1 use crate::utils::span_lint;
3 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
4 use rustc::{declare_lint_pass, declare_tool_lint};
5 use std::f64::consts as f64;
6 use syntax::ast::{FloatTy, LitKind};
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)
14 /// [`std::f64::consts`](https://doc.rust-lang.org/stable/std/f64/consts/#constants),
15 /// respectively, suggesting to use the predefined constant.
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).
22 /// **Known problems:** None.
30 "the approximate of a known float constant (in `std::fXX::consts`)"
33 // Tuples are of the form (constant, name, min_digits)
34 const KNOWN_CONSTS: [(f64, &str, usize); 16] = [
36 (f64::FRAC_1_PI, "FRAC_1_PI", 4),
37 (f64::FRAC_1_SQRT_2, "FRAC_1_SQRT_2", 5),
38 (f64::FRAC_2_PI, "FRAC_2_PI", 5),
39 (f64::FRAC_2_SQRT_PI, "FRAC_2_SQRT_PI", 5),
40 (f64::FRAC_PI_2, "FRAC_PI_2", 5),
41 (f64::FRAC_PI_3, "FRAC_PI_3", 5),
42 (f64::FRAC_PI_4, "FRAC_PI_4", 5),
43 (f64::FRAC_PI_6, "FRAC_PI_6", 5),
44 (f64::FRAC_PI_8, "FRAC_PI_8", 5),
45 (f64::LN_10, "LN_10", 5),
46 (f64::LN_2, "LN_2", 5),
47 (f64::LOG10_E, "LOG10_E", 5),
48 (f64::LOG2_E, "LOG2_E", 5),
50 (f64::SQRT_2, "SQRT_2", 5),
53 declare_lint_pass!(ApproxConstant => [APPROX_CONSTANT]);
55 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ApproxConstant {
56 fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
57 if let ExprKind::Lit(lit) = &e.node {
58 check_lit(cx, &lit.node, e);
63 fn check_lit(cx: &LateContext<'_, '_>, lit: &LitKind, e: &Expr) {
65 LitKind::Float(s, FloatTy::F32) => check_known_consts(cx, e, s, "f32"),
66 LitKind::Float(s, FloatTy::F64) => check_known_consts(cx, e, s, "f64"),
67 LitKind::FloatUnsuffixed(s) => check_known_consts(cx, e, s, "f{32, 64}"),
72 fn check_known_consts(cx: &LateContext<'_, '_>, e: &Expr, s: symbol::Symbol, module: &str) {
74 if s.parse::<f64>().is_ok() {
75 for &(constant, name, min_digits) in &KNOWN_CONSTS {
76 if is_approx_const(constant, &s, min_digits) {
82 "approximate value of `{}::consts::{}` found. \
83 Consider using it directly",
93 /// Returns `false` if the number of significant figures in `value` are
94 /// less than `min_digits`; otherwise, returns true if `value` is equal
95 /// to `constant`, rounded to the number of digits present in `value`.
96 fn is_approx_const(constant: f64, value: &str, min_digits: usize) -> bool {
97 if value.len() <= min_digits {
100 let round_const = format!("{:.*}", value.len() - 2, constant);
102 let mut trunc_const = constant.to_string();
103 if trunc_const.len() > value.len() {
104 trunc_const.truncate(value.len());
107 (value == round_const) || (value == trunc_const)