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

   1 use rustc::lint::*;
   2 use rustc::hir::*;
   3 use std::f64::consts as f64;
   4 use syntax::ast::{Lit, LitKind, FloatTy};
   5 use syntax::symbol;
   6 use utils::span_lint;
   7
   8 /// **What it does:** Checks for floating point literals that approximate
   9 /// constants which are defined in
  10 /// [`std::f32::consts`](https://doc.rust-lang.
  11 /// org/stable/std/f32/consts/#constants)
  12 /// or
  13 /// [`std::f64::consts`](https://doc.rust-lang.
  14 /// 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:** If you happen to have a value that is within 1/8192 of a
  23 /// known constant, but is not *and should not* be the same, this lint will
  24 /// report your value anyway. We have not yet noticed any false positives in
  25 /// code we tested clippy with (this includes servo), but YMMV.
  26 ///
  27 /// **Example:**
  28 /// ```rust
  29 /// let x = 3.14;
  30 /// ```
  31 declare_lint! {
  32     pub APPROX_CONSTANT,
  33     Warn,
  34     "the approximate of a known float constant (in `std::fXX::consts`)"
  35 }
  36
  37 // Tuples are of the form (constant, name, min_digits)
  38 const KNOWN_CONSTS: &'static [(f64, &'static str, usize)] = &[
  39     (f64::E, "E", 4),
  40     (f64::FRAC_1_PI, "FRAC_1_PI", 4),
  41     (f64::FRAC_1_SQRT_2, "FRAC_1_SQRT_2", 5),
  42     (f64::FRAC_2_PI, "FRAC_2_PI", 5),
  43     (f64::FRAC_2_SQRT_PI, "FRAC_2_SQRT_PI", 5),
  44     (f64::FRAC_PI_2, "FRAC_PI_2", 5),
  45     (f64::FRAC_PI_3, "FRAC_PI_3", 5),
  46     (f64::FRAC_PI_4, "FRAC_PI_4", 5),
  47     (f64::FRAC_PI_6, "FRAC_PI_6", 5),
  48     (f64::FRAC_PI_8, "FRAC_PI_8", 5),
  49     (f64::LN_10, "LN_10", 5),
  50     (f64::LN_2, "LN_2", 5),
  51     (f64::LOG10_E, "LOG10_E", 5),
  52     (f64::LOG2_E, "LOG2_E", 5),
  53     (f64::PI, "PI", 3),
  54     (f64::SQRT_2, "SQRT_2", 5),
  55 ];
  56
  57 #[derive(Copy, Clone)]
  58 pub struct Pass;
  59
  60 impl LintPass for Pass {
  61     fn get_lints(&self) -> LintArray {
  62         lint_array!(APPROX_CONSTANT)
  63     }
  64 }
  65
  66 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
  67     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
  68         if let ExprLit(ref lit) = e.node {
  69             check_lit(cx, lit, e);
  70         }
  71     }
  72 }
  73
  74 fn check_lit(cx: &LateContext, lit: &Lit, e: &Expr) {
  75     match lit.node {
  76         LitKind::Float(ref s, FloatTy::F32) => check_known_consts(cx, e, s, "f32"),
  77         LitKind::Float(ref s, FloatTy::F64) => check_known_consts(cx, e, s, "f64"),
  78         LitKind::FloatUnsuffixed(ref s) => check_known_consts(cx, e, s, "f{32, 64}"),
  79         _ => (),
  80     }
  81 }
  82
  83 fn check_known_consts(cx: &LateContext, e: &Expr, s: &symbol::Symbol, module: &str) {
  84     let s = s.as_str();
  85     if s.parse::<f64>().is_ok() {
  86         for &(constant, name, min_digits) in KNOWN_CONSTS {
  87             if is_approx_const(constant, &s, min_digits) {
  88                 span_lint(
  89                     cx,
  90                     APPROX_CONSTANT,
  91                     e.span,
  92                     &format!(
  93                         "approximate value of `{}::consts::{}` found. \
  94                                     Consider using it directly",
  95                         module,
  96                         &name
  97                     ),
  98                 );
  99                 return;
 100             }
 101         }
 102     }
 103 }
 104
 105 /// Returns false if the number of significant figures in `value` are
 106 /// less than `min_digits`; otherwise, returns true if `value` is equal
 107 /// to `constant`, rounded to the number of digits present in `value`.
 108 fn is_approx_const(constant: f64, value: &str, min_digits: usize) -> bool {
 109     if value.len() <= min_digits {
 110         false
 111     } else {
 112         let round_const = format!("{:.*}", value.len() - 2, constant);
 113
 114         let mut trunc_const = constant.to_string();
 115         if trunc_const.len() > value.len() {
 116             trunc_const.truncate(value.len());
 117         }
 118
 119         (value == round_const) || (value == trunc_const)
 120     }
 121 }