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

   1 use clippy_utils::diagnostics::span_lint_and_help;
   2 use clippy_utils::{meets_msrv, msrvs};
   3 use rustc_ast::ast::{FloatTy, LitFloatType, LitKind};
   4 use rustc_hir::{Expr, ExprKind};
   5 use rustc_lint::{LateContext, LateLintPass};
   6 use rustc_semver::RustcVersion;
   7 use rustc_session::{declare_tool_lint, impl_lint_pass};
   8 use rustc_span::symbol;
   9 use std::f64::consts as f64;
  10
  11 declare_clippy_lint! {
  12     /// ### What it does
  13     /// Checks for floating point literals that approximate
  14     /// constants which are defined in
  15     /// [`std::f32::consts`](https://doc.rust-lang.org/stable/std/f32/consts/#constants)
  16     /// or
  17     /// [`std::f64::consts`](https://doc.rust-lang.org/stable/std/f64/consts/#constants),
  18     /// respectively, suggesting to use the predefined constant.
  19     ///
  20     /// ### Why is this bad?
  21     /// Usually, the definition in the standard library is more
  22     /// precise than what people come up with. If you find that your definition is
  23     /// actually more precise, please [file a Rust
  24     /// issue](https://github.com/rust-lang/rust/issues).
  25     ///
  26     /// ### Example
  27     /// ```rust
  28     /// let x = 3.14;
  29     /// let y = 1_f64 / x;
  30     /// ```
  31     /// Use instead:
  32     /// ```rust
  33     /// let x = std::f32::consts::PI;
  34     /// let y = std::f64::consts::FRAC_1_PI;
  35     /// ```
  36     #[clippy::version = "pre 1.29.0"]
  37     pub APPROX_CONSTANT,
  38     correctness,
  39     "the approximate of a known float constant (in `std::fXX::consts`)"
  40 }
  41
  42 // Tuples are of the form (constant, name, min_digits, msrv)
  43 const KNOWN_CONSTS: [(f64, &str, usize, Option<RustcVersion>); 19] = [
  44     (f64::E, "E", 4, None),
  45     (f64::FRAC_1_PI, "FRAC_1_PI", 4, None),
  46     (f64::FRAC_1_SQRT_2, "FRAC_1_SQRT_2", 5, None),
  47     (f64::FRAC_2_PI, "FRAC_2_PI", 5, None),
  48     (f64::FRAC_2_SQRT_PI, "FRAC_2_SQRT_PI", 5, None),
  49     (f64::FRAC_PI_2, "FRAC_PI_2", 5, None),
  50     (f64::FRAC_PI_3, "FRAC_PI_3", 5, None),
  51     (f64::FRAC_PI_4, "FRAC_PI_4", 5, None),
  52     (f64::FRAC_PI_6, "FRAC_PI_6", 5, None),
  53     (f64::FRAC_PI_8, "FRAC_PI_8", 5, None),
  54     (f64::LN_2, "LN_2", 5, None),
  55     (f64::LN_10, "LN_10", 5, None),
  56     (f64::LOG2_10, "LOG2_10", 5, Some(msrvs::LOG2_10)),
  57     (f64::LOG2_E, "LOG2_E", 5, None),
  58     (f64::LOG10_2, "LOG10_2", 5, Some(msrvs::LOG10_2)),
  59     (f64::LOG10_E, "LOG10_E", 5, None),
  60     (f64::PI, "PI", 3, None),
  61     (f64::SQRT_2, "SQRT_2", 5, None),
  62     (f64::TAU, "TAU", 3, Some(msrvs::TAU)),
  63 ];
  64
  65 pub struct ApproxConstant {
  66     msrv: Option<RustcVersion>,
  67 }
  68
  69 impl ApproxConstant {
  70     #[must_use]
  71     pub fn new(msrv: Option<RustcVersion>) -> Self {
  72         Self { msrv }
  73     }
  74
  75     fn check_lit(&self, cx: &LateContext<'_>, lit: &LitKind, e: &Expr<'_>) {
  76         match *lit {
  77             LitKind::Float(s, LitFloatType::Suffixed(fty)) => match fty {
  78                 FloatTy::F32 => self.check_known_consts(cx, e, s, "f32"),
  79                 FloatTy::F64 => self.check_known_consts(cx, e, s, "f64"),
  80             },
  81             LitKind::Float(s, LitFloatType::Unsuffixed) => self.check_known_consts(cx, e, s, "f{32, 64}"),
  82             _ => (),
  83         }
  84     }
  85
  86     fn check_known_consts(&self, cx: &LateContext<'_>, e: &Expr<'_>, s: symbol::Symbol, module: &str) {
  87         let s = s.as_str();
  88         if s.parse::<f64>().is_ok() {
  89             for &(constant, name, min_digits, msrv) in &KNOWN_CONSTS {
  90                 if is_approx_const(constant, s, min_digits) && msrv.map_or(true, |msrv| meets_msrv(self.msrv, msrv)) {
  91                     span_lint_and_help(
  92                         cx,
  93                         APPROX_CONSTANT,
  94                         e.span,
  95                         &format!("approximate value of `{}::consts::{}` found", module, &name),
  96                         None,
  97                         "consider using the constant directly",
  98                     );
  99                     return;
 100                 }
 101             }
 102         }
 103     }
 104 }
 105
 106 impl_lint_pass!(ApproxConstant => [APPROX_CONSTANT]);
 107
 108 impl<'tcx> LateLintPass<'tcx> for ApproxConstant {
 109     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
 110         if let ExprKind::Lit(lit) = &e.kind {
 111             self.check_lit(cx, &lit.node, e);
 112         }
 113     }
 114
 115     extract_msrv_attr!(LateContext);
 116 }
 117
 118 /// Returns `false` if the number of significant figures in `value` are
 119 /// less than `min_digits`; otherwise, returns true if `value` is equal
 120 /// to `constant`, rounded to the number of digits present in `value`.
 121 #[must_use]
 122 fn is_approx_const(constant: f64, value: &str, min_digits: usize) -> bool {
 123     if value.len() <= min_digits {
 124         false
 125     } else if constant.to_string().starts_with(value) {
 126         // The value is a truncated constant
 127         true
 128     } else {
 129         let round_const = format!("{:.*}", value.len() - 2, constant);
 130         value == round_const
 131     }
 132 }