New lint: precedence_bits, with recent additions to precedence

samueltardieu · samueltardieu · commit 8db9ecfd7473 · 2025-01-30T17:07:50.000+01:00
Commit 2550530 has extended the `precedence` lint to include bitmasking and shift operations. The lint is warn by default, and this generates many hits, especially in embedded or system code, where it is very idiomatic to use expressions such as `1 << 3 | 1 << 5` without parentheses. This commit splits the recent addition into a new lint, which is put into the "restriction" category, while the original one stays in "complexity", because mixing bitmasking and arithmetic operations is less typical.
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -5954,6 +5954,7 @@ Released 2018-09-13
 [`positional_named_format_parameters`]: https://rust-lang.github.io/rust-clippy/master/index.html#positional_named_format_parameters
 [`possible_missing_comma`]: https://rust-lang.github.io/rust-clippy/master/index.html#possible_missing_comma
 [`precedence`]: https://rust-lang.github.io/rust-clippy/master/index.html#precedence
+[`precedence_bits`]: https://rust-lang.github.io/rust-clippy/master/index.html#precedence_bits
 [`print_in_format_impl`]: https://rust-lang.github.io/rust-clippy/master/index.html#print_in_format_impl
 [`print_literal`]: https://rust-lang.github.io/rust-clippy/master/index.html#print_literal
 [`print_stderr`]: https://rust-lang.github.io/rust-clippy/master/index.html#print_stderr
diff --git a/clippy_lints/src/declared_lints.rs b/clippy_lints/src/declared_lints.rs
@@ -626,6 +626,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
     crate::permissions_set_readonly_false::PERMISSIONS_SET_READONLY_FALSE_INFO,
     crate::pointers_in_nomem_asm_block::POINTERS_IN_NOMEM_ASM_BLOCK_INFO,
     crate::precedence::PRECEDENCE_INFO,
+    crate::precedence::PRECEDENCE_BITS_INFO,
     crate::ptr::CMP_NULL_INFO,
     crate::ptr::INVALID_NULL_PTR_USAGE_INFO,
     crate::ptr::MUT_FROM_REF_INFO,
diff --git a/clippy_lints/src/precedence.rs b/clippy_lints/src/precedence.rs
@@ -3,33 +3,47 @@ use clippy_utils::source::snippet_with_applicability;
 use rustc_ast::ast::BinOpKind::{Add, BitAnd, BitOr, BitXor, Div, Mul, Rem, Shl, Shr, Sub};
 use rustc_ast::ast::{BinOpKind, Expr, ExprKind};
 use rustc_errors::Applicability;
-use rustc_lint::{EarlyContext, EarlyLintPass};
+use rustc_lint::{EarlyContext, EarlyLintPass, Lint};
 use rustc_session::declare_lint_pass;
 use rustc_span::source_map::Spanned;
 
 declare_clippy_lint! {
     /// ### What it does
-    /// Checks for operations where precedence may be unclear
-    /// and suggests to add parentheses. Currently it catches the following:
-    /// * mixed usage of arithmetic and bit shifting/combining operators without
-    /// parentheses
-    /// * mixed usage of bitmasking and bit shifting operators without parentheses
+    /// Checks for operations where precedence may be unclear and suggests to add parentheses.
+    /// It catches a mixed usage of arithmetic and bit shifting/combining operators without parentheses
     ///
     /// ### Why is this bad?
     /// Not everyone knows the precedence of those operators by
     /// heart, so expressions like these may trip others trying to reason about the
     /// code.
     ///
     /// ### Example
-    /// * `1 << 2 + 3` equals 32, while `(1 << 2) + 3` equals 7
-    /// * `0x2345 & 0xF000 >> 12` equals 5, while `(0x2345 & 0xF000) >> 12` equals 2
+    /// `1 << 2 + 3` equals 32, while `(1 << 2) + 3` equals 7
     #[clippy::version = "pre 1.29.0"]
     pub PRECEDENCE,
     complexity,
     "operations where precedence may be unclear"
 }
 
-declare_lint_pass!(Precedence => [PRECEDENCE]);
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for bit shifting operations combined with bit masking/combining operators
+    /// and suggest using parentheses.
+    ///
+    /// ### Why restrict this?
+    /// Not everyone knows the precedence of those operators by
+    /// heart, so expressions like these may trip others trying to reason about the
+    /// code.
+    ///
+    /// ### Example
+    /// `0x2345 & 0xF000 >> 12` equals 5, while `(0x2345 & 0xF000) >> 12` equals 2
+    #[clippy::version = "1.86.0"]
+    pub PRECEDENCE_BITS,
+    restriction,
+    "operations mixing bit shifting with bit combining/masking"
+}
+
+declare_lint_pass!(Precedence => [PRECEDENCE, PRECEDENCE_BITS]);
 
 impl EarlyLintPass for Precedence {
     fn check_expr(&mut self, cx: &EarlyContext<'_>, expr: &Expr) {
@@ -38,10 +52,10 @@ impl EarlyLintPass for Precedence {
         }
 
         if let ExprKind::Binary(Spanned { node: op, .. }, ref left, ref right) = expr.kind {
-            let span_sugg = |expr: &Expr, sugg, appl| {
+            let span_sugg = |lint: &'static Lint, expr: &Expr, sugg, appl| {
                 span_lint_and_sugg(
                     cx,
-                    PRECEDENCE,
+                    lint,
                     expr.span,
                     "operator precedence might not be obvious",
                     "consider parenthesizing your expression",
@@ -57,37 +71,41 @@ impl EarlyLintPass for Precedence {
             match (op, get_bin_opt(left), get_bin_opt(right)) {
                 (
                     BitAnd | BitOr | BitXor,
-                    Some(Shl | Shr | Add | Div | Mul | Rem | Sub),
-                    Some(Shl | Shr | Add | Div | Mul | Rem | Sub),
+                    Some(left_op @ (Shl | Shr | Add | Div | Mul | Rem | Sub)),
+                    Some(right_op @ (Shl | Shr | Add | Div | Mul | Rem | Sub)),
                 )
-                | (Shl | Shr, Some(Add | Div | Mul | Rem | Sub), Some(Add | Div | Mul | Rem | Sub)) => {
+                | (
+                    Shl | Shr,
+                    Some(left_op @ (Add | Div | Mul | Rem | Sub)),
+                    Some(right_op @ (Add | Div | Mul | Rem | Sub)),
+                ) => {
                     let sugg = format!(
                         "({}) {} ({})",
                         snippet_with_applicability(cx, left.span, "..", &mut applicability),
                         op.as_str(),
                         snippet_with_applicability(cx, right.span, "..", &mut applicability)
                     );
-                    span_sugg(expr, sugg, applicability);
+                    span_sugg(lint_for(&[op, left_op, right_op]), expr, sugg, applicability);
                 },
-                (BitAnd | BitOr | BitXor, Some(Shl | Shr | Add | Div | Mul | Rem | Sub), _)
-                | (Shl | Shr, Some(Add | Div | Mul | Rem | Sub), _) => {
+                (BitAnd | BitOr | BitXor, Some(side_op @ (Shl | Shr | Add | Div | Mul | Rem | Sub)), _)
+                | (Shl | Shr, Some(side_op @ (Add | Div | Mul | Rem | Sub)), _) => {
                     let sugg = format!(
                         "({}) {} {}",
                         snippet_with_applicability(cx, left.span, "..", &mut applicability),
                         op.as_str(),
                         snippet_with_applicability(cx, right.span, "..", &mut applicability)
                     );
-                    span_sugg(expr, sugg, applicability);
+                    span_sugg(lint_for(&[op, side_op]), expr, sugg, applicability);
                 },
-                (BitAnd | BitOr | BitXor, _, Some(Shl | Shr | Add | Div | Mul | Rem | Sub))
-                | (Shl | Shr, _, Some(Add | Div | Mul | Rem | Sub)) => {
+                (BitAnd | BitOr | BitXor, _, Some(side_op @ (Shl | Shr | Add | Div | Mul | Rem | Sub)))
+                | (Shl | Shr, _, Some(side_op @ (Add | Div | Mul | Rem | Sub))) => {
                     let sugg = format!(
                         "{} {} ({})",
                         snippet_with_applicability(cx, left.span, "..", &mut applicability),
                         op.as_str(),
                         snippet_with_applicability(cx, right.span, "..", &mut applicability)
                     );
-                    span_sugg(expr, sugg, applicability);
+                    span_sugg(lint_for(&[op, side_op]), expr, sugg, applicability);
                 },
                 _ => (),
             }
@@ -106,3 +124,11 @@ fn get_bin_opt(expr: &Expr) -> Option<BinOpKind> {
 fn is_bit_op(op: BinOpKind) -> bool {
     matches!(op, BitXor | BitAnd | BitOr | Shl | Shr)
 }
+
+fn lint_for(ops: &[BinOpKind]) -> &'static Lint {
+    if ops.iter().all(|op| is_bit_op(*op)) {
+        PRECEDENCE_BITS
+    } else {
+        PRECEDENCE
+    }
+}
diff --git a/tests/ui/precedence.fixed b/tests/ui/precedence.fixed
@@ -20,10 +20,10 @@ fn main() {
     1 ^ (1 - 1);
     3 | (2 - 1);
     3 & (5 - 2);
-    0x0F00 & (0x00F0 << 4);
-    0x0F00 & (0xF000 >> 4);
-    (0x0F00 << 1) ^ 3;
-    (0x0F00 << 1) | 2;
+    0x0F00 & 0x00F0 << 4;
+    0x0F00 & 0xF000 >> 4;
+    0x0F00 << 1 ^ 3;
+    0x0F00 << 1 | 2;
 
     let b = 3;
     trip!(b * 8);
diff --git a/tests/ui/precedence.stderr b/tests/ui/precedence.stderr
@@ -43,29 +43,5 @@ error: operator precedence might not be obvious
 LL |     3 & 5 - 2;
    |     ^^^^^^^^^ help: consider parenthesizing your expression: `3 & (5 - 2)`
 
-error: operator precedence might not be obvious
-  --> tests/ui/precedence.rs:23:5
-   |
-LL |     0x0F00 & 0x00F0 << 4;
-   |     ^^^^^^^^^^^^^^^^^^^^ help: consider parenthesizing your expression: `0x0F00 & (0x00F0 << 4)`
-
-error: operator precedence might not be obvious
-  --> tests/ui/precedence.rs:24:5
-   |
-LL |     0x0F00 & 0xF000 >> 4;
-   |     ^^^^^^^^^^^^^^^^^^^^ help: consider parenthesizing your expression: `0x0F00 & (0xF000 >> 4)`
-
-error: operator precedence might not be obvious
-  --> tests/ui/precedence.rs:25:5
-   |
-LL |     0x0F00 << 1 ^ 3;
-   |     ^^^^^^^^^^^^^^^ help: consider parenthesizing your expression: `(0x0F00 << 1) ^ 3`
-
-error: operator precedence might not be obvious
-  --> tests/ui/precedence.rs:26:5
-   |
-LL |     0x0F00 << 1 | 2;
-   |     ^^^^^^^^^^^^^^^ help: consider parenthesizing your expression: `(0x0F00 << 1) | 2`
-
-error: aborting due to 11 previous errors
+error: aborting due to 7 previous errors
 
diff --git a/tests/ui/precedence_bits.fixed b/tests/ui/precedence_bits.fixed
@@ -0,0 +1,35 @@
+#![warn(clippy::precedence_bits)]
+#![allow(
+    unused_must_use,
+    clippy::no_effect,
+    clippy::unnecessary_operation,
+    clippy::precedence
+)]
+#![allow(clippy::identity_op)]
+#![allow(clippy::eq_op)]
+
+macro_rules! trip {
+    ($a:expr) => {
+        match $a & 0b1111_1111u8 {
+            0 => println!("a is zero ({})", $a),
+            _ => println!("a is {}", $a),
+        }
+    };
+}
+
+fn main() {
+    1 << 2 + 3;
+    1 + 2 << 3;
+    4 >> 1 + 1;
+    1 + 3 >> 2;
+    1 ^ 1 - 1;
+    3 | 2 - 1;
+    3 & 5 - 2;
+    0x0F00 & (0x00F0 << 4);
+    0x0F00 & (0xF000 >> 4);
+    (0x0F00 << 1) ^ 3;
+    (0x0F00 << 1) | 2;
+
+    let b = 3;
+    trip!(b * 8);
+}
diff --git a/tests/ui/precedence_bits.rs b/tests/ui/precedence_bits.rs
@@ -0,0 +1,35 @@
+#![warn(clippy::precedence_bits)]
+#![allow(
+    unused_must_use,
+    clippy::no_effect,
+    clippy::unnecessary_operation,
+    clippy::precedence
+)]
+#![allow(clippy::identity_op)]
+#![allow(clippy::eq_op)]
+
+macro_rules! trip {
+    ($a:expr) => {
+        match $a & 0b1111_1111u8 {
+            0 => println!("a is zero ({})", $a),
+            _ => println!("a is {}", $a),
+        }
+    };
+}
+
+fn main() {
+    1 << 2 + 3;
+    1 + 2 << 3;
+    4 >> 1 + 1;
+    1 + 3 >> 2;
+    1 ^ 1 - 1;
+    3 | 2 - 1;
+    3 & 5 - 2;
+    0x0F00 & 0x00F0 << 4;
+    0x0F00 & 0xF000 >> 4;
+    0x0F00 << 1 ^ 3;
+    0x0F00 << 1 | 2;
+
+    let b = 3;
+    trip!(b * 8);
+}
diff --git a/tests/ui/precedence_bits.stderr b/tests/ui/precedence_bits.stderr
@@ -0,0 +1,29 @@
+error: operator precedence might not be obvious
+  --> tests/ui/precedence_bits.rs:28:5
+   |
+LL |     0x0F00 & 0x00F0 << 4;
+   |     ^^^^^^^^^^^^^^^^^^^^ help: consider parenthesizing your expression: `0x0F00 & (0x00F0 << 4)`
+   |
+   = note: `-D clippy::precedence-bits` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::precedence_bits)]`
+
+error: operator precedence might not be obvious
+  --> tests/ui/precedence_bits.rs:29:5
+   |
+LL |     0x0F00 & 0xF000 >> 4;
+   |     ^^^^^^^^^^^^^^^^^^^^ help: consider parenthesizing your expression: `0x0F00 & (0xF000 >> 4)`
+
+error: operator precedence might not be obvious
+  --> tests/ui/precedence_bits.rs:30:5
+   |
+LL |     0x0F00 << 1 ^ 3;
+   |     ^^^^^^^^^^^^^^^ help: consider parenthesizing your expression: `(0x0F00 << 1) ^ 3`
+
+error: operator precedence might not be obvious
+  --> tests/ui/precedence_bits.rs:31:5
+   |
+LL |     0x0F00 << 1 | 2;
+   |     ^^^^^^^^^^^^^^^ help: consider parenthesizing your expression: `(0x0F00 << 1) | 2`
+
+error: aborting due to 4 previous errors
+
