Add new lint (modulo_arithmetic)

clippy_lints/src/modulo_arithmetic.rs

@@ -0,0 +1,149 @@
+use crate::consts::{constant, Constant};
+use crate::utils::{sext, span_lint_and_then};
+use if_chain::if_chain;
+use rustc::declare_lint_pass;
+use rustc::hir::*;
+use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
+use rustc::ty::{self};
+use rustc_session::declare_tool_lint;
+use std::fmt::Display;
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for modulo arithemtic.
+    ///
+    /// **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`.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **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.tables, operand) {
+        Some((Constant::Int(v), _)) => match cx.tables.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<'a, 'tcx>(
+    cx: &LateContext<'a, '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()
+            ),
+            |db| {
+                db.note("double check for expected result especially when interoperating with different languages");
+                if lhs_operand.is_integral {
+                    db.note("or consider using `rem_euclid` or similar function");
+                }
+            },
+        );
+    }
+}
+
+fn check_non_const_operands<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr, operand: &Expr) {
+    let operand_type = cx.tables.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",
+            |db| {
+                db.note("double check for expected result especially when interoperating with different languages");
+                if operand_type.is_integral() {
+                    db.note("or consider using `rem_euclid` or similar function");
+                }
+            },
+        );
+    }
+}
+
+impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ModuloArithmetic {
+    fn check_expr(&mut self, cx: &LateContext<'a, '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);
+                        }
+                    }
+                };
+            },
+            _ => {},
+        }
+    }
+}