mirror of
https://github.com/rust-lang/rust-clippy
synced 2024-12-21 02:23:19 +00:00
119 lines
3.9 KiB
Rust
119 lines
3.9 KiB
Rust
use rustc::lint::*;
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use rustc::hir::*;
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use std::f64::consts as f64;
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use syntax::ast::{FloatTy, Lit, LitKind};
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use syntax::symbol;
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use utils::span_lint;
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/// **What it does:** Checks for floating point literals that approximate
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/// constants which are defined in
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/// [`std::f32::consts`](https://doc.rust-lang.org/stable/std/f32/consts/#constants)
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/// or
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/// [`std::f64::consts`](https://doc.rust-lang.org/stable/std/f64/consts/#constants),
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/// 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
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/// precise than what people come up with. If you find that your definition is
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/// actually more precise, please [file a Rust
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/// 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
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/// known constant, but is not *and should not* be the same, this lint will
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/// report your value anyway. We have not yet noticed any false positives in
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/// code we tested clippy with (this includes servo), but YMMV.
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///
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/// **Example:**
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/// ```rust
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/// let x = 3.14;
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/// ```
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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::fXX::consts`)"
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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)] = &[
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(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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];
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#[derive(Copy, Clone)]
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pub struct Pass;
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impl LintPass for Pass {
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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<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
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fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx 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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LitKind::Float(ref s, FloatTy::F32) => check_known_consts(cx, e, s, "f32"),
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LitKind::Float(ref s, FloatTy::F64) => check_known_consts(cx, e, s, "f64"),
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LitKind::FloatUnsuffixed(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: &symbol::Symbol, module: &str) {
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let s = s.as_str();
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if s.parse::<f64>().is_ok() {
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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(
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cx,
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APPROX_CONSTANT,
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e.span,
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&format!(
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"approximate value of `{}::consts::{}` found. \
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Consider using it directly",
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module,
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&name
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),
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);
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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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