src/tools/clippy/clippy_lints/src/approx_const.rs

   1 use clippy_utils::diagnostics::span_lint;
   2 use rustc_ast::ast::{FloatTy, LitFloatType, LitKind};
   3 use rustc_hir::{Expr, ExprKind};
   4 use rustc_lint::{LateContext, LateLintPass};
   5 use rustc_session::{declare_lint_pass, declare_tool_lint};
   6 use rustc_span::symbol;
   7 use std::f64::consts as f64;
   8
   9 declare_clippy_lint! {
  10     /// **What it does:** Checks for floating point literals that approximate
  11     /// constants which are defined in
  12     /// [`std::f32::consts`](https://doc.rust-lang.org/stable/std/f32/consts/#constants)
  13     /// or
  14     /// [`std::f64::consts`](https://doc.rust-lang.org/stable/std/f64/consts/#constants),
  15     /// respectively, suggesting to use the predefined constant.
  16     ///
  17     /// **Why is this bad?** Usually, the definition in the standard library is more
  18     /// precise than what people come up with. If you find that your definition is
  19     /// actually more precise, please [file a Rust
  20     /// issue](https://github.com/rust-lang/rust/issues).
  21     ///
  22     /// **Known problems:** None.
  23     ///
  24     /// **Example:**
  25     /// ```rust
  26     /// let x = 3.14;
  27     /// let y = 1_f64 / x;
  28     /// ```
  29     /// Use predefined constants instead:
  30     /// ```rust
  31     /// let x = std::f32::consts::PI;
  32     /// let y = std::f64::consts::FRAC_1_PI;
  33     /// ```
  34     pub APPROX_CONSTANT,
  35     correctness,
  36     "the approximate of a known float constant (in `std::fXX::consts`)"
  37 }
  38
  39 // Tuples are of the form (constant, name, min_digits)
  40 const KNOWN_CONSTS: [(f64, &str, usize); 18] = [
  41     (f64::E, "E", 4),
  42     (f64::FRAC_1_PI, "FRAC_1_PI", 4),
  43     (f64::FRAC_1_SQRT_2, "FRAC_1_SQRT_2", 5),
  44     (f64::FRAC_2_PI, "FRAC_2_PI", 5),
  45     (f64::FRAC_2_SQRT_PI, "FRAC_2_SQRT_PI", 5),
  46     (f64::FRAC_PI_2, "FRAC_PI_2", 5),
  47     (f64::FRAC_PI_3, "FRAC_PI_3", 5),
  48     (f64::FRAC_PI_4, "FRAC_PI_4", 5),
  49     (f64::FRAC_PI_6, "FRAC_PI_6", 5),
  50     (f64::FRAC_PI_8, "FRAC_PI_8", 5),
  51     (f64::LN_10, "LN_10", 5),
  52     (f64::LN_2, "LN_2", 5),
  53     (f64::LOG10_E, "LOG10_E", 5),
  54     (f64::LOG2_E, "LOG2_E", 5),
  55     (f64::LOG2_10, "LOG2_10", 5),
  56     (f64::LOG10_2, "LOG10_2", 5),
  57     (f64::PI, "PI", 3),
  58     (f64::SQRT_2, "SQRT_2", 5),
  59 ];
  60
  61 declare_lint_pass!(ApproxConstant => [APPROX_CONSTANT]);
  62
  63 impl<'tcx> LateLintPass<'tcx> for ApproxConstant {
  64     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
  65         if let ExprKind::Lit(lit) = &e.kind {
  66             check_lit(cx, &lit.node, e);
  67         }
  68     }
  69 }
  70
  71 fn check_lit(cx: &LateContext<'_>, lit: &LitKind, e: &Expr<'_>) {
  72     match *lit {
  73         LitKind::Float(s, LitFloatType::Suffixed(fty)) => match fty {
  74             FloatTy::F32 => check_known_consts(cx, e, s, "f32"),
  75             FloatTy::F64 => check_known_consts(cx, e, s, "f64"),
  76         },
  77         LitKind::Float(s, LitFloatType::Unsuffixed) => check_known_consts(cx, e, s, "f{32, 64}"),
  78         _ => (),
  79     }
  80 }
  81
  82 fn check_known_consts(cx: &LateContext<'_>, e: &Expr<'_>, s: symbol::Symbol, module: &str) {
  83     let s = s.as_str();
  84     if s.parse::<f64>().is_ok() {
  85         for &(constant, name, min_digits) in &KNOWN_CONSTS {
  86             if is_approx_const(constant, &s, min_digits) {
  87                 span_lint(
  88                     cx,
  89                     APPROX_CONSTANT,
  90                     e.span,
  91                     &format!(
  92                         "approximate value of `{}::consts::{}` found. \
  93                          Consider using it directly",
  94                         module, &name
  95                     ),
  96                 );
  97                 return;
  98             }
  99         }
 100     }
 101 }
 102
 103 /// Returns `false` if the number of significant figures in `value` are
 104 /// less than `min_digits`; otherwise, returns true if `value` is equal
 105 /// to `constant`, rounded to the number of digits present in `value`.
 106 #[must_use]
 107 fn is_approx_const(constant: f64, value: &str, min_digits: usize) -> bool {
 108     if value.len() <= min_digits {
 109         false
 110     } else if constant.to_string().starts_with(value) {
 111         // The value is a truncated constant
 112         true
 113     } else {
 114         let round_const = format!("{:.*}", value.len() - 2, constant);
 115         value == round_const
 116     }
 117 }