compiler/rustc_lint/src/array_into_iter.rs

   1 use crate::{LateContext, LateLintPass, LintContext};
   2 use rustc_errors::Applicability;
   3 use rustc_hir as hir;
   4 use rustc_middle::ty;
   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;
   9 use rustc_span::Span;
  10
  11 declare_lint! {
  12     /// The `array_into_iter` lint detects calling `into_iter` on arrays.
  13     ///
  14     /// ### Example
  15     ///
  16     /// ```rust,edition2018
  17     /// # #![allow(unused)]
  18     /// [1, 2, 3].into_iter().for_each(|n| { *n; });
  19     /// ```
  20     ///
  21     /// {{produces}}
  22     ///
  23     /// ### Explanation
  24     ///
  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)`.
  31     pub ARRAY_INTO_ITER,
  32     Warn,
  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),
  37     };
  38 }
  39
  40 #[derive(Copy, Clone, Default)]
  41 pub struct ArrayIntoIter {
  42     for_expr_span: Span,
  43 }
  44
  45 impl_lint_pass!(ArrayIntoIter => [ARRAY_INTO_ITER]);
  46
  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,
  55                     _,
  56                 )) = &path.kind
  57                 {
  58                     self.for_expr_span = arg.span;
  59                 }
  60             }
  61         }
  62
  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 {
  66                 return;
  67             }
  68
  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) => {}
  74                 _ => return,
  75             };
  76
  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);
  81
  82             let target = match adjustments.last() {
  83                 Some(Adjustment { kind: Adjust::Borrow(_), target }) => target,
  84                 _ => return,
  85             };
  86
  87             let types =
  88                 std::iter::once(receiver_ty).chain(adjustments.iter().map(|adj| adj.target));
  89
  90             let mut found_array = false;
  91
  92             for ty in types {
  93                 match ty.kind() {
  94                     // If we run into a &[T; N] or &[T] first, there's nothing to warn about.
  95                     // It'll resolve to the reference version.
  96                     ty::Ref(_, inner_ty, _) if inner_ty.is_array() => return,
  97                     ty::Ref(_, inner_ty, _) if matches!(inner_ty.kind(), ty::Slice(..)) => return,
  98                     // Found an actual array type without matching a &[T; N] first.
  99                     // This is the problematic case.
 100                     ty::Array(..) => {
 101                         found_array = true;
 102                         break;
 103                     }
 104                     _ => {}
 105                 }
 106             }
 107
 108             if !found_array {
 109                 return;
 110             }
 111
 112             // Emit lint diagnostic.
 113             let target = match *target.kind() {
 114                 ty::Ref(_, inner_ty, _) if inner_ty.is_array() => "[T; N]",
 115                 ty::Ref(_, inner_ty, _) if matches!(inner_ty.kind(), ty::Slice(..)) => "[T]",
 116                 // We know the original first argument type is an array type,
 117                 // we know that the first adjustment was an autoref coercion
 118                 // and we know that `IntoIterator` is the trait involved. The
 119                 // array cannot be coerced to something other than a reference
 120                 // to an array or to a slice.
 121                 _ => bug!("array type coerced to something other than array or slice"),
 122             };
 123             cx.struct_span_lint(ARRAY_INTO_ITER, *span, |lint| {
 124                 let mut diag = lint.build(&format!(
 125                     "this method call resolves to `<&{} as IntoIterator>::into_iter` \
 126                     (due to backwards compatibility), \
 127                     but will resolve to <{} as IntoIterator>::into_iter in Rust 2021",
 128                     target, target,
 129                 ));
 130                 diag.span_suggestion(
 131                     call.ident.span,
 132                     "use `.iter()` instead of `.into_iter()` to avoid ambiguity",
 133                     "iter".into(),
 134                     Applicability::MachineApplicable,
 135                 );
 136                 if self.for_expr_span == expr.span {
 137                     diag.span_suggestion(
 138                         receiver_arg.span.shrink_to_hi().to(expr.span.shrink_to_hi()),
 139                         "or remove `.into_iter()` to iterate by value",
 140                         String::new(),
 141                         Applicability::MaybeIncorrect,
 142                     );
 143                 } else if receiver_ty.is_array() {
 144                     diag.multipart_suggestion(
 145                         "or use `IntoIterator::into_iter(..)` instead of `.into_iter()` to explicitly iterate by value",
 146                         vec![
 147                             (expr.span.shrink_to_lo(), "IntoIterator::into_iter(".into()),
 148                             (receiver_arg.span.shrink_to_hi().to(expr.span.shrink_to_hi()), ")".into()),
 149                         ],
 150                         Applicability::MaybeIncorrect,
 151                     );
 152                 }
 153                 diag.emit();
 154             })
 155         }
 156     }
 157 }