rust/clippy_lints/src/approx_const.rs

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use crate::utils::span_lint;
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use rustc_ast::ast::{FloatTy, LitFloatType, LitKind};
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use rustc_hir::{Expr, ExprKind};
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use rustc_lint::{LateContext, LateLintPass};
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use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::symbol;
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use std::f64::consts as f64;
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declare_clippy_lint! {
/// **What it does:** 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.
///
/// **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).
///
/// **Known problems:** None.
///
/// **Example:**
/// ```rust
/// let x = 3.14;
/// let y = 1_f64 / x;
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/// ```
/// Use predefined constants instead:
/// ```rust
/// let x = std::f32::consts::PI;
/// let y = std::f64::consts::FRAC_1_PI;
/// ```
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pub APPROX_CONSTANT,
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correctness,
"the approximate of a known float constant (in `std::fXX::consts`)"
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}
// Tuples are of the form (constant, name, min_digits)
const KNOWN_CONSTS: [(f64, &str, usize); 18] = [
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(f64::E, "E", 4),
(f64::FRAC_1_PI, "FRAC_1_PI", 4),
(f64::FRAC_1_SQRT_2, "FRAC_1_SQRT_2", 5),
(f64::FRAC_2_PI, "FRAC_2_PI", 5),
(f64::FRAC_2_SQRT_PI, "FRAC_2_SQRT_PI", 5),
(f64::FRAC_PI_2, "FRAC_PI_2", 5),
(f64::FRAC_PI_3, "FRAC_PI_3", 5),
(f64::FRAC_PI_4, "FRAC_PI_4", 5),
(f64::FRAC_PI_6, "FRAC_PI_6", 5),
(f64::FRAC_PI_8, "FRAC_PI_8", 5),
(f64::LN_10, "LN_10", 5),
(f64::LN_2, "LN_2", 5),
(f64::LOG10_E, "LOG10_E", 5),
(f64::LOG2_E, "LOG2_E", 5),
(f64::LOG2_10, "LOG2_10", 5),
(f64::LOG10_2, "LOG10_2", 5),
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(f64::PI, "PI", 3),
(f64::SQRT_2, "SQRT_2", 5),
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];
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declare_lint_pass!(ApproxConstant => [APPROX_CONSTANT]);
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impl<'tcx> LateLintPass<'tcx> for ApproxConstant {
fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
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if let ExprKind::Lit(lit) = &e.kind {
check_lit(cx, &lit.node, e);
}
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}
}
fn check_lit(cx: &LateContext<'_>, lit: &LitKind, e: &Expr<'_>) {
match *lit {
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LitKind::Float(s, LitFloatType::Suffixed(fty)) => match fty {
FloatTy::F32 => check_known_consts(cx, e, s, "f32"),
FloatTy::F64 => check_known_consts(cx, e, s, "f64"),
},
LitKind::Float(s, LitFloatType::Unsuffixed) => check_known_consts(cx, e, s, "f{32, 64}"),
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_ => (),
}
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}
fn check_known_consts(cx: &LateContext<'_>, e: &Expr<'_>, s: symbol::Symbol, module: &str) {
let s = s.as_str();
if s.parse::<f64>().is_ok() {
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for &(constant, name, min_digits) in &KNOWN_CONSTS {
if is_approx_const(constant, &s, min_digits) {
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span_lint(
cx,
APPROX_CONSTANT,
e.span,
&format!(
"approximate value of `{}::consts::{}` found. \
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Consider using it directly",
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module, &name
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),
);
return;
}
}
}
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}
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/// Returns `false` if the number of significant figures in `value` are
/// less than `min_digits`; otherwise, returns true if `value` is equal
/// to `constant`, rounded to the number of digits present in `value`.
#[must_use]
fn is_approx_const(constant: f64, value: &str, min_digits: usize) -> bool {
if value.len() <= min_digits {
false
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} else if constant.to_string().starts_with(value) {
// The value is a truncated constant
true
} else {
let round_const = format!("{:.*}", value.len() - 2, constant);
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value == round_const
}
}