clippy_lints/src/methods/into_iter_on_ref.rs

use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::res::{MaybeDef, MaybeTypeckRes};
use clippy_utils::ty::has_iter_method;
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_lint::LateContext;
use rustc_middle::ty::{self, Ty};
use rustc_span::Span;
use rustc_span::symbol::{Symbol, sym};

use super::INTO_ITER_ON_REF;

pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, method_span: Span, receiver: &hir::Expr<'_>) {
    let self_ty = cx.typeck_results().expr_ty_adjusted(receiver);
    if let ty::Ref(..) = self_ty.kind()
        && cx.ty_based_def(expr).opt_parent(cx).is_diag_item(cx, sym::IntoIterator)
        && let Some((kind, method_name)) = ty_has_iter_method(cx, self_ty)
    {
        span_lint_and_sugg(
            cx,
            INTO_ITER_ON_REF,
            method_span,
            format!("this `.into_iter()` call is equivalent to `.{method_name}()` and will not consume the `{kind}`",),
            "call directly",
            method_name.to_string(),
            Applicability::MachineApplicable,
        );
    }
}

fn ty_has_iter_method(cx: &LateContext<'_>, self_ref_ty: Ty<'_>) -> Option<(Symbol, &'static str)> {
    has_iter_method(cx, self_ref_ty).map(|ty_name| {
        let ty::Ref(_, _, mutbl) = self_ref_ty.kind() else {
            unreachable!()
        };
        let method_name = match mutbl {
            hir::Mutability::Not => "iter",
            hir::Mutability::Mut => "iter_mut",
        };
        (ty_name, method_name)
    })
}