clippy_lints/src/neg_multiply.rs

   1 use clippy_utils::consts::{self, Constant};
   2 use clippy_utils::diagnostics::span_lint_and_sugg;
   3 use clippy_utils::source::snippet_with_applicability;
   4 use if_chain::if_chain;
   5 use rustc_errors::Applicability;
   6 use rustc_hir::{BinOpKind, Expr, ExprKind, UnOp};
   7 use rustc_lint::{LateContext, LateLintPass};
   8 use rustc_session::{declare_lint_pass, declare_tool_lint};
   9 use rustc_span::source_map::Span;
  10
  11 declare_clippy_lint! {
  12     /// ### What it does
  13     /// Checks for multiplication by -1 as a form of negation.
  14     ///
  15     /// ### Why is this bad?
  16     /// It's more readable to just negate.
  17     ///
  18     /// ### Known problems
  19     /// This only catches integers (for now).
  20     ///
  21     /// ### Example
  22     /// ```ignore
  23     /// // Bad
  24     /// let a = x * -1;
  25     ///
  26     /// // Good
  27     /// let b = -x;
  28     /// ```
  29     #[clippy::version = "pre 1.29.0"]
  30     pub NEG_MULTIPLY,
  31     style,
  32     "multiplying integers by `-1`"
  33 }
  34
  35 declare_lint_pass!(NegMultiply => [NEG_MULTIPLY]);
  36
  37 #[allow(clippy::match_same_arms)]
  38 impl<'tcx> LateLintPass<'tcx> for NegMultiply {
  39     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
  40         if let ExprKind::Binary(ref op, left, right) = e.kind {
  41             if BinOpKind::Mul == op.node {
  42                 match (&left.kind, &right.kind) {
  43                     (&ExprKind::Unary(..), &ExprKind::Unary(..)) => {},
  44                     (&ExprKind::Unary(UnOp::Neg, lit), _) => check_mul(cx, e.span, lit, right),
  45                     (_, &ExprKind::Unary(UnOp::Neg, lit)) => check_mul(cx, e.span, lit, left),
  46                     _ => {},
  47                 }
  48             }
  49         }
  50     }
  51 }
  52
  53 fn check_mul(cx: &LateContext<'_>, span: Span, lit: &Expr<'_>, exp: &Expr<'_>) {
  54     if_chain! {
  55         if let ExprKind::Lit(ref l) = lit.kind;
  56         if consts::lit_to_constant(&l.node, cx.typeck_results().expr_ty_opt(lit)) == Constant::Int(1);
  57         if cx.typeck_results().expr_ty(exp).is_integral();
  58
  59         then {
  60             let mut applicability = Applicability::MachineApplicable;
  61             let suggestion = format!("-{}", snippet_with_applicability(cx, exp.span, "..", &mut applicability));
  62             span_lint_and_sugg(
  63                     cx,
  64                     NEG_MULTIPLY,
  65                     span,
  66                     "this multiplication by -1 can be written more succinctly",
  67                     "consider using",
  68                     suggestion,
  69                     applicability,
  70                 );
  71         }
  72     }
  73 }