Add Box<[T; N]>: IntoIterator without any method dispatch hacks

library/alloc/src/boxed.rs

@@ -165,6 +165,7 @@ use core::ops::{
 };
 use core::pin::Pin;
 use core::ptr::{self, addr_of_mut, NonNull, Unique};
+use core::slice;
 use core::task::{Context, Poll};
 
 #[cfg(not(no_global_oom_handling))]
@@ -177,6 +178,7 @@ use crate::raw_vec::RawVec;
 use crate::str::from_boxed_utf8_unchecked;
 #[cfg(not(no_global_oom_handling))]
 use crate::string::String;
+use crate::vec;
 #[cfg(not(no_global_oom_handling))]
 use crate::vec::Vec;
 
@@ -2080,6 +2082,51 @@ impl<I> FromIterator<I> for Box<[I]> {
     }
 }
 
+/// This implementation is required to make sure that the `Box<[I; N]>: IntoIterator`
+/// implementation doesn't overlap with `IntoIterator for T where T: Iterator` blanket.
+#[stable(feature = "boxed_slice_into_iter", since = "CURRENT_RUSTC_VERSION")]
+impl<I, const N: usize, A: Allocator> !Iterator for Box<[I; N], A> {}
+
+/// This implementation is required to make sure that the `&Box<[I; N]>: IntoIterator`
+/// implementation doesn't overlap with `IntoIterator for T where T: Iterator` blanket.
+#[stable(feature = "boxed_slice_into_iter", since = "CURRENT_RUSTC_VERSION")]
+impl<'a, const N: usize, I, A: Allocator> !Iterator for &'a Box<[I; N], A> {}
+
+/// This implementation is required to make sure that the `&mut Box<[I; N]>: IntoIterator`
+/// implementation doesn't overlap with `IntoIterator for T where T: Iterator` blanket.
+#[stable(feature = "boxed_slice_into_iter", since = "CURRENT_RUSTC_VERSION")]
+impl<'a, const N: usize, I, A: Allocator> !Iterator for &'a mut Box<[I; N], A> {}
+
+// Note: the `#[rustc_skip_during_method_dispatch(boxed_slice)]` on `trait IntoIterator`
+// hides this implementation from explicit `.into_iter()` calls on editions < 2024,
+// so those calls will still resolve to the slice implementation, by reference.
+#[stable(feature = "boxed_slice_into_iter", since = "CURRENT_RUSTC_VERSION")]
+impl<I, const N: usize, A: Allocator> IntoIterator for Box<[I; N], A> {
+    type IntoIter = vec::IntoIter<I, A>;
+    type Item = I;
+    fn into_iter(self) -> vec::IntoIter<I, A> {
+        (self as Box<[I], A>).into_vec().into_iter()
+    }
+}
+
+#[stable(feature = "boxed_slice_into_iter", since = "CURRENT_RUSTC_VERSION")]
+impl<'a, I, const N: usize, A: Allocator> IntoIterator for &'a Box<[I; N], A> {
+    type IntoIter = slice::Iter<'a, I>;
+    type Item = &'a I;
+    fn into_iter(self) -> slice::Iter<'a, I> {
+        self.iter()
+    }
+}
+
+#[stable(feature = "boxed_slice_into_iter", since = "CURRENT_RUSTC_VERSION")]
+impl<'a, I, const N: usize, A: Allocator> IntoIterator for &'a mut Box<[I; N], A> {
+    type IntoIter = slice::IterMut<'a, I>;
+    type Item = &'a mut I;
+    fn into_iter(self) -> slice::IterMut<'a, I> {
+        self.iter_mut()
+    }
+}
+
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {

library/core/src/slice/iter.rs

@@ -16,6 +16,9 @@ use crate::ptr::{self, without_provenance, without_provenance_mut, NonNull};
 
 use super::{from_raw_parts, from_raw_parts_mut};
 
+#[stable(feature = "boxed_slice_into_iter", since = "CURRENT_RUSTC_VERSION")]
+impl<T, const N: usize> !Iterator for [T; N] {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, T> IntoIterator for &'a [T] {
     type Item = &'a T;

tests/ui/iterators/into-iter-on-arrays-2018.rs

@@ -5,6 +5,7 @@ use std::array::IntoIter;
 use std::ops::Deref;
 use std::rc::Rc;
 use std::slice::Iter;
+use std::vec;
 
 fn main() {
     let array = [0; 10];
@@ -15,10 +16,6 @@ fn main() {
     //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
     //~| WARNING this changes meaning
 
-    let _: Iter<'_, i32> = Box::new(array).into_iter();
-    //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
-    //~| WARNING this changes meaning
-
     let _: Iter<'_, i32> = Rc::new(array).into_iter();
     //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
     //~| WARNING this changes meaning

tests/ui/iterators/into-iter-on-arrays-2018.stderr

@@ -1,5 +1,5 @@
 warning: this method call resolves to `<&[T; N] as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to <[T; N] as IntoIterator>::into_iter in Rust 2021
-  --> $DIR/into-iter-on-arrays-2018.rs:14:34
+  --> $DIR/into-iter-on-arrays-2018.rs:15:34
    |
 LL |     let _: Iter<'_, i32> = array.into_iter();
    |                                  ^^^^^^^^^
@@ -17,16 +17,7 @@ LL |     let _: Iter<'_, i32> = IntoIterator::into_iter(array);
    |                            ++++++++++++++++++++++++     ~
 
 warning: this method call resolves to `<&[T; N] as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to <[T; N] as IntoIterator>::into_iter in Rust 2021
-  --> $DIR/into-iter-on-arrays-2018.rs:18:44
-   |
-LL |     let _: Iter<'_, i32> = Box::new(array).into_iter();
-   |                                            ^^^^^^^^^ help: use `.iter()` instead of `.into_iter()` to avoid ambiguity: `iter`
-   |
-   = warning: this changes meaning in Rust 2021
-   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/IntoIterator-for-arrays.html>
-
-warning: this method call resolves to `<&[T; N] as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to <[T; N] as IntoIterator>::into_iter in Rust 2021
-  --> $DIR/into-iter-on-arrays-2018.rs:22:43
+  --> $DIR/into-iter-on-arrays-2018.rs:19:43
    |
 LL |     let _: Iter<'_, i32> = Rc::new(array).into_iter();
    |                                           ^^^^^^^^^ help: use `.iter()` instead of `.into_iter()` to avoid ambiguity: `iter`
@@ -35,7 +26,7 @@ LL |     let _: Iter<'_, i32> = Rc::new(array).into_iter();
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/IntoIterator-for-arrays.html>
 
 warning: this method call resolves to `<&[T; N] as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to <[T; N] as IntoIterator>::into_iter in Rust 2021
-  --> $DIR/into-iter-on-arrays-2018.rs:25:41
+  --> $DIR/into-iter-on-arrays-2018.rs:22:41
    |
 LL |     let _: Iter<'_, i32> = Array(array).into_iter();
    |                                         ^^^^^^^^^ help: use `.iter()` instead of `.into_iter()` to avoid ambiguity: `iter`
@@ -44,7 +35,7 @@ LL |     let _: Iter<'_, i32> = Array(array).into_iter();
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/IntoIterator-for-arrays.html>
 
 warning: this method call resolves to `<&[T; N] as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to <[T; N] as IntoIterator>::into_iter in Rust 2021
-  --> $DIR/into-iter-on-arrays-2018.rs:32:24
+  --> $DIR/into-iter-on-arrays-2018.rs:29:24
    |
 LL |     for _ in [1, 2, 3].into_iter() {}
    |                        ^^^^^^^^^
@@ -61,5 +52,5 @@ LL -     for _ in [1, 2, 3].into_iter() {}
 LL +     for _ in [1, 2, 3] {}
    |
 
-warning: 5 warnings emitted
+warning: 4 warnings emitted
 

tests/ui/iterators/into-iter-on-arrays-2021.rs

@@ -4,13 +4,15 @@
 use std::array::IntoIter;
 use std::ops::Deref;
 use std::rc::Rc;
+use std::vec;
 
 fn main() {
     let array = [0; 10];
 
     // In 2021, the method dispatches to `IntoIterator for [T; N]`.
     let _: IntoIter<i32, 10> = array.into_iter();
-    let _: IntoIter<i32, 10> = Box::new(array).into_iter();
+    // and for `Box<[T; N]>`, we use `vec::IntoIter`, since #124108.
+    let _: vec::IntoIter<i32> = Box::new(array).into_iter();
 
     // The `array_into_iter` lint doesn't cover other wrappers that deref to an array.
     let _: IntoIter<i32, 10> = Rc::new(array).into_iter();

tests/ui/iterators/into-iter-on-arrays-lint.fixed

@@ -21,32 +21,6 @@ fn main() {
     //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
     //~| WARNING this changes meaning
 
-    Box::new(small).iter();
-    //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
-    //~| WARNING this changes meaning
-    Box::new([1, 2]).iter();
-    //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
-    //~| WARNING this changes meaning
-    Box::new(big).iter();
-    //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
-    //~| WARNING this changes meaning
-    Box::new([0u8; 33]).iter();
-    //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
-    //~| WARNING this changes meaning
-
-    Box::new(Box::new(small)).iter();
-    //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
-    //~| WARNING this changes meaning
-    Box::new(Box::new([1, 2])).iter();
-    //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
-    //~| WARNING this changes meaning
-    Box::new(Box::new(big)).iter();
-    //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
-    //~| WARNING this changes meaning
-    Box::new(Box::new([0u8; 33])).iter();
-    //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
-    //~| WARNING this changes meaning
-
     // Expressions that should not
     (&[1, 2]).into_iter();
     (&small).into_iter();

tests/ui/iterators/into-iter-on-arrays-lint.rs

@@ -21,32 +21,6 @@ fn main() {
     //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
     //~| WARNING this changes meaning
 
-    Box::new(small).into_iter();
-    //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
-    //~| WARNING this changes meaning
-    Box::new([1, 2]).into_iter();
-    //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
-    //~| WARNING this changes meaning
-    Box::new(big).into_iter();
-    //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
-    //~| WARNING this changes meaning
-    Box::new([0u8; 33]).into_iter();
-    //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
-    //~| WARNING this changes meaning
-
-    Box::new(Box::new(small)).into_iter();
-    //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
-    //~| WARNING this changes meaning
-    Box::new(Box::new([1, 2])).into_iter();
-    //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
-    //~| WARNING this changes meaning
-    Box::new(Box::new(big)).into_iter();
-    //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
-    //~| WARNING this changes meaning
-    Box::new(Box::new([0u8; 33])).into_iter();
-    //~^ WARNING this method call resolves to `<&[T; N] as IntoIterator>::into_iter`
-    //~| WARNING this changes meaning
-
     // Expressions that should not
     (&[1, 2]).into_iter();
     (&small).into_iter();

tests/ui/iterators/into-iter-on-arrays-lint.stderr

@@ -67,77 +67,5 @@ help: or use `IntoIterator::into_iter(..)` instead of `.into_iter()` to explicit
 LL |     IntoIterator::into_iter([0u8; 33]);
    |     ++++++++++++++++++++++++         ~
 
-warning: this method call resolves to `<&[T; N] as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to <[T; N] as IntoIterator>::into_iter in Rust 2021
-  --> $DIR/into-iter-on-arrays-lint.rs:24:21
-   |
-LL |     Box::new(small).into_iter();
-   |                     ^^^^^^^^^ help: use `.iter()` instead of `.into_iter()` to avoid ambiguity: `iter`
-   |
-   = warning: this changes meaning in Rust 2021
-   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/IntoIterator-for-arrays.html>
-
-warning: this method call resolves to `<&[T; N] as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to <[T; N] as IntoIterator>::into_iter in Rust 2021
-  --> $DIR/into-iter-on-arrays-lint.rs:27:22
-   |
-LL |     Box::new([1, 2]).into_iter();
-   |                      ^^^^^^^^^ help: use `.iter()` instead of `.into_iter()` to avoid ambiguity: `iter`
-   |
-   = warning: this changes meaning in Rust 2021
-   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/IntoIterator-for-arrays.html>
-
-warning: this method call resolves to `<&[T; N] as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to <[T; N] as IntoIterator>::into_iter in Rust 2021
-  --> $DIR/into-iter-on-arrays-lint.rs:30:19
-   |
-LL |     Box::new(big).into_iter();
-   |                   ^^^^^^^^^ help: use `.iter()` instead of `.into_iter()` to avoid ambiguity: `iter`
-   |
-   = warning: this changes meaning in Rust 2021
-   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/IntoIterator-for-arrays.html>
-
-warning: this method call resolves to `<&[T; N] as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to <[T; N] as IntoIterator>::into_iter in Rust 2021
-  --> $DIR/into-iter-on-arrays-lint.rs:33:25
-   |
-LL |     Box::new([0u8; 33]).into_iter();
-   |                         ^^^^^^^^^ help: use `.iter()` instead of `.into_iter()` to avoid ambiguity: `iter`
-   |
-   = warning: this changes meaning in Rust 2021
-   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/IntoIterator-for-arrays.html>
-
-warning: this method call resolves to `<&[T; N] as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to <[T; N] as IntoIterator>::into_iter in Rust 2021
-  --> $DIR/into-iter-on-arrays-lint.rs:37:31
-   |
-LL |     Box::new(Box::new(small)).into_iter();
-   |                               ^^^^^^^^^ help: use `.iter()` instead of `.into_iter()` to avoid ambiguity: `iter`
-   |
-   = warning: this changes meaning in Rust 2021
-   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/IntoIterator-for-arrays.html>
-
-warning: this method call resolves to `<&[T; N] as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to <[T; N] as IntoIterator>::into_iter in Rust 2021
-  --> $DIR/into-iter-on-arrays-lint.rs:40:32
-   |
-LL |     Box::new(Box::new([1, 2])).into_iter();
-   |                                ^^^^^^^^^ help: use `.iter()` instead of `.into_iter()` to avoid ambiguity: `iter`
-   |
-   = warning: this changes meaning in Rust 2021
-   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/IntoIterator-for-arrays.html>
-
-warning: this method call resolves to `<&[T; N] as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to <[T; N] as IntoIterator>::into_iter in Rust 2021
-  --> $DIR/into-iter-on-arrays-lint.rs:43:29
-   |
-LL |     Box::new(Box::new(big)).into_iter();
-   |                             ^^^^^^^^^ help: use `.iter()` instead of `.into_iter()` to avoid ambiguity: `iter`
-   |
-   = warning: this changes meaning in Rust 2021
-   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/IntoIterator-for-arrays.html>
-
-warning: this method call resolves to `<&[T; N] as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to <[T; N] as IntoIterator>::into_iter in Rust 2021
-  --> $DIR/into-iter-on-arrays-lint.rs:46:35
-   |
-LL |     Box::new(Box::new([0u8; 33])).into_iter();
-   |                                   ^^^^^^^^^ help: use `.iter()` instead of `.into_iter()` to avoid ambiguity: `iter`
-   |
-   = warning: this changes meaning in Rust 2021
-   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/IntoIterator-for-arrays.html>
-
-warning: 12 warnings emitted
+warning: 4 warnings emitted
 

tests/ui/traits/method-on-unbounded-type-param.stderr

@@ -76,8 +76,8 @@ LL |     x.cmp(&x);
            which is required by `&mut dyn T: Iterator`
    = help: items from traits can only be used if the trait is implemented and in scope
    = note: the following traits define an item `cmp`, perhaps you need to implement one of them:
-           candidate #1: `Iterator`
-           candidate #2: `Ord`
+           candidate #1: `Ord`
+   = note: the trait `Iterator` defines an item `cmp`, but is explicitly unimplemented
 
 error: aborting due to 4 previous errors