src/tools/clippy/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 clippy_utils::sugg::has_enclosing_paren;
   5 use if_chain::if_chain;
   6 use rustc_ast::util::parser::PREC_PREFIX;
   7 use rustc_errors::Applicability;
   8 use rustc_hir::{BinOpKind, Expr, ExprKind, UnOp};
   9 use rustc_lint::{LateContext, LateLintPass};
  10 use rustc_session::{declare_lint_pass, declare_tool_lint};
  11 use rustc_span::source_map::Span;
  12
  13 declare_clippy_lint! {
  14     /// ### What it does
  15     /// Checks for multiplication by -1 as a form of negation.
  16     ///
  17     /// ### Why is this bad?
  18     /// It's more readable to just negate.
  19     ///
  20     /// ### Known problems
  21     /// This only catches integers (for now).
  22     ///
  23     /// ### Example
  24     /// ```rust,ignore
  25     /// let a = x * -1;
  26     /// ```
  27     ///
  28     /// Use instead:
  29     /// ```rust,ignore
  30     /// let a = -x;
  31     /// ```
  32     #[clippy::version = "pre 1.29.0"]
  33     pub NEG_MULTIPLY,
  34     style,
  35     "multiplying integers by `-1`"
  36 }
  37
  38 declare_lint_pass!(NegMultiply => [NEG_MULTIPLY]);
  39
  40 impl<'tcx> LateLintPass<'tcx> for NegMultiply {
  41     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
  42         if let ExprKind::Binary(ref op, left, right) = e.kind {
  43             if BinOpKind::Mul == op.node {
  44                 match (&left.kind, &right.kind) {
  45                     (&ExprKind::Unary(..), &ExprKind::Unary(..)) => {},
  46                     (&ExprKind::Unary(UnOp::Neg, lit), _) => check_mul(cx, e.span, lit, right),
  47                     (_, &ExprKind::Unary(UnOp::Neg, lit)) => check_mul(cx, e.span, lit, left),
  48                     _ => {},
  49                 }
  50             }
  51         }
  52     }
  53 }
  54
  55 fn check_mul(cx: &LateContext<'_>, span: Span, lit: &Expr<'_>, exp: &Expr<'_>) {
  56     if_chain! {
  57         if let ExprKind::Lit(ref l) = lit.kind;
  58         if consts::lit_to_mir_constant(&l.node, cx.typeck_results().expr_ty_opt(lit)) == Constant::Int(1);
  59         if cx.typeck_results().expr_ty(exp).is_integral();
  60
  61         then {
  62             let mut applicability = Applicability::MachineApplicable;
  63             let snip = snippet_with_applicability(cx, exp.span, "..", &mut applicability);
  64             let suggestion = if exp.precedence().order() < PREC_PREFIX && !has_enclosing_paren(&snip) {
  65                 format!("-({snip})")
  66             } else {
  67                 format!("-{snip}")
  68             };
  69             span_lint_and_sugg(
  70                     cx,
  71                     NEG_MULTIPLY,
  72                     span,
  73                     "this multiplication by -1 can be written more succinctly",
  74                     "consider using",
  75                     suggestion,
  76                     applicability,
  77                 );
  78         }
  79     }
  80 }