2 use rustc_front::hir::*;
3 use std::f64::consts as f64;
5 use syntax::ast::Lit_::*;
7 use syntax::ast::FloatTy::*;
9 /// **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. This lint is `Warn` by default.
11 /// **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).
13 /// **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.
15 /// **Example:** `let x = 3.14;`
19 "the approximate of a known float constant (in `std::f64::consts` or `std::f32::consts`) \
20 is found; suggests to use the constant"
23 // Tuples are of the form (constant, name, min_digits)
24 const KNOWN_CONSTS : &'static [(f64, &'static str, usize)] = &[
26 (f64::FRAC_1_PI, "FRAC_1_PI", 4),
27 (f64::FRAC_1_SQRT_2, "FRAC_1_SQRT_2", 5),
28 (f64::FRAC_2_PI, "FRAC_2_PI", 5),
29 (f64::FRAC_2_SQRT_PI, "FRAC_2_SQRT_PI", 5),
30 (f64::FRAC_PI_2, "FRAC_PI_2", 5),
31 (f64::FRAC_PI_3, "FRAC_PI_3", 5),
32 (f64::FRAC_PI_4, "FRAC_PI_4", 5),
33 (f64::FRAC_PI_6, "FRAC_PI_6", 5),
34 (f64::FRAC_PI_8, "FRAC_PI_8", 5),
35 (f64::LN_10, "LN_10", 5),
36 (f64::LN_2, "LN_2", 5),
37 (f64::LOG10_E, "LOG10_E", 5),
38 (f64::LOG2_E, "LOG2_E", 5),
40 (f64::SQRT_2, "SQRT_2", 5),
44 pub struct ApproxConstant;
46 impl LintPass for ApproxConstant {
47 fn get_lints(&self) -> LintArray {
48 lint_array!(APPROX_CONSTANT)
52 impl LateLintPass for ApproxConstant {
53 fn check_expr(&mut self, cx: &LateContext, e: &Expr) {
54 if let ExprLit(ref lit) = e.node {
55 check_lit(cx, lit, e);
60 fn check_lit(cx: &LateContext, lit: &Lit, e: &Expr) {
62 LitFloat(ref s, TyF32) => check_known_consts(cx, e, s, "f32"),
63 LitFloat(ref s, TyF64) => check_known_consts(cx, e, s, "f64"),
64 LitFloatUnsuffixed(ref s) =>
65 check_known_consts(cx, e, s, "f{32, 64}"),
70 fn check_known_consts(cx: &LateContext, e: &Expr, s: &str, module: &str) {
71 if let Ok(_) = s.parse::<f64>() {
72 for &(constant, name, min_digits) in KNOWN_CONSTS {
73 if is_approx_const(constant, s, min_digits) {
74 span_lint(cx, APPROX_CONSTANT, e.span, &format!(
75 "approximate value of `{}::{}` found. \
76 Consider using it directly", module, &name));
83 /// Returns false if the number of significant figures in `value` are
84 /// less than `min_digits`; otherwise, returns true if `value` is equal
85 /// to `constant`, rounded to the number of digits present in `value`.
86 fn is_approx_const(constant: f64, value: &str, min_digits: usize) -> bool {
87 if value.len() <= min_digits {
90 let round_const = format!("{:.*}", value.len() - 2, constant);
92 let mut trunc_const = constant.to_string();
93 if trunc_const.len() > value.len() {
94 trunc_const.truncate(value.len());
97 (value == round_const) || (value == trunc_const)