rust/clippy_lints/src/methods/iter_nth_zero.rs

use clippy_utils::consts::{constant, Constant};
use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::is_trait_method;
use clippy_utils::source::snippet_with_applicability;
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_lint::LateContext;
use rustc_span::sym;

use super::ITER_NTH_ZERO;

pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &hir::Expr<'_>, recv: &hir::Expr<'_>, arg: &hir::Expr<'_>) {
    if_chain! {
        if is_trait_method(cx, expr, sym::Iterator);
        if let Some((Constant::Int(0), _)) = constant(cx, cx.typeck_results(), arg);
        then {
            let mut applicability = Applicability::MachineApplicable;
            span_lint_and_sugg(
                cx,
                ITER_NTH_ZERO,
                expr.span,
                "called `.nth(0)` on a `std::iter::Iterator`, when `.next()` is equivalent",
                "try calling `.next()` instead of `.nth(0)`",
                format!("{}.next()", snippet_with_applicability(cx, recv.span, "..", &mut applicability)),
                applicability,
            );
        }
    }
}