1111 != 0b1110; //~ bad_bit_mask
+    let _ = x | 0b1111 != 0b1010; //~ bad_bit_mask
+
+    let _ = 0b0010 & x == 0b0001; //~ bad_bit_mask
+    let _ = 0b0001 == x & 0b0010; //~ bad_bit_mask
+    let _ = 0b0001 == 0b0010 & x; //~ bad_bit_mask
+
+    let _ = x & (0b0100 | 0b0010) == (0b0111 ^ 0b1000); //~ bad_bit_mask
+
+    external! {
+        let x = black_box(5u32);
+        let _ = x & 0b0010 == 0b0001;
+    }
+    with_span! {
+        sp
+        let x = black_box(5u32);
+        let _ = x & 0b0010 == 0b0001;
+    }
+
+    {
+        const C: i32 = 0b0011;
+
+        let x = black_box(5i32);
+        let _ = x & C == 0b0011;
+        let _ = x & C == 0b0100; //~ bad_bit_mask
+        let _ = x & 0b0001 == C; //~ bad_bit_mask
+    }
+
+    {
+        // Bits shifted out.
+        let _ = black_box(1u8) & 0xf0 == 0x11 << 4;
+        let _ = black_box(1i8) & 0x70 == 0x11 << 4;
+        let _ = black_box(1u16) & 0xf000 == 0x11 << 12;
+        let _ = black_box(1i16) & 0x7000 == 0x11 << 12;
+    }
+
+    {
+        struct S(u32);
+        impl BitAnd<u32> for S {
+            type Output = Self;
+            fn bitand(self, _: u32) -> Self {
+                self
+            }
+        }
+        impl PartialEq<u32> for S {
+            fn eq(&self, _: &u32) -> bool {
+                true
+            }
+        }
+        let _ = black_box(S(0)) & 0x1 != 0;
+    }
+}