1 use crate::utils::span_lint;
2 use rustc_ast::ast::{FloatTy, LitFloatType, LitKind};
3 use rustc_hir::{Expr, ExprKind};
4 use rustc_lint::{LateContext, LateLintPass};
5 use rustc_session::{declare_lint_pass, declare_tool_lint};
6 use rustc_span::symbol;
7 use std::f64::consts as f64;
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.
27 /// let y = 1_f64 / x;
29 /// Use predefined constants instead:
31 /// let x = std::f32::consts::PI;
32 /// let y = std::f64::consts::FRAC_1_PI;
36 "the approximate of a known float constant (in `std::fXX::consts`)"
39 // Tuples are of the form (constant, name, min_digits)
40 const KNOWN_CONSTS: [(f64, &str, usize); 18] = [
42 (f64::FRAC_1_PI, "FRAC_1_PI", 4),
43 (f64::FRAC_1_SQRT_2, "FRAC_1_SQRT_2", 5),
44 (f64::FRAC_2_PI, "FRAC_2_PI", 5),
45 (f64::FRAC_2_SQRT_PI, "FRAC_2_SQRT_PI", 5),
46 (f64::FRAC_PI_2, "FRAC_PI_2", 5),
47 (f64::FRAC_PI_3, "FRAC_PI_3", 5),
48 (f64::FRAC_PI_4, "FRAC_PI_4", 5),
49 (f64::FRAC_PI_6, "FRAC_PI_6", 5),
50 (f64::FRAC_PI_8, "FRAC_PI_8", 5),
51 (f64::LN_10, "LN_10", 5),
52 (f64::LN_2, "LN_2", 5),
53 (f64::LOG10_E, "LOG10_E", 5),
54 (f64::LOG2_E, "LOG2_E", 5),
55 (f64::LOG2_10, "LOG2_10", 5),
56 (f64::LOG10_2, "LOG10_2", 5),
58 (f64::SQRT_2, "SQRT_2", 5),
61 declare_lint_pass!(ApproxConstant => [APPROX_CONSTANT]);
63 impl<'tcx> LateLintPass<'tcx> for ApproxConstant {
64 fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
65 if let ExprKind::Lit(lit) = &e.kind {
66 check_lit(cx, &lit.node, e);
71 fn check_lit(cx: &LateContext<'_>, lit: &LitKind, e: &Expr<'_>) {
73 LitKind::Float(s, LitFloatType::Suffixed(fty)) => match fty {
74 FloatTy::F32 => check_known_consts(cx, e, s, "f32"),
75 FloatTy::F64 => check_known_consts(cx, e, s, "f64"),
77 LitKind::Float(s, LitFloatType::Unsuffixed) => 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`.
107 fn is_approx_const(constant: f64, value: &str, min_digits: usize) -> bool {
108 if value.len() <= min_digits {
110 } else if constant.to_string().starts_with(value) {
111 // The value is a truncated constant
114 let round_const = format!("{:.*}", value.len() - 2, constant);