1 use crate::utils::span_lint;
4 use rustc::{declare_lint, lint_array};
5 use std::f64::consts as f64;
6 use syntax::ast::{FloatTy, Lit, LitKind};
9 /// **What it does:** Checks for floating point literals that approximate
10 /// constants which are defined in
11 /// [`std::f32::consts`](https://doc.rust-lang.org/stable/std/f32/consts/#constants)
13 /// [`std::f64::consts`](https://doc.rust-lang.org/stable/std/f64/consts/#constants),
14 /// respectively, suggesting to use the predefined constant.
16 /// **Why is this bad?** Usually, the definition in the standard library is more
17 /// precise than what people come up with. If you find that your definition is
18 /// actually more precise, please [file a Rust
19 /// issue](https://github.com/rust-lang/rust/issues).
21 /// **Known problems:** If you happen to have a value that is within 1/8192 of a
22 /// known constant, but is not *and should not* be the same, this lint will
23 /// report your value anyway. We have not yet noticed any false positives in
24 /// code we tested clippy with (this includes servo), but YMMV.
30 declare_clippy_lint! {
33 "the approximate of a known float constant (in `std::fXX::consts`)"
36 // Tuples are of the form (constant, name, min_digits)
37 const KNOWN_CONSTS: &[(f64, &str, usize)] = &[
39 (f64::FRAC_1_PI, "FRAC_1_PI", 4),
40 (f64::FRAC_1_SQRT_2, "FRAC_1_SQRT_2", 5),
41 (f64::FRAC_2_PI, "FRAC_2_PI", 5),
42 (f64::FRAC_2_SQRT_PI, "FRAC_2_SQRT_PI", 5),
43 (f64::FRAC_PI_2, "FRAC_PI_2", 5),
44 (f64::FRAC_PI_3, "FRAC_PI_3", 5),
45 (f64::FRAC_PI_4, "FRAC_PI_4", 5),
46 (f64::FRAC_PI_6, "FRAC_PI_6", 5),
47 (f64::FRAC_PI_8, "FRAC_PI_8", 5),
48 (f64::LN_10, "LN_10", 5),
49 (f64::LN_2, "LN_2", 5),
50 (f64::LOG10_E, "LOG10_E", 5),
51 (f64::LOG2_E, "LOG2_E", 5),
53 (f64::SQRT_2, "SQRT_2", 5),
56 #[derive(Copy, Clone)]
59 impl LintPass for Pass {
60 fn get_lints(&self) -> LintArray {
61 lint_array!(APPROX_CONSTANT)
65 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
66 fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
67 if let ExprKind::Lit(ref lit) = e.node {
68 check_lit(cx, lit, e);
73 fn check_lit(cx: &LateContext<'_, '_>, lit: &Lit, e: &Expr) {
75 LitKind::Float(s, FloatTy::F32) => check_known_consts(cx, e, s, "f32"),
76 LitKind::Float(s, FloatTy::F64) => check_known_consts(cx, e, s, "f64"),
77 LitKind::FloatUnsuffixed(s) => check_known_consts(cx, e, s, "f{32, 64}"),
82 fn check_known_consts(cx: &LateContext<'_, '_>, e: &Expr, s: symbol::Symbol, module: &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) {
92 "approximate value of `{}::consts::{}` found. \
93 Consider using it directly",
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 fn is_approx_const(constant: f64, value: &str, min_digits: usize) -> bool {
107 if value.len() <= min_digits {
110 let round_const = format!("{:.*}", value.len() - 2, constant);
112 let mut trunc_const = constant.to_string();
113 if trunc_const.len() > value.len() {
114 trunc_const.truncate(value.len());
117 (value == round_const) || (value == trunc_const)