3 use std::f64::consts as f64;
4 use syntax::ast::{Lit, LitKind, FloatTy};
7 /// **What it does:** This lint checks for floating point literals that approximate constants which are defined in [`std::f32::consts`](https://doc.rust-lang.org/stable/std/f32/consts/#constants) or [`std::f64::consts`](https://doc.rust-lang.org/stable/std/f64/consts/#constants), respectively, suggesting to use the predefined constant.
9 /// **Why is this bad?** Usually, the definition in the standard library is more precise than what people come up with. If you find that your definition is actually more precise, please [file a Rust issue](https://github.com/rust-lang/rust/issues).
11 /// **Known problems:** If you happen to have a value that is within 1/8192 of a known constant, but is not *and should not* be the same, this lint will report your value anyway. We have not yet noticed any false positives in code we tested clippy with (this includes servo), but YMMV.
13 /// **Example:** `let x = 3.14;`
17 "the approximate of a known float constant (in `std::f64::consts` or `std::f32::consts`) \
18 is found; suggests to use the constant"
21 // Tuples are of the form (constant, name, min_digits)
22 const KNOWN_CONSTS: &'static [(f64, &'static str, usize)] = &[(f64::E, "E", 4),
23 (f64::FRAC_1_PI, "FRAC_1_PI", 4),
24 (f64::FRAC_1_SQRT_2, "FRAC_1_SQRT_2", 5),
25 (f64::FRAC_2_PI, "FRAC_2_PI", 5),
26 (f64::FRAC_2_SQRT_PI, "FRAC_2_SQRT_PI", 5),
27 (f64::FRAC_PI_2, "FRAC_PI_2", 5),
28 (f64::FRAC_PI_3, "FRAC_PI_3", 5),
29 (f64::FRAC_PI_4, "FRAC_PI_4", 5),
30 (f64::FRAC_PI_6, "FRAC_PI_6", 5),
31 (f64::FRAC_PI_8, "FRAC_PI_8", 5),
32 (f64::LN_10, "LN_10", 5),
33 (f64::LN_2, "LN_2", 5),
34 (f64::LOG10_E, "LOG10_E", 5),
35 (f64::LOG2_E, "LOG2_E", 5),
37 (f64::SQRT_2, "SQRT_2", 5)];
40 pub struct ApproxConstant;
42 impl LintPass for ApproxConstant {
43 fn get_lints(&self) -> LintArray {
44 lint_array!(APPROX_CONSTANT)
48 impl LateLintPass for ApproxConstant {
49 fn check_expr(&mut self, cx: &LateContext, e: &Expr) {
50 if let ExprLit(ref lit) = e.node {
51 check_lit(cx, lit, e);
56 fn check_lit(cx: &LateContext, lit: &Lit, e: &Expr) {
58 LitKind::Float(ref s, FloatTy::F32) => check_known_consts(cx, e, s, "f32"),
59 LitKind::Float(ref s, FloatTy::F64) => check_known_consts(cx, e, s, "f64"),
60 LitKind::FloatUnsuffixed(ref s) => check_known_consts(cx, e, s, "f{32, 64}"),
65 fn check_known_consts(cx: &LateContext, e: &Expr, s: &str, module: &str) {
66 if let Ok(_) = s.parse::<f64>() {
67 for &(constant, name, min_digits) in KNOWN_CONSTS {
68 if is_approx_const(constant, s, min_digits) {
72 &format!("approximate value of `{}::{}` found. Consider using it directly", module, &name));
79 /// Returns false if the number of significant figures in `value` are
80 /// less than `min_digits`; otherwise, returns true if `value` is equal
81 /// to `constant`, rounded to the number of digits present in `value`.
82 fn is_approx_const(constant: f64, value: &str, min_digits: usize) -> bool {
83 if value.len() <= min_digits {
86 let round_const = format!("{:.*}", value.len() - 2, constant);
88 let mut trunc_const = constant.to_string();
89 if trunc_const.len() > value.len() {
90 trunc_const.truncate(value.len());
93 (value == round_const) || (value == trunc_const)