rust-clippy/clippy_lints/src/modulo_arithmetic.rs

use clippy_utils::consts::{constant, Constant};
use clippy_utils::diagnostics::span_lint_and_then;
use clippy_utils::sext;
use if_chain::if_chain;
use rustc_hir::{BinOpKind, Expr, ExprKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty;
use rustc_session::{declare_lint_pass, declare_tool_lint};
use std::fmt::Display;

declare_clippy_lint! {
    /// ### What it does
    /// Checks for modulo arithmetic.
    ///
    /// ### Why is this bad?
    /// The results of modulo (%) operation might differ
    /// depending on the language, when negative numbers are involved.
    /// If you interop with different languages it might be beneficial
    /// to double check all places that use modulo arithmetic.
    ///
    /// For example, in Rust `17 % -3 = 2`, but in Python `17 % -3 = -1`.
    ///
    /// ### Example
    /// ```rust
    /// let x = -17 % 3;
    /// ```
    pub MODULO_ARITHMETIC,
    restriction,
    "any modulo arithmetic statement"
}

declare_lint_pass!(ModuloArithmetic => [MODULO_ARITHMETIC]);

struct OperandInfo {
    string_representation: Option<String>,
    is_negative: bool,
    is_integral: bool,
}

fn analyze_operand(operand: &Expr<'_>, cx: &LateContext<'_>, expr: &Expr<'_>) -> Option<OperandInfo> {
    match constant(cx, cx.typeck_results(), operand) {
        Some((Constant::Int(v), _)) => match *cx.typeck_results().expr_ty(expr).kind() {
            ty::Int(ity) => {
                let value = sext(cx.tcx, v, ity);
                return Some(OperandInfo {
                    string_representation: Some(value.to_string()),
                    is_negative: value < 0,
                    is_integral: true,
                });
            },
            ty::Uint(_) => {
                return Some(OperandInfo {
                    string_representation: None,
                    is_negative: false,
                    is_integral: true,
                });
            },
            _ => {},
        },
        Some((Constant::F32(f), _)) => {
            return Some(floating_point_operand_info(&f));
        },
        Some((Constant::F64(f), _)) => {
            return Some(floating_point_operand_info(&f));
        },
        _ => {},
    }
    None
}

fn floating_point_operand_info<T: Display + PartialOrd + From<f32>>(f: &T) -> OperandInfo {
    OperandInfo {
        string_representation: Some(format!("{:.3}", *f)),
        is_negative: *f < 0.0.into(),
        is_integral: false,
    }
}

fn might_have_negative_value(t: &ty::TyS<'_>) -> bool {
    t.is_signed() || t.is_floating_point()
}

fn check_const_operands<'tcx>(
    cx: &LateContext<'tcx>,
    expr: &'tcx Expr<'_>,
    lhs_operand: &OperandInfo,
    rhs_operand: &OperandInfo,
) {
    if lhs_operand.is_negative ^ rhs_operand.is_negative {
        span_lint_and_then(
            cx,
            MODULO_ARITHMETIC,
            expr.span,
            &format!(
                "you are using modulo operator on constants with different signs: `{} % {}`",
                lhs_operand.string_representation.as_ref().unwrap(),
                rhs_operand.string_representation.as_ref().unwrap()
            ),
            |diag| {
                diag.note("double check for expected result especially when interoperating with different languages");
                if lhs_operand.is_integral {
                    diag.note("or consider using `rem_euclid` or similar function");
                }
            },
        );
    }
}

fn check_non_const_operands<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, operand: &Expr<'_>) {
    let operand_type = cx.typeck_results().expr_ty(operand);
    if might_have_negative_value(operand_type) {
        span_lint_and_then(
            cx,
            MODULO_ARITHMETIC,
            expr.span,
            "you are using modulo operator on types that might have different signs",
            |diag| {
                diag.note("double check for expected result especially when interoperating with different languages");
                if operand_type.is_integral() {
                    diag.note("or consider using `rem_euclid` or similar function");
                }
            },
        );
    }
}

impl<'tcx> LateLintPass<'tcx> for ModuloArithmetic {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        match &expr.kind {
            ExprKind::Binary(op, lhs, rhs) | ExprKind::AssignOp(op, lhs, rhs) => {
                if let BinOpKind::Rem = op.node {
                    let lhs_operand = analyze_operand(lhs, cx, expr);
                    let rhs_operand = analyze_operand(rhs, cx, expr);
                    if_chain! {
                        if let Some(lhs_operand) = lhs_operand;
                        if let Some(rhs_operand) = rhs_operand;
                        then {
                            check_const_operands(cx, expr, &lhs_operand, &rhs_operand);
                        }
                        else {
                            check_non_const_operands(cx, expr, lhs);
                        }
                    }
                };
            },
            _ => {},
        }
    }
}