1 use crate::utils::span_lint;
2 use rustc::declare_lint_pass;
4 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
5 use rustc_session::declare_tool_lint;
6 use std::f64::consts as f64;
7 use syntax::ast::{FloatTy, LitFloatType, LitKind};
10 declare_clippy_lint! {
11 /// **What it does:** Checks for floating point literals that approximate
12 /// constants which are defined in
13 /// [`std::f32::consts`](https://doc.rust-lang.org/stable/std/f32/consts/#constants)
15 /// [`std::f64::consts`](https://doc.rust-lang.org/stable/std/f64/consts/#constants),
16 /// respectively, suggesting to use the predefined constant.
18 /// **Why is this bad?** Usually, the definition in the standard library is more
19 /// precise than what people come up with. If you find that your definition is
20 /// actually more precise, please [file a Rust
21 /// issue](https://github.com/rust-lang/rust/issues).
23 /// **Known problems:** None.
29 /// let y = 1_f64 / x;
32 /// let x = std::f32::consts::PI;
33 /// let y = std::f64::consts::FRAC_1_PI;
37 "the approximate of a known float constant (in `std::fXX::consts`)"
40 // Tuples are of the form (constant, name, min_digits)
41 const KNOWN_CONSTS: [(f64, &str, usize); 16] = [
43 (f64::FRAC_1_PI, "FRAC_1_PI", 4),
44 (f64::FRAC_1_SQRT_2, "FRAC_1_SQRT_2", 5),
45 (f64::FRAC_2_PI, "FRAC_2_PI", 5),
46 (f64::FRAC_2_SQRT_PI, "FRAC_2_SQRT_PI", 5),
47 (f64::FRAC_PI_2, "FRAC_PI_2", 5),
48 (f64::FRAC_PI_3, "FRAC_PI_3", 5),
49 (f64::FRAC_PI_4, "FRAC_PI_4", 5),
50 (f64::FRAC_PI_6, "FRAC_PI_6", 5),
51 (f64::FRAC_PI_8, "FRAC_PI_8", 5),
52 (f64::LN_10, "LN_10", 5),
53 (f64::LN_2, "LN_2", 5),
54 (f64::LOG10_E, "LOG10_E", 5),
55 (f64::LOG2_E, "LOG2_E", 5),
57 (f64::SQRT_2, "SQRT_2", 5),
60 declare_lint_pass!(ApproxConstant => [APPROX_CONSTANT]);
62 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ApproxConstant {
63 fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr<'_>) {
64 if let ExprKind::Lit(lit) = &e.kind {
65 check_lit(cx, &lit.node, e);
70 fn check_lit(cx: &LateContext<'_, '_>, lit: &LitKind, e: &Expr<'_>) {
72 LitKind::Float(s, LitFloatType::Suffixed(fty)) => match fty {
73 FloatTy::F32 => check_known_consts(cx, e, s, "f32"),
74 FloatTy::F64 => check_known_consts(cx, e, s, "f64"),
76 LitKind::Float(s, LitFloatType::Unsuffixed) => check_known_consts(cx, e, s, "f{32, 64}"),
81 fn check_known_consts(cx: &LateContext<'_, '_>, e: &Expr<'_>, s: symbol::Symbol, module: &str) {
83 if s.parse::<f64>().is_ok() {
84 for &(constant, name, min_digits) in &KNOWN_CONSTS {
85 if is_approx_const(constant, &s, min_digits) {
91 "approximate value of `{}::consts::{}` found. \
92 Consider using it directly",
102 /// Returns `false` if the number of significant figures in `value` are
103 /// less than `min_digits`; otherwise, returns true if `value` is equal
104 /// 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 {
109 } else if constant.to_string().starts_with(value) {
110 // The value is a truncated constant
113 let round_const = format!("{:.*}", value.len() - 2, constant);