clippy_lints/src/neg_cmp_op_on_partial_ord.rs

   1 use rustc::hir::*;
   2 use rustc::lint::*;
   3
   4 use crate::utils::{self, paths, span_lint, in_external_macro};
   5
   6 /// **What it does:**
   7 /// Checks for the usage of negated comparision operators on types which only implement
   8 /// `PartialOrd` (e.g. `f64`).
   9 ///
  10 /// **Why is this bad?**
  11 /// These operators make it easy to forget that the underlying types actually allow not only three
  12 /// potential Orderings (Less, Equal, Greater) but also a forth one (Uncomparable). Escpeccially if
  13 /// the operator based comparision result is negated it is easy to miss that fact.
  14 ///
  15 /// **Known problems:** None.
  16 ///
  17 /// **Example:**
  18 ///
  19 /// ```rust
  20 /// use core::cmp::Ordering;
  21 ///
  22 /// // Bad
  23 /// let a = 1.0;
  24 /// let b = std::f64::NAN;
  25 ///
  26 /// let _not_less_or_equal = !(a <= b);
  27 ///
  28 /// // Good
  29 /// let a = 1.0;
  30 /// let b = std::f64::NAN;
  31 ///
  32 /// let _not_less_or_equal = match a.partial_cmp(&b) {
  33 ///     None | Some(Ordering::Greater) => true,
  34 ///     _ => false,
  35 /// };
  36 /// ```
  37 declare_clippy_lint! {
  38     pub NEG_CMP_OP_ON_PARTIAL_ORD,
  39     complexity,
  40     "The use of negated comparision operators on partially orded types may produce confusing code."
  41 }
  42
  43 pub struct NoNegCompOpForPartialOrd;
  44
  45 impl LintPass for NoNegCompOpForPartialOrd {
  46     fn get_lints(&self) -> LintArray {
  47         lint_array!(NEG_CMP_OP_ON_PARTIAL_ORD)
  48     }
  49 }
  50
  51 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NoNegCompOpForPartialOrd {
  52
  53     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
  54         if_chain! {
  55
  56             if !in_external_macro(cx, expr.span);
  57             if let ExprKind::Unary(UnOp::UnNot, ref inner) = expr.node;
  58             if let ExprKind::Binary(ref op, ref left, _) = inner.node;
  59             if let BinOpKind::Le | BinOpKind::Ge | BinOpKind::Lt | BinOpKind::Gt = op.node;
  60
  61             then {
  62
  63                 let ty = cx.tables.expr_ty(left);
  64
  65                 let implements_ord = {
  66                     if let Some(id) = utils::get_trait_def_id(cx, &paths::ORD) {
  67                         utils::implements_trait(cx, ty, id, &[])
  68                     } else {
  69                         return;
  70                     }
  71                 };
  72
  73                 let implements_partial_ord = {
  74                     if let Some(id) = utils::get_trait_def_id(cx, &paths::PARTIAL_ORD) {
  75                         utils::implements_trait(cx, ty, id, &[])
  76                     } else {
  77                         return;
  78                     }
  79                 };
  80
  81                 if implements_partial_ord && !implements_ord {
  82                     span_lint(
  83                         cx,
  84                         NEG_CMP_OP_ON_PARTIAL_ORD,
  85                         expr.span,
  86                         "The use of negated comparision operators on partially orded \
  87                         types produces code that is hard to read and refactor. Please \
  88                         consider to use the `partial_cmp` instead, to make it clear \
  89                         that the two values could be incomparable."
  90                     )
  91                 }
  92             }
  93         }
  94     }
  95 }