clippy_lints/src/precedence.rs

   1 use rustc::lint::*;
   2 use syntax::ast::*;
   3 use syntax::codemap::Spanned;
   4 use utils::{span_lint, snippet};
   5
   6 /// **What it does:** Checks for operations where precedence may be unclear
   7 /// and suggests to add parentheses. Currently it catches the following:
   8 /// * mixed usage of arithmetic and bit shifting/combining operators without parentheses
   9 /// * a "negative" numeric literal (which is really a unary `-` followed by a numeric literal)
  10 ///   followed by a method call
  11 ///
  12 /// **Why is this bad?** Not everyone knows the precedence of those operators by
  13 /// heart, so expressions like these may trip others trying to reason about the
  14 /// code.
  15 ///
  16 /// **Known problems:** None.
  17 ///
  18 /// **Example:**
  19 /// * `1 << 2 + 3` equals 32, while `(1 << 2) + 3` equals 7
  20 /// * `-1i32.abs()` equals -1, while `(-1i32).abs()` equals 1
  21 declare_lint! {
  22     pub PRECEDENCE,
  23     Warn,
  24     "operations where precedence may be unclear"
  25 }
  26
  27 #[derive(Copy,Clone)]
  28 pub struct Precedence;
  29
  30 impl LintPass for Precedence {
  31     fn get_lints(&self) -> LintArray {
  32         lint_array!(PRECEDENCE)
  33     }
  34 }
  35
  36 impl EarlyLintPass for Precedence {
  37     fn check_expr(&mut self, cx: &EarlyContext, expr: &Expr) {
  38         if let ExprKind::Binary(Spanned { node: op, .. }, ref left, ref right) = expr.node {
  39             if !is_bit_op(op) {
  40                 return;
  41             }
  42             match (is_arith_expr(left), is_arith_expr(right)) {
  43                 (true, true) => {
  44                     span_lint(cx,
  45                               PRECEDENCE,
  46                               expr.span,
  47                               &format!("operator precedence can trip the unwary. Consider parenthesizing your \
  48                                         expression:`({}) {} ({})`",
  49                                        snippet(cx, left.span, ".."),
  50                                        op.to_string(),
  51                                        snippet(cx, right.span, "..")));
  52                 },
  53                 (true, false) => {
  54                     span_lint(cx,
  55                               PRECEDENCE,
  56                               expr.span,
  57                               &format!("operator precedence can trip the unwary. Consider parenthesizing your \
  58                                         expression:`({}) {} {}`",
  59                                        snippet(cx, left.span, ".."),
  60                                        op.to_string(),
  61                                        snippet(cx, right.span, "..")));
  62                 },
  63                 (false, true) => {
  64                     span_lint(cx,
  65                               PRECEDENCE,
  66                               expr.span,
  67                               &format!("operator precedence can trip the unwary. Consider parenthesizing your \
  68                                         expression:`{} {} ({})`",
  69                                        snippet(cx, left.span, ".."),
  70                                        op.to_string(),
  71                                        snippet(cx, right.span, "..")));
  72                 },
  73                 _ => (),
  74             }
  75         }
  76
  77         if let ExprKind::Unary(UnOp::Neg, ref rhs) = expr.node {
  78             if let ExprKind::MethodCall(_, _, ref args) = rhs.node {
  79                 if let Some(slf) = args.first() {
  80                     if let ExprKind::Lit(ref lit) = slf.node {
  81                         match lit.node {
  82                             LitKind::Int(..) |
  83                             LitKind::Float(..) |
  84                             LitKind::FloatUnsuffixed(..) => {
  85                                 span_lint(cx,
  86                                           PRECEDENCE,
  87                                           expr.span,
  88                                           &format!("unary minus has lower precedence than method call. Consider \
  89                                                     adding parentheses to clarify your intent: -({})",
  90                                                    snippet(cx, rhs.span, "..")));
  91                             },
  92                             _ => (),
  93                         }
  94                     }
  95                 }
  96             }
  97         }
  98     }
  99 }
 100
 101 fn is_arith_expr(expr: &Expr) -> bool {
 102     match expr.node {
 103         ExprKind::Binary(Spanned { node: op, .. }, _, _) => is_arith_op(op),
 104         _ => false,
 105     }
 106 }
 107
 108 fn is_bit_op(op: BinOpKind) -> bool {
 109     use syntax::ast::BinOpKind::*;
 110     match op {
 111         BitXor | BitAnd | BitOr | Shl | Shr => true,
 112         _ => false,
 113     }
 114 }
 115
 116 fn is_arith_op(op: BinOpKind) -> bool {
 117     use syntax::ast::BinOpKind::*;
 118     match op {
 119         Add | Sub | Mul | Div | Rem => true,
 120         _ => false,
 121     }
 122 }