clippy_lints/src/methods/iter_nth_zero.rs

use clippy_utils::consts::{ConstEvalCtxt, Constant};
use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::is_lang_item_or_ctor;
use clippy_utils::res::{MaybeDef, MaybeTypeckRes};
use clippy_utils::source::snippet_with_applicability;
use hir::{LangItem, OwnerNode};
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(cx: &LateContext<'_>, expr: &hir::Expr<'_>, recv: &hir::Expr<'_>, arg: &hir::Expr<'_>) {
    if let OwnerNode::Item(item) = cx.tcx.hir_owner_node(cx.tcx.hir_get_parent_item(expr.hir_id))
        && let def_id = item.owner_id.to_def_id()
        && cx.ty_based_def(expr).opt_parent(cx).is_diag_item(cx, sym::Iterator)
        && let Some(Constant::Int(0)) = ConstEvalCtxt::new(cx).eval_local(arg, expr.span.ctxt())
        && !is_lang_item_or_ctor(cx, def_id, LangItem::IteratorNext)
    {
        let mut app = 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 app)),
            app,
        );
    }
}