use rustc::lint::*;
use rustc::hir::*;
use std::f64::consts as f64;
use syntax::ast::{FloatTy, Lit, LitKind};
use syntax::symbol;
use utils::span_lint;
declare_lint! {
pub APPROX_CONSTANT,
Warn,
"the approximate of a known float constant (in `std::fXX::consts`)"
}
const KNOWN_CONSTS: &[(f64, &str, usize)] = &[
(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::PI, "PI", 3),
(f64::SQRT_2, "SQRT_2", 5),
];
#[derive(Copy, Clone)]
pub struct Pass;
impl LintPass for Pass {
fn get_lints(&self) -> LintArray {
lint_array!(APPROX_CONSTANT)
}
}
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
if let ExprLit(ref lit) = e.node {
check_lit(cx, lit, e);
}
}
}
fn check_lit(cx: &LateContext, lit: &Lit, e: &Expr) {
match lit.node {
LitKind::Float(ref s, FloatTy::F32) => check_known_consts(cx, e, s, "f32"),
LitKind::Float(ref s, FloatTy::F64) => check_known_consts(cx, e, s, "f64"),
LitKind::FloatUnsuffixed(ref s) => check_known_consts(cx, e, s, "f{32, 64}"),
_ => (),
}
}
fn check_known_consts(cx: &LateContext, e: &Expr, s: &symbol::Symbol, module: &str) {
let s = s.as_str();
if s.parse::<f64>().is_ok() {
for &(constant, name, min_digits) in KNOWN_CONSTS {
if is_approx_const(constant, &s, min_digits) {
span_lint(
cx,
APPROX_CONSTANT,
e.span,
&format!(
"approximate value of `{}::consts::{}` found. \
Consider using it directly",
module,
&name
),
);
return;
}
}
}
}
fn is_approx_const(constant: f64, value: &str, min_digits: usize) -> bool {
if value.len() <= min_digits {
false
} else {
let round_const = format!("{:.*}", value.len() - 2, constant);
let mut trunc_const = constant.to_string();
if trunc_const.len() > value.len() {
trunc_const.truncate(value.len());
}
(value == round_const) || (value == trunc_const)
}
}