src/tools/clippy/clippy_lints/src/modulo_arithmetic.rs

   1 use clippy_utils::consts::{constant, Constant};
   2 use clippy_utils::diagnostics::span_lint_and_then;
   3 use clippy_utils::sext;
   4 use if_chain::if_chain;
   5 use rustc_hir::{BinOpKind, Expr, ExprKind};
   6 use rustc_lint::{LateContext, LateLintPass};
   7 use rustc_middle::ty;
   8 use rustc_session::{declare_lint_pass, declare_tool_lint};
   9 use std::fmt::Display;
  10
  11 declare_clippy_lint! {
  12     /// ### What it does
  13     /// Checks for modulo arithmetic.
  14     ///
  15     /// ### Why is this bad?
  16     /// The results of modulo (%) operation might differ
  17     /// depending on the language, when negative numbers are involved.
  18     /// If you interop with different languages it might be beneficial
  19     /// to double check all places that use modulo arithmetic.
  20     ///
  21     /// For example, in Rust `17 % -3 = 2`, but in Python `17 % -3 = -1`.
  22     ///
  23     /// ### Example
  24     /// ```rust
  25     /// let x = -17 % 3;
  26     /// ```
  27     #[clippy::version = "1.42.0"]
  28     pub MODULO_ARITHMETIC,
  29     restriction,
  30     "any modulo arithmetic statement"
  31 }
  32
  33 declare_lint_pass!(ModuloArithmetic => [MODULO_ARITHMETIC]);
  34
  35 struct OperandInfo {
  36     string_representation: Option<String>,
  37     is_negative: bool,
  38     is_integral: bool,
  39 }
  40
  41 fn analyze_operand(operand: &Expr<'_>, cx: &LateContext<'_>, expr: &Expr<'_>) -> Option<OperandInfo> {
  42     match constant(cx, cx.typeck_results(), operand) {
  43         Some((Constant::Int(v), _)) => match *cx.typeck_results().expr_ty(expr).kind() {
  44             ty::Int(ity) => {
  45                 let value = sext(cx.tcx, v, ity);
  46                 return Some(OperandInfo {
  47                     string_representation: Some(value.to_string()),
  48                     is_negative: value < 0,
  49                     is_integral: true,
  50                 });
  51             },
  52             ty::Uint(_) => {
  53                 return Some(OperandInfo {
  54                     string_representation: None,
  55                     is_negative: false,
  56                     is_integral: true,
  57                 });
  58             },
  59             _ => {},
  60         },
  61         Some((Constant::F32(f), _)) => {
  62             return Some(floating_point_operand_info(&f));
  63         },
  64         Some((Constant::F64(f), _)) => {
  65             return Some(floating_point_operand_info(&f));
  66         },
  67         _ => {},
  68     }
  69     None
  70 }
  71
  72 fn floating_point_operand_info<T: Display + PartialOrd + From<f32>>(f: &T) -> OperandInfo {
  73     OperandInfo {
  74         string_representation: Some(format!("{:.3}", *f)),
  75         is_negative: *f < 0.0.into(),
  76         is_integral: false,
  77     }
  78 }
  79
  80 fn might_have_negative_value(t: &ty::TyS<'_>) -> bool {
  81     t.is_signed() || t.is_floating_point()
  82 }
  83
  84 fn check_const_operands<'tcx>(
  85     cx: &LateContext<'tcx>,
  86     expr: &'tcx Expr<'_>,
  87     lhs_operand: &OperandInfo,
  88     rhs_operand: &OperandInfo,
  89 ) {
  90     if lhs_operand.is_negative ^ rhs_operand.is_negative {
  91         span_lint_and_then(
  92             cx,
  93             MODULO_ARITHMETIC,
  94             expr.span,
  95             &format!(
  96                 "you are using modulo operator on constants with different signs: `{} % {}`",
  97                 lhs_operand.string_representation.as_ref().unwrap(),
  98                 rhs_operand.string_representation.as_ref().unwrap()
  99             ),
 100             |diag| {
 101                 diag.note("double check for expected result especially when interoperating with different languages");
 102                 if lhs_operand.is_integral {
 103                     diag.note("or consider using `rem_euclid` or similar function");
 104                 }
 105             },
 106         );
 107     }
 108 }
 109
 110 fn check_non_const_operands<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, operand: &Expr<'_>) {
 111     let operand_type = cx.typeck_results().expr_ty(operand);
 112     if might_have_negative_value(operand_type) {
 113         span_lint_and_then(
 114             cx,
 115             MODULO_ARITHMETIC,
 116             expr.span,
 117             "you are using modulo operator on types that might have different signs",
 118             |diag| {
 119                 diag.note("double check for expected result especially when interoperating with different languages");
 120                 if operand_type.is_integral() {
 121                     diag.note("or consider using `rem_euclid` or similar function");
 122                 }
 123             },
 124         );
 125     }
 126 }
 127
 128 impl<'tcx> LateLintPass<'tcx> for ModuloArithmetic {
 129     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
 130         match &expr.kind {
 131             ExprKind::Binary(op, lhs, rhs) | ExprKind::AssignOp(op, lhs, rhs) => {
 132                 if op.node == BinOpKind::Rem {
 133                     let lhs_operand = analyze_operand(lhs, cx, expr);
 134                     let rhs_operand = analyze_operand(rhs, cx, expr);
 135                     if_chain! {
 136                         if let Some(lhs_operand) = lhs_operand;
 137                         if let Some(rhs_operand) = rhs_operand;
 138                         then {
 139                             check_const_operands(cx, expr, &lhs_operand, &rhs_operand);
 140                         }
 141                         else {
 142                             check_non_const_operands(cx, expr, lhs);
 143                         }
 144                     }
 145                 };
 146             },
 147             _ => {},
 148         }
 149     }
 150 }