refactor(core): replace MyIterator with standard Iterator

[rust-lang/rust#106541][1] added `#[const_trait]` to `Iterator` as well
as internal attribute to bypass the constness check for other methods
than `Iterator::next`. As a result, user code can now implement `const
Iterator` in a normal way, i.e., just by implementing `Iterator::next`.

[1]: rust-lang/rust#106541

Author: yvt

doc/toolchain_limitations.md

@@ -106,7 +106,7 @@ type Alias<T> = Struct<{<T as Trait>::N}>;
 
 ### `[tag:const_for]` `for` loops are unusable in `const fn`
 
-Technically it's available under the compiler feature `const_for`, but the lack of essential trait implementations (e.g., `[ref:range_const_iterator]`, `[ref:const_slice_iter]`) and above all the inability of implementing `const Iterator` (`[ref:iterator_const_default]`) make it unusable.
+Technically it's available under the compiler feature `const_for`, but the lack of essential trait implementations (e.g., `[ref:range_const_iterator]`, `[ref:const_slice_iter]`) makes it unusable in many cases.
 
 
 ### `[tag:const_static_item_ref]` `const`s and `const fn`s can't refer to `static`s
@@ -449,25 +449,6 @@ const _: () = assert!(matches!((2..4).next(), Some(2)));
 ```
 
 
-### `[tag:iterator_const_default]` `Iterator` lack `#[const_trait]`
-
-```rust,compile_fail
-#![feature(const_trait_impl)]
-#![feature(const_mut_refs)]
-
-struct MyIterator;
-
-// error: const `impl` for trait `Iterator` which is not marked with `#[const_trait]`
-impl const Iterator for MyIterator {
-    type Item = u32;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        Some(42)
-    }
-}
-```
-
-
 ### `[tag:const_assert_eq]` `assert_eq!` and similar macros are unusable in `const fn`
 
 ```rust,compile_fail,E0015

src/r3_core/src/bind/sorter.rs

@@ -318,7 +318,7 @@ pub(super) const fn sort_bindings<Callback, SorterUseInfoList, VertexList>(
         End,
     }
 
-    impl const MyIterator for VertexIter<'_> {
+    impl const Iterator for VertexIter<'_> {
         type Item = Vertex;
 
         fn next(&mut self) -> Option<Self::Item> {
@@ -369,7 +369,7 @@ pub(super) const fn sort_bindings<Callback, SorterUseInfoList, VertexList>(
         End,
     }
 
-    impl<Callback> const MyIterator for SuccessorIter<'_, Callback>
+    impl<Callback> const Iterator for SuccessorIter<'_, Callback>
     where
         Callback: ~const SorterCallback,
     {
@@ -533,23 +533,14 @@ pub(super) const fn sort_bindings<Callback, SorterUseInfoList, VertexList>(
 // Helper traits
 // --------------------------------------------------------------------------
 
-// `const Iterator` is currently very hard to implement
-// [ref:iterator_const_default]
-/// An [`Iterator`][] usable in `const fn`.
-#[const_trait]
-trait MyIterator {
-    type Item;
-    fn next(&mut self) -> Option<Self::Item>;
-}
-
 #[const_trait]
 trait GraphAccess<VertexRef> {
-    type VertexIter<'a>: ~const MyIterator<Item = VertexRef> + ~const Destruct + 'a
+    type VertexIter<'a>: ~const Iterator<Item = VertexRef> + ~const Destruct + 'a
     where
         Self: 'a;
     fn vertices(&self) -> Self::VertexIter<'_>;
 
-    type SuccessorIter<'a>: ~const MyIterator<Item = VertexRef> + ~const Destruct + 'a
+    type SuccessorIter<'a>: ~const Iterator<Item = VertexRef> + ~const Destruct + 'a
     where
         Self: 'a;
     fn successors(&self, v: &VertexRef) -> Self::SuccessorIter<'_>;