Auto merge of rust-lang#12722 - MATSMACKE:master, r=Alexendoo

Fix false positive in `cast_possible_truncation`

Fixes [rust-lang#12721](rust-lang/rust-clippy#12721)

changelog: [`cast_possible_truncation`]:  Separated checking whether integer constant has sufficient leading zeroes to be safely casted when getting remainder from bitwise and, since the latter allows a constant on either side of the operator to increase the number of leading zeroes that can be guaranteed.

Author: bors

clippy_lints/src/casts/cast_possible_truncation.rs

@@ -40,9 +40,14 @@ fn apply_reductions(cx: &LateContext<'_>, nbits: u64, expr: &Expr<'_>, signed: b
                     get_constant_bits(cx, right).map_or(0, |b| b.saturating_sub(1))
                 })
             },
-            BinOpKind::Rem | BinOpKind::BitAnd => get_constant_bits(cx, right)
+            BinOpKind::Rem => get_constant_bits(cx, right)
                 .unwrap_or(u64::MAX)
                 .min(apply_reductions(cx, nbits, left, signed)),
+            BinOpKind::BitAnd => get_constant_bits(cx, right)
+                .unwrap_or(u64::MAX)
+                .min(get_constant_bits(cx, left).unwrap_or(u64::MAX))
+                .min(apply_reductions(cx, nbits, right, signed))
+                .min(apply_reductions(cx, nbits, left, signed)),
             BinOpKind::Shr => apply_reductions(cx, nbits, left, signed)
                 .saturating_sub(constant_int(cx, right).map_or(0, |s| u64::try_from(s).unwrap_or_default())),
             _ => nbits,

tests/ui/cast.rs

@@ -13,7 +13,8 @@
     clippy::cast_abs_to_unsigned,
     clippy::no_effect,
     clippy::unnecessary_operation,
-    clippy::unnecessary_literal_unwrap
+    clippy::unnecessary_literal_unwrap,
+    clippy::identity_op
 )]
 
 fn main() {
@@ -479,3 +480,21 @@ fn issue12506() -> usize {
     let bar: Result<Option<i64>, u32> = Ok(Some(10));
     bar.unwrap().unwrap() as usize
 }
+
+fn issue12721() {
+    fn x() -> u64 {
+        u64::MAX
+    }
+
+    // Don't lint.
+    (255 & 999999u64) as u8;
+    // Don't lint.
+    let _ = ((x() & 255) & 999999) as u8;
+    // Don't lint.
+    let _ = (999999 & (x() & 255)) as u8;
+
+    (256 & 999999u64) as u8;
+    //~^ ERROR: casting `u64` to `u8` may truncate the value
+    (255 % 999999u64) as u8;
+    //~^ ERROR: casting `u64` to `u8` may truncate the value
+}