Constify Extend and FromIterator

library/core/src/iter/traits/collect.rs

@@ -1,4 +1,5 @@
 use super::TrustedLen;
+use crate::marker::Destruct;
 
 /// Conversion from an [`Iterator`].
 ///
@@ -131,7 +132,8 @@ use super::TrustedLen;
     label = "value of type `{Self}` cannot be built from `std::iter::Iterator<Item={A}>`"
 )]
 #[rustc_diagnostic_item = "FromIterator"]
-pub trait FromIterator<A>: Sized {
+#[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+pub const trait FromIterator<A>: Sized {
     /// Creates a value from an iterator.
     ///
     /// See the [module-level documentation] for more.
@@ -288,7 +290,7 @@ pub const trait IntoIterator {
 
     /// Which kind of iterator are we turning this into?
     #[stable(feature = "rust1", since = "1.0.0")]
-    type IntoIter: Iterator<Item = Self::Item>;
+    type IntoIter: [const] Iterator<Item = Self::Item>;
 
     /// Creates an iterator from a value.
     ///
@@ -393,8 +395,9 @@ const impl<I: [const] Iterator> IntoIterator for I {
 /// // we've added these elements onto the end
 /// assert_eq!("MyCollection([5, 6, 7, 1, 2, 3])", format!("{c:?}"));
 /// ```
+#[rustc_const_unstable(feature = "const_iter", issue = "92476")]
 #[stable(feature = "rust1", since = "1.0.0")]
-pub trait Extend<A> {
+pub const trait Extend<A> {
     /// Extends a collection with the contents of an iterator.
     ///
     /// As this is the only required method for this trait, the [trait-level] docs
@@ -413,11 +416,16 @@ pub trait Extend<A> {
     /// assert_eq!("abcdef", &message);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    fn extend<T: IntoIterator<Item = A>>(&mut self, iter: T);
+    fn extend<T: [const] IntoIterator<Item = A>>(&mut self, iter: T)
+    where
+        T::IntoIter: [const] Destruct;
 
     /// Extends a collection with exactly one element.
     #[unstable(feature = "extend_one", issue = "72631")]
-    fn extend_one(&mut self, item: A) {
+    fn extend_one(&mut self, item: A)
+    where
+        A: [const] Destruct,
+    {
         self.extend(Some(item));
     }
 
@@ -445,6 +453,7 @@ pub trait Extend<A> {
     unsafe fn extend_one_unchecked(&mut self, item: A)
     where
         Self: Sized,
+        A: [const] Destruct,
     {
         self.extend_one(item);
     }
@@ -540,41 +549,54 @@ where
 
 /// An implementation of [`extend`](Extend::extend) that calls `extend_one` or
 /// `extend_one_unchecked` for each element of the iterator.
-fn default_extend<ExtendT, I, T>(collection: &mut ExtendT, iter: I)
+
+#[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+const fn default_extend<ExtendT, I, T>(collection: &mut ExtendT, iter: I)
 where
-    ExtendT: Extend<T>,
-    I: IntoIterator<Item = T>,
+    ExtendT: [const] Extend<T>,
+    T: [const] Destruct,
+    I: [const] IntoIterator<Item = T>,
+    I::IntoIter: [const] Destruct,
 {
     // Specialize on `TrustedLen` and call `extend_one_unchecked` where
     // applicable.
-    trait SpecExtend<I> {
+    #[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+    const trait SpecExtend<I> {
         fn extend(&mut self, iter: I);
     }
 
     // Extracting these to separate functions avoid monomorphising the closures
     // for every iterator type.
-    fn extender<ExtendT, T>(collection: &mut ExtendT) -> impl FnMut(T) + use<'_, ExtendT, T>
+    #[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+    const fn extender<ExtendT, T>(
+        collection: &mut ExtendT,
+    ) -> impl [const] FnMut(T) + use<'_, ExtendT, T> + [const] Destruct
     where
-        ExtendT: Extend<T>,
+        ExtendT: [const] Extend<T>,
+        T: [const] Destruct,
     {
-        move |item| collection.extend_one(item)
+        const move |item| collection.extend_one(item)
     }
 
-    unsafe fn unchecked_extender<ExtendT, T>(
+    #[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+    const unsafe fn unchecked_extender<ExtendT, T>(
         collection: &mut ExtendT,
-    ) -> impl FnMut(T) + use<'_, ExtendT, T>
+    ) -> impl [const] FnMut(T) + [const] Destruct + use<'_, ExtendT, T>
     where
-        ExtendT: Extend<T>,
+        ExtendT: [const] Extend<T>,
+        T: [const] Destruct,
     {
         // SAFETY: we make sure that there is enough space at the callsite of
         // this function.
-        move |item| unsafe { collection.extend_one_unchecked(item) }
+        const move |item| unsafe { collection.extend_one_unchecked(item) }
     }
 
-    impl<ExtendT, I, T> SpecExtend<I> for ExtendT
+    #[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+    const impl<ExtendT, I, T> SpecExtend<I> for ExtendT
     where
-        ExtendT: Extend<T>,
-        I: Iterator<Item = T>,
+        ExtendT: [const] Extend<T>,
+        T: [const] Destruct,
+        I: [const] Iterator<Item = T> + [const] Destruct,
     {
         default fn extend(&mut self, iter: I) {
             let (lower_bound, _) = iter.size_hint();
@@ -586,10 +608,12 @@ where
         }
     }
 
-    impl<ExtendT, I, T> SpecExtend<I> for ExtendT
+    #[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+    const impl<ExtendT, I, T> SpecExtend<I> for ExtendT
     where
-        ExtendT: Extend<T>,
-        I: TrustedLen<Item = T>,
+        ExtendT: [const] Extend<T>,
+        T: [const] Destruct,
+        I: [const] TrustedLen<Item = T> + [const] Destruct,
     {
         fn extend(&mut self, iter: I) {
             let (lower_bound, upper_bound) = iter.size_hint();
@@ -616,11 +640,14 @@ macro_rules! impl_extend_tuple {
     ($(($ty:tt, $extend_ty:tt, $index:tt)),+) => {
         #[doc(hidden)]
         #[stable(feature = "extend_for_tuple", since = "1.56.0")]
-        impl<$($ty,)+ $($extend_ty,)+> Extend<($($ty,)+)> for ($($extend_ty,)+)
+        #[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+        const impl<$($ty,)+ $($extend_ty,)+> Extend<($($ty,)+)> for ($($extend_ty,)+)
         where
-            $($extend_ty: Extend<$ty>,)+
+            $($ty: [const] Destruct,)+
+            $($extend_ty: [const] Extend<$ty>,)+
         {
-            fn extend<T: IntoIterator<Item = ($($ty,)+)>>(&mut self, iter: T) {
+            fn extend<T: [const] IntoIterator<Item = ($($ty,)+)>>(&mut self, iter: T)
+            where T::IntoIter: [const] Destruct{
                 default_extend(self, iter)
             }
 

library/core/src/iter/traits/iterator.rs

@@ -873,15 +873,17 @@ pub const trait Iterator {
     /// ```
     #[inline]
     #[stable(feature = "iterator_for_each", since = "1.21.0")]
-    #[rustc_non_const_trait_method]
     fn for_each<F>(self, f: F)
     where
-        Self: Sized,
-        F: FnMut(Self::Item),
+        Self: Sized + [const] Destruct,
+        F: [const] FnMut(Self::Item) + [const] Destruct,
     {
         #[inline]
-        fn call<T>(mut f: impl FnMut(T)) -> impl FnMut((), T) {
-            move |(), item| f(item)
+        #[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+        const fn call<T>(
+            mut f: impl [const] FnMut(T) + [const] Destruct,
+        ) -> impl [const] FnMut((), T) + [const] Destruct {
+            const move |(), item| f(item)
         }
 
         self.fold((), call(f));