mirror of
https://github.com/rust-lang/rust-clippy
synced 2024-12-24 20:13:24 +00:00
95 lines
4.6 KiB
Rust
95 lines
4.6 KiB
Rust
use rustc::lint::*;
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use rustc_front::hir::*;
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use std::f64::consts as f64;
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use utils::span_lint;
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use syntax::ast::{Lit, Lit_, FloatTy};
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/// **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.
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///
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/// **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).
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///
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/// **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.
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///
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/// **Example:** `let x = 3.14;`
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declare_lint! {
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pub APPROX_CONSTANT,
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Warn,
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"the approximate of a known float constant (in `std::f64::consts` or `std::f32::consts`) \
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is found; suggests to use the constant"
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}
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// Tuples are of the form (constant, name, min_digits)
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const KNOWN_CONSTS: &'static [(f64, &'static str, usize)] = &[(f64::E, "E", 4),
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(f64::FRAC_1_PI, "FRAC_1_PI", 4),
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(f64::FRAC_1_SQRT_2, "FRAC_1_SQRT_2", 5),
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(f64::FRAC_2_PI, "FRAC_2_PI", 5),
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(f64::FRAC_2_SQRT_PI, "FRAC_2_SQRT_PI", 5),
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(f64::FRAC_PI_2, "FRAC_PI_2", 5),
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(f64::FRAC_PI_3, "FRAC_PI_3", 5),
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(f64::FRAC_PI_4, "FRAC_PI_4", 5),
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(f64::FRAC_PI_6, "FRAC_PI_6", 5),
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(f64::FRAC_PI_8, "FRAC_PI_8", 5),
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(f64::LN_10, "LN_10", 5),
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(f64::LN_2, "LN_2", 5),
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(f64::LOG10_E, "LOG10_E", 5),
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(f64::LOG2_E, "LOG2_E", 5),
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(f64::PI, "PI", 3),
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(f64::SQRT_2, "SQRT_2", 5)];
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#[derive(Copy,Clone)]
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pub struct ApproxConstant;
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impl LintPass for ApproxConstant {
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fn get_lints(&self) -> LintArray {
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lint_array!(APPROX_CONSTANT)
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}
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}
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impl LateLintPass for ApproxConstant {
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fn check_expr(&mut self, cx: &LateContext, e: &Expr) {
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if let ExprLit(ref lit) = e.node {
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check_lit(cx, lit, e);
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}
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}
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}
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fn check_lit(cx: &LateContext, lit: &Lit, e: &Expr) {
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match lit.node {
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Lit_::LitFloat(ref s, FloatTy::TyF32) => check_known_consts(cx, e, s, "f32"),
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Lit_::LitFloat(ref s, FloatTy::TyF64) => check_known_consts(cx, e, s, "f64"),
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Lit_::LitFloatUnsuffixed(ref s) => check_known_consts(cx, e, s, "f{32, 64}"),
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_ => (),
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}
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}
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fn check_known_consts(cx: &LateContext, e: &Expr, s: &str, module: &str) {
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if let Ok(_) = s.parse::<f64>() {
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for &(constant, name, min_digits) in KNOWN_CONSTS {
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if is_approx_const(constant, s, min_digits) {
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span_lint(cx,
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APPROX_CONSTANT,
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e.span,
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&format!("approximate value of `{}::{}` found. Consider using it directly", module, &name));
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return;
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}
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}
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}
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}
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/// Returns false if the number of significant figures in `value` are
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/// less than `min_digits`; otherwise, returns true if `value` is equal
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/// to `constant`, rounded to the number of digits present in `value`.
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fn is_approx_const(constant: f64, value: &str, min_digits: usize) -> bool {
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if value.len() <= min_digits {
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false
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} else {
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let round_const = format!("{:.*}", value.len() - 2, constant);
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let mut trunc_const = constant.to_string();
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if trunc_const.len() > value.len() {
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trunc_const.truncate(value.len());
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}
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(value == round_const) || (value == trunc_const)
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}
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}
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