clippy_lints/src/neg_cmp_op_on_partial_ord.rs

   1 use rustc::hir::*;
   2 use rustc::lint::*;
   3 use rustc::{declare_lint, lint_array};
   4
   5 use crate::utils::{self, paths, span_lint, in_external_macro};
   6
   7 /// **What it does:**
   8 /// Checks for the usage of negated comparision operators on types which only implement
   9 /// `PartialOrd` (e.g. `f64`).
  10 ///
  11 /// **Why is this bad?**
  12 /// These operators make it easy to forget that the underlying types actually allow not only three
  13 /// potential Orderings (Less, Equal, Greater) but also a forth one (Uncomparable). Escpeccially if
  14 /// the operator based comparision result is negated it is easy to miss that fact.
  15 ///
  16 /// **Known problems:** None.
  17 ///
  18 /// **Example:**
  19 ///
  20 /// ```rust
  21 /// use core::cmp::Ordering;
  22 ///
  23 /// // Bad
  24 /// let a = 1.0;
  25 /// let b = std::f64::NAN;
  26 ///
  27 /// let _not_less_or_equal = !(a <= b);
  28 ///
  29 /// // Good
  30 /// let a = 1.0;
  31 /// let b = std::f64::NAN;
  32 ///
  33 /// let _not_less_or_equal = match a.partial_cmp(&b) {
  34 ///     None | Some(Ordering::Greater) => true,
  35 ///     _ => false,
  36 /// };
  37 /// ```
  38 declare_clippy_lint! {
  39     pub NEG_CMP_OP_ON_PARTIAL_ORD,
  40     complexity,
  41     "The use of negated comparision operators on partially orded types may produce confusing code."
  42 }
  43
  44 pub struct NoNegCompOpForPartialOrd;
  45
  46 impl LintPass for NoNegCompOpForPartialOrd {
  47     fn get_lints(&self) -> LintArray {
  48         lint_array!(NEG_CMP_OP_ON_PARTIAL_ORD)
  49     }
  50 }
  51
  52 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NoNegCompOpForPartialOrd {
  53
  54     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
  55         if_chain! {
  56
  57             if !in_external_macro(cx, expr.span);
  58             if let ExprKind::Unary(UnOp::UnNot, ref inner) = expr.node;
  59             if let ExprKind::Binary(ref op, ref left, _) = inner.node;
  60             if let BinOpKind::Le | BinOpKind::Ge | BinOpKind::Lt | BinOpKind::Gt = op.node;
  61
  62             then {
  63
  64                 let ty = cx.tables.expr_ty(left);
  65
  66                 let implements_ord = {
  67                     if let Some(id) = utils::get_trait_def_id(cx, &paths::ORD) {
  68                         utils::implements_trait(cx, ty, id, &[])
  69                     } else {
  70                         return;
  71                     }
  72                 };
  73
  74                 let implements_partial_ord = {
  75                     if let Some(id) = utils::get_trait_def_id(cx, &paths::PARTIAL_ORD) {
  76                         utils::implements_trait(cx, ty, id, &[])
  77                     } else {
  78                         return;
  79                     }
  80                 };
  81
  82                 if implements_partial_ord && !implements_ord {
  83                     span_lint(
  84                         cx,
  85                         NEG_CMP_OP_ON_PARTIAL_ORD,
  86                         expr.span,
  87                         "The use of negated comparision operators on partially orded \
  88                         types produces code that is hard to read and refactor. Please \
  89                         consider to use the `partial_cmp` instead, to make it clear \
  90                         that the two values could be incomparable."
  91                     )
  92                 }
  93             }
  94         }
  95     }
  96 }