Auto merge of rust-lang#124097 - compiler-errors:box-into-iter, r=WaffleLapkin

Add `IntoIterator` for `Box<[T]>` + edition 2024-specific lints

* Adds a similar method probe opt-out mechanism to the `[T;N]: IntoIterator` implementation for edition 2021.
* Adjusts the relevant lints (shadowed `.into_iter()` calls, new source of method ambiguity).
* Adds some tests.
* Took the liberty to rework the logic in the `ARRAY_INTO_ITER` lint, since it was kind of confusing.

Based mostly off of rust-lang#116607.

ACP: rust-lang/libs-team#263
References rust-lang#59878
Tracking for Rust 2024: rust-lang#123759

Crater run was done here: rust-lang#116607 (comment)
Consensus afaict was that there is too much breakage, so let's do this in an edition-dependent way much like `[T; N]: IntoIterator`.

Author: bors

compiler/rustc_ast/src/ptr.rs

@@ -184,7 +184,7 @@ impl<'a, T> IntoIterator for &'a P<[T]> {
     type Item = &'a T;
     type IntoIter = slice::Iter<'a, T>;
     fn into_iter(self) -> Self::IntoIter {
-        self.ptr.into_iter()
+        self.ptr.iter()
     }
 }
 

compiler/rustc_feature/src/builtin_attrs.rs

@@ -899,10 +899,11 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         "the `#[rustc_main]` attribute is used internally to specify test entry point function",
     ),
     rustc_attr!(
-        rustc_skip_array_during_method_dispatch, Normal, template!(Word),
-        WarnFollowing, EncodeCrossCrate::No,
-        "the `#[rustc_skip_array_during_method_dispatch]` attribute is used to exclude a trait \
-        from method dispatch when the receiver is an array, for compatibility in editions < 2021."
+        rustc_skip_during_method_dispatch, Normal, template!(List: "array, boxed_slice"), WarnFollowing,
+        EncodeCrossCrate::No,
+        "the `#[rustc_skip_during_method_dispatch]` attribute is used to exclude a trait \
+        from method dispatch when the receiver is of the following type, for compatibility in \
+        editions < 2021 (array) or editions < 2024 (boxed_slice)."
     ),
     rustc_attr!(
         rustc_must_implement_one_of, Normal, template!(List: "function1, function2, ..."),

compiler/rustc_hir_analysis/src/collect.rs

@@ -1117,8 +1117,24 @@ fn trait_def(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::TraitDef {
 
     let is_marker = tcx.has_attr(def_id, sym::marker);
     let rustc_coinductive = tcx.has_attr(def_id, sym::rustc_coinductive);
-    let skip_array_during_method_dispatch =
-        tcx.has_attr(def_id, sym::rustc_skip_array_during_method_dispatch);
+
+    // FIXME: We could probably do way better attribute validation here.
+    let mut skip_array_during_method_dispatch = false;
+    let mut skip_boxed_slice_during_method_dispatch = false;
+    for attr in tcx.get_attrs(def_id, sym::rustc_skip_during_method_dispatch) {
+        if let Some(lst) = attr.meta_item_list() {
+            for item in lst {
+                if let Some(ident) = item.ident() {
+                    match ident.as_str() {
+                        "array" => skip_array_during_method_dispatch = true,
+                        "boxed_slice" => skip_boxed_slice_during_method_dispatch = true,
+                        _ => (),
+                    }
+                }
+            }
+        }
+    }
+
     let specialization_kind = if tcx.has_attr(def_id, sym::rustc_unsafe_specialization_marker) {
         ty::trait_def::TraitSpecializationKind::Marker
     } else if tcx.has_attr(def_id, sym::rustc_specialization_trait) {
@@ -1253,6 +1269,7 @@ fn trait_def(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::TraitDef {
         is_marker,
         is_coinductive: rustc_coinductive || is_auto,
         skip_array_during_method_dispatch,
+        skip_boxed_slice_during_method_dispatch,
         specialization_kind,
         must_implement_one_of,
         implement_via_object,

compiler/rustc_hir_typeck/src/method/mod.rs

@@ -3,7 +3,7 @@
 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/method-lookup.html
 
 mod confirm;
-mod prelude2021;
+mod prelude_edition_lints;
 pub mod probe;
 mod suggest;
 
@@ -186,7 +186,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let pick =
             self.lookup_probe(segment.ident, self_ty, call_expr, ProbeScope::TraitsInScope)?;
 
-        self.lint_dot_call_from_2018(self_ty, segment, span, call_expr, self_expr, &pick, args);
+        self.lint_edition_dependent_dot_call(
+            self_ty, segment, span, call_expr, self_expr, &pick, args,
+        );
 
         for &import_id in &pick.import_ids {
             debug!("used_trait_import: {:?}", import_id);

compiler/rustc_hir_typeck/src/method/prelude2021.rs renamed to compiler/rustc_hir_typeck/src/method/prelude_edition_lints.rs

@@ -1,12 +1,12 @@
-use crate::{
-    method::probe::{self, Pick},
-    FnCtxt,
-};
+use crate::method::probe::{self, Pick};
+use crate::FnCtxt;
+
 use hir::def_id::DefId;
 use hir::HirId;
 use hir::ItemKind;
 use rustc_errors::Applicability;
 use rustc_hir as hir;
+use rustc_lint::{ARRAY_INTO_ITER, BOXED_SLICE_INTO_ITER};
 use rustc_middle::span_bug;
 use rustc_middle::ty::{self, Ty};
 use rustc_session::lint::builtin::RUST_2021_PRELUDE_COLLISIONS;
@@ -17,7 +17,7 @@ use rustc_trait_selection::infer::InferCtxtExt;
 use std::fmt::Write;
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
-    pub(super) fn lint_dot_call_from_2018(
+    pub(super) fn lint_edition_dependent_dot_call(
         &self,
         self_ty: Ty<'tcx>,
         segment: &hir::PathSegment<'_>,
@@ -32,22 +32,32 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             segment.ident, self_ty, call_expr, self_expr
         );
 
-        // Rust 2021 and later is already using the new prelude
-        if span.at_least_rust_2021() {
-            return;
-        }
-
-        let prelude_or_array_lint = match segment.ident.name {
+        let (prelude_or_array_lint, edition) = match segment.ident.name {
             // `try_into` was added to the prelude in Rust 2021.
-            sym::try_into => RUST_2021_PRELUDE_COLLISIONS,
+            sym::try_into if !span.at_least_rust_2021() => (RUST_2021_PRELUDE_COLLISIONS, "2021"),
             // `into_iter` wasn't added to the prelude,
             // but `[T; N].into_iter()` doesn't resolve to IntoIterator::into_iter
             // before Rust 2021, which results in the same problem.
             // It is only a problem for arrays.
-            sym::into_iter if let ty::Array(..) = self_ty.kind() => {
-                // In this case, it wasn't really a prelude addition that was the problem.
-                // Instead, the problem is that the array-into_iter hack will no longer apply in Rust 2021.
-                rustc_lint::ARRAY_INTO_ITER
+            sym::into_iter => {
+                if let ty::Array(..) = self_ty.kind()
+                    && !span.at_least_rust_2021()
+                {
+                    // In this case, it wasn't really a prelude addition that was the problem.
+                    // Instead, the problem is that the array-into_iter hack will no longer
+                    // apply in Rust 2021.
+                    (ARRAY_INTO_ITER, "2021")
+                } else if self_ty.is_box()
+                    && self_ty.boxed_ty().is_slice()
+                    && !span.at_least_rust_2024()
+                {
+                    // In this case, it wasn't really a prelude addition that was the problem.
+                    // Instead, the problem is that the boxed-slice-into_iter hack will no
+                    // longer apply in Rust 2024.
+                    (BOXED_SLICE_INTO_ITER, "2024")
+                } else {
+                    return;
+                }
             }
             _ => return,
         };
@@ -81,7 +91,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 prelude_or_array_lint,
                 self_expr.hir_id,
                 self_expr.span,
-                format!("trait method `{}` will become ambiguous in Rust 2021", segment.ident.name),
+                format!(
+                    "trait method `{}` will become ambiguous in Rust {edition}",
+                    segment.ident.name
+                ),
                 |lint| {
                     let sp = self_expr.span;
 
@@ -131,7 +144,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 prelude_or_array_lint,
                 call_expr.hir_id,
                 call_expr.span,
-                format!("trait method `{}` will become ambiguous in Rust 2021", segment.ident.name),
+                format!(
+                    "trait method `{}` will become ambiguous in Rust {edition}",
+                    segment.ident.name
+                ),
                 |lint| {
                     let sp = call_expr.span;
                     let trait_name = self.trait_path_or_bare_name(

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -1444,6 +1444,18 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                                 return ProbeResult::NoMatch;
                             }
                         }
+
+                        // Some trait methods are excluded for boxed slices before 2024.
+                        // (`boxed_slice.into_iter()` wants a slice iterator for compatibility.)
+                        if self_ty.is_box()
+                            && self_ty.boxed_ty().is_slice()
+                            && !method_name.span.at_least_rust_2024()
+                        {
+                            let trait_def = self.tcx.trait_def(poly_trait_ref.def_id());
+                            if trait_def.skip_boxed_slice_during_method_dispatch {
+                                return ProbeResult::NoMatch;
+                            }
+                        }
                     }
 
                     let trait_ref = self.instantiate_binder_with_fresh_vars(

compiler/rustc_lint/messages.ftl

@@ -2,13 +2,6 @@ lint_ambiguous_wide_pointer_comparisons = ambiguous wide pointer comparison, the
     .addr_metadata_suggestion = use explicit `std::ptr::eq` method to compare metadata and addresses
     .addr_suggestion = use `std::ptr::addr_eq` or untyped pointers to only compare their addresses
 
-lint_array_into_iter =
-    this method call resolves to `<&{$target} as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to <{$target} as IntoIterator>::into_iter in Rust 2021
-    .use_iter_suggestion = use `.iter()` instead of `.into_iter()` to avoid ambiguity
-    .remove_into_iter_suggestion = or remove `.into_iter()` to iterate by value
-    .use_explicit_into_iter_suggestion =
-        or use `IntoIterator::into_iter(..)` instead of `.into_iter()` to explicitly iterate by value
-
 lint_async_fn_in_trait = use of `async fn` in public traits is discouraged as auto trait bounds cannot be specified
     .note = you can suppress this lint if you plan to use the trait only in your own code, or do not care about auto traits like `Send` on the `Future`
     .suggestion = you can alternatively desugar to a normal `fn` that returns `impl Future` and add any desired bounds such as `Send`, but these cannot be relaxed without a breaking API change
@@ -565,6 +558,13 @@ lint_renamed_lint = lint `{$name}` has been renamed to `{$replace}`
 
 lint_requested_level = requested on the command line with `{$level} {$lint_name}`
 
+lint_shadowed_into_iter =
+    this method call resolves to `<&{$target} as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to `<{$target} as IntoIterator>::into_iter` in Rust {$edition}
+    .use_iter_suggestion = use `.iter()` instead of `.into_iter()` to avoid ambiguity
+    .remove_into_iter_suggestion = or remove `.into_iter()` to iterate by value
+    .use_explicit_into_iter_suggestion =
+        or use `IntoIterator::into_iter(..)` instead of `.into_iter()` to explicitly iterate by value
+
 lint_span_use_eq_ctxt = use `.eq_ctxt()` instead of `.ctxt() == .ctxt()`
 
 lint_supertrait_as_deref_target = this `Deref` implementation is covered by an implicit supertrait coercion