Have FromIterator receive an IntoIterator type.

Instead of receiving an IteratorBase<Item> type. This allows us to
eliminate IteratorBase and drop virtual on Iterator::next().

The downside is the template implementation of Option::from_iter()
and Result::from_iter() is much more complex, as you can't deduce
the type inside a concept you're receiving as a parameter.

Author: danakj

subdoc/lib/path.h

@@ -89,7 +89,7 @@ class RecordIter final
     return RecordIter(decl);
   }
 
-  sus::Option<std::string_view> next() noexcept final {
+  sus::Option<std::string_view> next() noexcept {
     if (next_decl_) {
       clang::RecordDecl* cur_decl = next_decl_;
 
@@ -134,7 +134,7 @@ class NamespaceIter final
     return NamespaceIter(decl);
   }
 
-  sus::Option<Namespace> next() noexcept final {
+  sus::Option<Namespace> next() noexcept {
     if (next_ndecl_) {
       clang::NamespaceDecl* cur_ndecl = next_ndecl_;
       next_ndecl_ =

subspace/CMakeLists.txt

@@ -51,12 +51,14 @@ target_sources(subspace PUBLIC
     "fn/fn_bind.h"
     "fn/fn_defn.h"
     "fn/fn_impl.h"
+    "iter/__private/into_iterator_archetype.h"
     "iter/__private/iterator_end.h"
     "iter/__private/iterator_loop.h"
     "iter/__private/step.h"
     "iter/boxed_iterator.h"
     "iter/filter.h"
     "iter/from_iterator.h"
+    "iter/into_iterator.h"
     "iter/iterator.h"
     "iter/iterator_concept.h"
     "iter/iterator_defn.h"

subspace/containers/__private/array_iter.h

@@ -40,7 +40,7 @@ struct ArrayIntoIter final
     return ArrayIntoIter(::sus::move(array));
   }
 
-  Option<Item> next() noexcept final {
+  Option<Item> next() noexcept {
     if (front_index_ == back_index_) [[unlikely]]
       return Option<Item>::none();
     // SAFETY: The front and back indicies are kept within the length of the

subspace/containers/__private/slice_iter.h

@@ -50,7 +50,7 @@ struct [[sus_trivial_abi]] SliceIter final
     return SliceIter(start, len);
   }
 
-  Option<Item> next() noexcept final {
+  Option<Item> next() noexcept {
     if (ptr_ == end_) [[unlikely]]
       return Option<Item>::none();
     // SAFETY: end_ is always > ptr_ when we get here (this was checked by the
@@ -121,7 +121,7 @@ struct [[sus_trivial_abi]] SliceIterMut final
   }
 
   // sus::iter::Iterator trait.
-  Option<Item> next() noexcept final {
+  Option<Item> next() noexcept {
     if (ptr_ == end_) [[unlikely]]
       return Option<Item>::none();
     // SAFETY: end_ is always > ptr_ when we get here (this was checked by the

subspace/containers/__private/vec_iter.h

@@ -41,7 +41,7 @@ struct VecIntoIter final
   }
 
   /// sus::iter::Iterator trait.
-  Option<Item> next() noexcept final {
+  Option<Item> next() noexcept {
     if (front_index_ == back_index_) [[unlikely]]
       return Option<Item>::none();
     // SAFETY: This class owns the Vec and does not expose it, so its length is

subspace/containers/vec.h

@@ -24,6 +24,7 @@
 #include "subspace/containers/__private/vec_marker.h"
 #include "subspace/containers/slice.h"
 #include "subspace/iter/from_iterator.h"
+#include "subspace/iter/into_iterator.h"
 #include "subspace/macros/compiler.h"
 #include "subspace/mem/move.h"
 #include "subspace/mem/relocate.h"
@@ -110,9 +111,10 @@ class Vec {
   /// Constructs a vector by taking all the elements from the iterator.
   ///
   /// sus::iter::FromIterator trait.
-  static constexpr Vec from_iter(::sus::iter::IteratorBase<T>&& iter) noexcept
+  static constexpr Vec from_iter(::sus::iter::IntoIterator<T> auto&& into_iter) noexcept
     requires(::sus::mem::Move<T> && !std::is_reference_v<T>)
   {
+    auto&& iter = sus::move(into_iter).into_iter();
     auto [lower, upper] = iter.size_hint();
     auto v = Vec::with_capacity(::sus::move(upper).unwrap_or(lower));
     for (T t : iter) v.push(::sus::move(t));

subspace/iter/__private/into_iterator_archetype.h

@@ -0,0 +1,43 @@
+// Copyright 2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#pragma once
+
+#include "subspace/iter/into_iterator.h"
+
+namespace sus::option {
+template <class T>
+class Option;
+}
+
+namespace sus::iter {
+template <class SubclassT, class ItemT>
+class IteratorImpl;
+}  // namespace sus::iter
+
+namespace sus::iter::__private {
+
+template <class T>
+struct IntoIteratorArchetype {
+  template <class Item>
+  struct Iter final : public IteratorImpl<Iter<Item>, Item> {
+    ::sus::option::Option<Item> next() noexcept {
+      return ::sus::option::Option<Item>::none();
+    }
+  };
+  Iter<T> into_iter() && { return Iter<T>(); }
+  static_assert(::sus::iter::IntoIterator<Iter<T>, T>);
+};
+
+}  // namespace sus::iter::__private

subspace/iter/boxed_iterator.h

@@ -22,8 +22,6 @@
 
 namespace sus::iter {
 
-template <class Item>
-class IteratorBase;
 template <class Iter, class Item>
 class IteratorImpl;
 
@@ -91,7 +89,7 @@ class [[nodiscard]] [[sus_trivial_abi]] BoxedIterator final
   }
 
   // sus::iter::Iterator trait.
-  Option<Item> next() noexcept final { return next_(iter_); }
+  Option<Item> next() noexcept { return next_(iter_); }
   // sus::iter::DoubleEndedIterator trait.
   Option<Item> next_back() noexcept
     requires(DoubleEnded)

subspace/iter/empty.h

@@ -29,7 +29,7 @@ class [[nodiscard]] [[sus_trivial_abi]] Empty final
   constexpr Empty() = default;
 
   // sus::iter::Iterator trait.
-  constexpr Option<Item> next() noexcept final {
+  constexpr Option<Item> next() noexcept {
     return sus::Option<Item>::none();
   }
   // sus::iter::DoubleEndedIterator trait.

subspace/iter/filter.h

@@ -40,7 +40,7 @@ class [[nodiscard]] [[sus_trivial_abi]] Filter final
   }
 
   // sus::iter::Iterator trait.
-  Option<Item> next() noexcept final {
+  Option<Item> next() noexcept {
     InnerSizedIter& iter = next_iter_;
     Pred& pred = pred_;