rust-clippy/clippy_lints/src/methods/into_iter_on_ref.rs

use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::ty::has_iter_method;
use clippy_utils::{match_trait_method, paths};
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_lint::LateContext;
use rustc_middle::ty::{self, Ty};
use rustc_span::source_map::Span;
use rustc_span::symbol::{sym, Symbol};

use super::INTO_ITER_ON_REF;

pub(super) fn check(
    cx: &LateContext<'_>,
    expr: &hir::Expr<'_>,
    method_span: Span,
    method_name: Symbol,
    args: &[hir::Expr<'_>],
) {
    let self_ty = cx.typeck_results().expr_ty_adjusted(&args[0]);
    if_chain! {
        if let ty::Ref(..) = self_ty.kind();
        if method_name == sym::into_iter;
        if match_trait_method(cx, expr, &paths::INTO_ITERATOR);
        if let Some((kind, method_name)) = ty_has_iter_method(cx, self_ty);
        then {
            span_lint_and_sugg(
                cx,
                INTO_ITER_ON_REF,
                method_span,
                &format!(
                    "this `.into_iter()` call is equivalent to `.{}()` and will not consume the `{}`",
                    method_name, 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 mutbl = match self_ref_ty.kind() {
            ty::Ref(_, _, mutbl) => mutbl,
            _ => unreachable!(),
        };
        let method_name = match mutbl {
            hir::Mutability::Not => "iter",
            hir::Mutability::Mut => "iter_mut",
        };
        (ty_name, method_name)
    })
}