clippy_lints/src/methods/suspicious_splitn.rs

use clippy_utils::diagnostics::span_lint_and_note;
use rustc_ast::LitKind;
use rustc_hir::{Expr, ExprKind};
use rustc_lint::LateContext;
use rustc_span::{Spanned, Symbol};

use super::SUSPICIOUS_SPLITN;

pub(super) fn check(cx: &LateContext<'_>, method_name: Symbol, expr: &Expr<'_>, self_arg: &Expr<'_>, count: u128) {
    if count <= 1
        && let Some(call_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id)
        && let Some(impl_id) = cx.tcx.inherent_impl_of_assoc(call_id)
        && let self_ty = cx.tcx.type_of(impl_id).instantiate_identity()
        && (self_ty.is_slice() || self_ty.is_str())
    {
        // Ignore empty slice and string literals when used with a literal count.
        if matches!(self_arg.kind, ExprKind::Array([]))
            || matches!(self_arg.kind, ExprKind::Lit(Spanned { node: LitKind::Str(s, _), .. }) if s.is_empty())
        {
            return;
        }

        let (msg, note_msg) = if count == 0 {
            (
                format!("`{method_name}` called with `0` splits"),
                "the resulting iterator will always return `None`",
            )
        } else {
            (
                format!("`{method_name}` called with `1` split"),
                if self_ty.is_slice() {
                    "the resulting iterator will always return the entire slice followed by `None`"
                } else {
                    "the resulting iterator will always return the entire string followed by `None`"
                },
            )
        };

        span_lint_and_note(cx, SUSPICIOUS_SPLITN, expr.span, msg, None, note_msg);
    }
}