1 use crate::{LateContext, LateLintPass, LintContext};
2 use rustc_errors::Applicability;
5 use rustc_middle::ty::adjustment::{Adjust, Adjustment};
6 use rustc_session::lint::FutureIncompatibilityReason;
7 use rustc_span::edition::Edition;
8 use rustc_span::symbol::sym;
12 /// The `array_into_iter` lint detects calling `into_iter` on arrays.
16 /// ```rust,edition2018
17 /// # #![allow(unused)]
18 /// [1, 2, 3].into_iter().for_each(|n| { *n; });
25 /// Since Rust 1.53, arrays implement `IntoIterator`. However, to avoid
26 /// breakage, `array.into_iter()` in Rust 2015 and 2018 code will still
27 /// behave as `(&array).into_iter()`, returning an iterator over
28 /// references, just like in Rust 1.52 and earlier.
29 /// This only applies to the method call syntax `array.into_iter()`, not to
30 /// any other syntax such as `for _ in array` or `IntoIterator::into_iter(array)`.
33 "detects calling `into_iter` on arrays in Rust 2015 and 2018",
34 @future_incompatible = FutureIncompatibleInfo {
35 reference: "<https://doc.rust-lang.org/nightly/edition-guide/rust-2021/IntoIterator-for-arrays.html>",
36 reason: FutureIncompatibilityReason::EditionSemanticsChange(Edition::Edition2021),
40 #[derive(Copy, Clone, Default)]
41 pub struct ArrayIntoIter {
45 impl_lint_pass!(ArrayIntoIter => [ARRAY_INTO_ITER]);
47 impl<'tcx> LateLintPass<'tcx> for ArrayIntoIter {
48 fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
49 // Save the span of expressions in `for _ in expr` syntax,
50 // so we can give a better suggestion for those later.
51 if let hir::ExprKind::Match(arg, [_], hir::MatchSource::ForLoopDesugar) = &expr.kind {
52 if let hir::ExprKind::Call(path, [arg]) = &arg.kind {
53 if let hir::ExprKind::Path(hir::QPath::LangItem(
54 hir::LangItem::IntoIterIntoIter,
58 self.for_expr_span = arg.span;
63 // We only care about method call expressions.
64 if let hir::ExprKind::MethodCall(call, span, args, _) = &expr.kind {
65 if call.ident.name != sym::into_iter {
69 // Check if the method call actually calls the libcore
70 // `IntoIterator::into_iter`.
71 let def_id = cx.typeck_results().type_dependent_def_id(expr.hir_id).unwrap();
72 match cx.tcx.trait_of_item(def_id) {
73 Some(trait_id) if cx.tcx.is_diagnostic_item(sym::IntoIterator, trait_id) => {}
77 // As this is a method call expression, we have at least one argument.
78 let receiver_arg = &args[0];
79 let receiver_ty = cx.typeck_results().expr_ty(receiver_arg);
80 let adjustments = cx.typeck_results().expr_adjustments(receiver_arg);
82 let Some(Adjustment { kind: Adjust::Borrow(_), target }) = adjustments.last() else {
87 std::iter::once(receiver_ty).chain(adjustments.iter().map(|adj| adj.target));
89 let mut found_array = false;
93 // If we run into a &[T; N] or &[T] first, there's nothing to warn about.
94 // It'll resolve to the reference version.
95 ty::Ref(_, inner_ty, _) if inner_ty.is_array() => return,
96 ty::Ref(_, inner_ty, _) if matches!(inner_ty.kind(), ty::Slice(..)) => return,
97 // Found an actual array type without matching a &[T; N] first.
98 // This is the problematic case.
111 // Emit lint diagnostic.
112 let target = match *target.kind() {
113 ty::Ref(_, inner_ty, _) if inner_ty.is_array() => "[T; N]",
114 ty::Ref(_, inner_ty, _) if matches!(inner_ty.kind(), ty::Slice(..)) => "[T]",
115 // We know the original first argument type is an array type,
116 // we know that the first adjustment was an autoref coercion
117 // and we know that `IntoIterator` is the trait involved. The
118 // array cannot be coerced to something other than a reference
119 // to an array or to a slice.
120 _ => bug!("array type coerced to something other than array or slice"),
122 cx.struct_span_lint(ARRAY_INTO_ITER, *span, |lint| {
123 let mut diag = lint.build(&format!(
124 "this method call resolves to `<&{} as IntoIterator>::into_iter` \
125 (due to backwards compatibility), \
126 but will resolve to <{} as IntoIterator>::into_iter in Rust 2021",
129 diag.span_suggestion(
131 "use `.iter()` instead of `.into_iter()` to avoid ambiguity",
133 Applicability::MachineApplicable,
135 if self.for_expr_span == expr.span {
136 diag.span_suggestion(
137 receiver_arg.span.shrink_to_hi().to(expr.span.shrink_to_hi()),
138 "or remove `.into_iter()` to iterate by value",
140 Applicability::MaybeIncorrect,
142 } else if receiver_ty.is_array() {
143 diag.multipart_suggestion(
144 "or use `IntoIterator::into_iter(..)` instead of `.into_iter()` to explicitly iterate by value",
146 (expr.span.shrink_to_lo(), "IntoIterator::into_iter(".into()),
147 (receiver_arg.span.shrink_to_hi().to(expr.span.shrink_to_hi()), ")".into()),
149 Applicability::MaybeIncorrect,