src/tools/clippy/clippy_lints/src/float_equality_without_abs.rs

   1 use crate::utils::{match_def_path, paths, span_lint_and_then, sugg};
   2 use if_chain::if_chain;
   3 use rustc_ast::util::parser::AssocOp;
   4 use rustc_errors::Applicability;
   5 use rustc_hir::def::{DefKind, Res};
   6 use rustc_hir::{BinOpKind, Expr, ExprKind};
   7 use rustc_lint::{LateContext, LateLintPass};
   8 use rustc_middle::ty;
   9 use rustc_session::{declare_lint_pass, declare_tool_lint};
  10 use rustc_span::source_map::Spanned;
  11
  12 declare_clippy_lint! {
  13     /// **What it does:** Checks for statements of the form `(a - b) < f32::EPSILON` or
  14      /// `(a - b) < f64::EPSILON`. Notes the missing `.abs()`.
  15      ///
  16      /// **Why is this bad?** The code without `.abs()` is more likely to have a bug.
  17      ///
  18      /// **Known problems:** If the user can ensure that b is larger than a, the `.abs()` is
  19      /// technically unneccessary. However, it will make the code more robust and doesn't have any
  20      /// large performance implications. If the abs call was deliberately left out for performance
  21      /// reasons, it is probably better to state this explicitly in the code, which then can be done
  22      /// with an allow.
  23      ///
  24      /// **Example:**
  25      ///
  26      /// ```rust
  27      /// pub fn is_roughly_equal(a: f32, b: f32) -> bool {
  28      ///     (a - b) < f32::EPSILON
  29      /// }
  30      /// ```
  31      /// Use instead:
  32      /// ```rust
  33      /// pub fn is_roughly_equal(a: f32, b: f32) -> bool {
  34      ///     (a - b).abs() < f32::EPSILON
  35      /// }
  36      /// ```
  37     pub FLOAT_EQUALITY_WITHOUT_ABS,
  38     correctness,
  39     "float equality check without `.abs()`"
  40 }
  41
  42 declare_lint_pass!(FloatEqualityWithoutAbs => [FLOAT_EQUALITY_WITHOUT_ABS]);
  43
  44 impl<'tcx> LateLintPass<'tcx> for FloatEqualityWithoutAbs {
  45     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  46         let lhs;
  47         let rhs;
  48
  49         // check if expr is a binary expression with a lt or gt operator
  50         if let ExprKind::Binary(op, ref left, ref right) = expr.kind {
  51             match op.node {
  52                 BinOpKind::Lt => {
  53                     lhs = left;
  54                     rhs = right;
  55                 },
  56                 BinOpKind::Gt => {
  57                     lhs = right;
  58                     rhs = left;
  59                 },
  60                 _ => return,
  61             };
  62         } else {
  63             return;
  64         }
  65
  66         if_chain! {
  67
  68             // left hand side is a substraction
  69             if let ExprKind::Binary(
  70                 Spanned {
  71                     node: BinOpKind::Sub,
  72                     ..
  73                 },
  74                 val_l,
  75                 val_r,
  76             ) = lhs.kind;
  77
  78             // right hand side matches either f32::EPSILON or f64::EPSILON
  79             if let ExprKind::Path(ref epsilon_path) = rhs.kind;
  80             if let Res::Def(DefKind::AssocConst, def_id) = cx.qpath_res(epsilon_path, rhs.hir_id);
  81             if match_def_path(cx, def_id, &paths::F32_EPSILON) || match_def_path(cx, def_id, &paths::F64_EPSILON);
  82
  83             // values of the substractions on the left hand side are of the type float
  84             let t_val_l = cx.typeck_results().expr_ty(val_l);
  85             let t_val_r = cx.typeck_results().expr_ty(val_r);
  86             if let ty::Float(_) = t_val_l.kind();
  87             if let ty::Float(_) = t_val_r.kind();
  88
  89             then {
  90                 let sug_l = sugg::Sugg::hir(cx, &val_l, "..");
  91                 let sug_r = sugg::Sugg::hir(cx, &val_r, "..");
  92                 // format the suggestion
  93                 let suggestion = format!("{}.abs()", sugg::make_assoc(AssocOp::Subtract, &sug_l, &sug_r).maybe_par());
  94                 // spans the lint
  95                 span_lint_and_then(
  96                     cx,
  97                     FLOAT_EQUALITY_WITHOUT_ABS,
  98                     expr.span,
  99                     "float equality check without `.abs()`",
 100                     | diag | {
 101                         diag.span_suggestion(
 102                             lhs.span,
 103                             "add `.abs()`",
 104                             suggestion,
 105                             Applicability::MaybeIncorrect,
 106                         );
 107                     }
 108                 );
 109             }
 110         }
 111     }
 112 }