clippy_lints/src/casts/cast_precision_loss.rs

use clippy_utils::diagnostics::span_lint;
use clippy_utils::ty::is_isize_or_usize;
use rustc_hir::Expr;
use rustc_lint::LateContext;
use rustc_middle::ty::{self, FloatTy, Ty};

use super::{CAST_PRECISION_LOSS, utils};

pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_from: Ty<'_>, cast_to: Ty<'_>) {
    let Some(from_nbits) = utils::int_ty_to_nbits(cx.tcx, cast_from) else {
        return;
    };

    // FIXME: handle `f16` and `f128`
    let to_nbits = match cast_to.kind() {
        ty::Float(f @ (FloatTy::F32 | FloatTy::F64)) => f.bit_width(),
        _ => return,
    };

    if !(is_isize_or_usize(cast_from) || from_nbits >= to_nbits) {
        return;
    }

    let cast_to_f64 = to_nbits == 64;
    let mantissa_nbits = if cast_to_f64 { 52 } else { 23 };

    let has_width = if is_isize_or_usize(cast_from) {
        "can be up to 64 bits wide depending on the target architecture".to_owned()
    } else {
        format!("is {from_nbits} bits wide")
    };

    span_lint(
        cx,
        CAST_PRECISION_LOSS,
        expr.span,
        format!(
            "casting `{cast_from}` to `{cast_to}` may cause a loss of precision \
            (`{cast_from}` {has_width}, \
             but `{cast_to}`'s mantissa is only {mantissa_nbits} bits wide)",
        ),
    );
}