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

   1 use clippy_utils::diagnostics::span_lint_and_then;
   2 use clippy_utils::{match_def_path, paths, sugg};
   3 use if_chain::if_chain;
   4 use rustc_ast::util::parser::AssocOp;
   5 use rustc_errors::Applicability;
   6 use rustc_hir::def::{DefKind, Res};
   7 use rustc_hir::{BinOpKind, Expr, ExprKind};
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_middle::ty;
  10 use rustc_session::{declare_lint_pass, declare_tool_lint};
  11 use rustc_span::source_map::Spanned;
  12
  13 declare_clippy_lint! {
  14     /// **What it does:** Checks for statements of the form `(a - b) < f32::EPSILON` or
  15      /// `(a - b) < f64::EPSILON`. Notes the missing `.abs()`.
  16      ///
  17      /// **Why is this bad?** The code without `.abs()` is more likely to have a bug.
  18      ///
  19      /// **Known problems:** If the user can ensure that b is larger than a, the `.abs()` is
  20      /// technically unneccessary. However, it will make the code more robust and doesn't have any
  21      /// large performance implications. If the abs call was deliberately left out for performance
  22      /// reasons, it is probably better to state this explicitly in the code, which then can be done
  23      /// with an allow.
  24      ///
  25      /// **Example:**
  26      ///
  27      /// ```rust
  28      /// pub fn is_roughly_equal(a: f32, b: f32) -> bool {
  29      ///     (a - b) < f32::EPSILON
  30      /// }
  31      /// ```
  32      /// Use instead:
  33      /// ```rust
  34      /// pub fn is_roughly_equal(a: f32, b: f32) -> bool {
  35      ///     (a - b).abs() < f32::EPSILON
  36      /// }
  37      /// ```
  38     pub FLOAT_EQUALITY_WITHOUT_ABS,
  39     correctness,
  40     "float equality check without `.abs()`"
  41 }
  42
  43 declare_lint_pass!(FloatEqualityWithoutAbs => [FLOAT_EQUALITY_WITHOUT_ABS]);
  44
  45 impl<'tcx> LateLintPass<'tcx> for FloatEqualityWithoutAbs {
  46     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  47         let lhs;
  48         let rhs;
  49
  50         // check if expr is a binary expression with a lt or gt operator
  51         if let ExprKind::Binary(op, ref left, ref right) = expr.kind {
  52             match op.node {
  53                 BinOpKind::Lt => {
  54                     lhs = left;
  55                     rhs = right;
  56                 },
  57                 BinOpKind::Gt => {
  58                     lhs = right;
  59                     rhs = left;
  60                 },
  61                 _ => return,
  62             };
  63         } else {
  64             return;
  65         }
  66
  67         if_chain! {
  68
  69             // left hand side is a substraction
  70             if let ExprKind::Binary(
  71                 Spanned {
  72                     node: BinOpKind::Sub,
  73                     ..
  74                 },
  75                 val_l,
  76                 val_r,
  77             ) = lhs.kind;
  78
  79             // right hand side matches either f32::EPSILON or f64::EPSILON
  80             if let ExprKind::Path(ref epsilon_path) = rhs.kind;
  81             if let Res::Def(DefKind::AssocConst, def_id) = cx.qpath_res(epsilon_path, rhs.hir_id);
  82             if match_def_path(cx, def_id, &paths::F32_EPSILON) || match_def_path(cx, def_id, &paths::F64_EPSILON);
  83
  84             // values of the substractions on the left hand side are of the type float
  85             let t_val_l = cx.typeck_results().expr_ty(val_l);
  86             let t_val_r = cx.typeck_results().expr_ty(val_r);
  87             if let ty::Float(_) = t_val_l.kind();
  88             if let ty::Float(_) = t_val_r.kind();
  89
  90             then {
  91                 let sug_l = sugg::Sugg::hir(cx, &val_l, "..");
  92                 let sug_r = sugg::Sugg::hir(cx, &val_r, "..");
  93                 // format the suggestion
  94                 let suggestion = format!("{}.abs()", sugg::make_assoc(AssocOp::Subtract, &sug_l, &sug_r).maybe_par());
  95                 // spans the lint
  96                 span_lint_and_then(
  97                     cx,
  98                     FLOAT_EQUALITY_WITHOUT_ABS,
  99                     expr.span,
 100                     "float equality check without `.abs()`",
 101                     | diag | {
 102                         diag.span_suggestion(
 103                             lhs.span,
 104                             "add `.abs()`",
 105                             suggestion,
 106                             Applicability::MaybeIncorrect,
 107                         );
 108                     }
 109                 );
 110             }
 111         }
 112     }
 113 }