mirror of
https://github.com/rust-lang/rust-clippy
synced 2024-12-21 10:33:27 +00:00
115 lines
3.9 KiB
Rust
115 lines
3.9 KiB
Rust
use crate::utils::span_lint;
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use rustc_hir::*;
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use rustc_lint::{LateContext, LateLintPass};
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use rustc_session::{declare_lint_pass, declare_tool_lint};
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use rustc_span::symbol;
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use std::f64::consts as f64;
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use syntax::ast::{FloatTy, LitFloatType, LitKind};
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declare_clippy_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:** None.
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///
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/// **Example:**
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/// ```rust
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/// // Bad
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/// let x = 3.14;
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/// let y = 1_f64 / x;
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///
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/// // Good
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/// let x = std::f32::consts::PI;
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/// let y = std::f64::consts::FRAC_1_PI;
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/// ```
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pub APPROX_CONSTANT,
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correctness,
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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: [(f64, &str, usize); 16] = [
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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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declare_lint_pass!(ApproxConstant => [APPROX_CONSTANT]);
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impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ApproxConstant {
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fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr<'_>) {
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if let ExprKind::Lit(lit) = &e.kind {
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check_lit(cx, &lit.node, e);
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}
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}
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}
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fn check_lit(cx: &LateContext<'_, '_>, lit: &LitKind, e: &Expr<'_>) {
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match *lit {
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LitKind::Float(s, LitFloatType::Suffixed(fty)) => match fty {
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FloatTy::F32 => check_known_consts(cx, e, s, "f32"),
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FloatTy::F64 => check_known_consts(cx, e, s, "f64"),
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},
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LitKind::Float(s, LitFloatType::Unsuffixed) => 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, &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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#[must_use]
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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 if constant.to_string().starts_with(value) {
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// The value is a truncated constant
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true
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} else {
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let round_const = format!("{:.*}", value.len() - 2, constant);
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value == round_const
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}
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}
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