rust/src/approx_const.rs

use rustc::lint::*;
use rustc_front::hir::*;
use std::f64::consts as f64;
use utils::span_lint;
use syntax::ast::Lit_::*;
use syntax::ast::Lit;
use syntax::ast::FloatTy::*;

declare_lint! {
    pub APPROX_CONSTANT,
    Warn,
    "the approximate of a known float constant (in `std::f64::consts` or `std::f32::consts`) \
     is found; suggests to use the constant"
}

// Tuples are of the form (constant, name, min_digits)
const KNOWN_CONSTS : &'static [(f64, &'static 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 ApproxConstant;

impl LintPass for ApproxConstant {
    fn get_lints(&self) -> LintArray {
        lint_array!(APPROX_CONSTANT)
    }
}

impl LateLintPass for ApproxConstant {
    fn check_expr(&mut self, cx: &LateContext, e: &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 {
        LitFloat(ref s, TyF32) => check_known_consts(cx, e, s, "f32"),
        LitFloat(ref s, TyF64) => check_known_consts(cx, e, s, "f64"),
        LitFloatUnsuffixed(ref s) =>
            check_known_consts(cx, e, s, "f{32, 64}"),
        _ => ()
    }
}

fn check_known_consts(cx: &LateContext, e: &Expr, s: &str, module: &str) {
    if let Ok(_) = s.parse::<f64>() {
        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 `{}::{}` found. \
                    Consider using it directly", module, &name));
                return;
            }
        }
    }
}

/// 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`.
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)
    }
}