Rollup merge of #154729 - Lars-Schumann:de-non_const-iterator, r=oli-obk

de-non_const some `Iterator` methods

#92476

r? oli-obk

this is essentially just #153708 again (which I accidentally closed...), but with the changes to `advance_by` omitted, since that seems to have caused the ICE.

Author: jhpratt

library/core/src/array/mod.rs

@@ -1005,9 +1005,10 @@ impl<T: [const] Destruct> const Drop for Guard<'_, T> {
 /// dropped.
 ///
 /// Used for [`Iterator::next_chunk`].
+#[rustc_const_unstable(feature = "const_iter", issue = "92476")]
 #[inline]
-pub(crate) fn iter_next_chunk<T, const N: usize>(
-    iter: &mut impl Iterator<Item = T>,
+pub(crate) const fn iter_next_chunk<T, const N: usize>(
+    iter: &mut impl [const] Iterator<Item = T>,
 ) -> Result<[T; N], IntoIter<T, N>> {
     let mut array = [const { MaybeUninit::uninit() }; N];
     let r = iter_next_chunk_erased(&mut array, iter);
@@ -1028,10 +1029,11 @@ pub(crate) fn iter_next_chunk<T, const N: usize>(
 ///
 /// Unfortunately this loop has two exit conditions, the buffer filling up
 /// or the iterator running out of items, making it tend to optimize poorly.
+#[rustc_const_unstable(feature = "const_iter", issue = "92476")]
 #[inline]
-fn iter_next_chunk_erased<T>(
+const fn iter_next_chunk_erased<T>(
     buffer: &mut [MaybeUninit<T>],
-    iter: &mut impl Iterator<Item = T>,
+    iter: &mut impl [const] Iterator<Item = T>,
 ) -> Result<(), usize> {
     // if `Iterator::next` panics, this guard will drop already initialized items
     let mut guard = Guard { array_mut: buffer, initialized: 0 };

library/core/src/iter/adapters/array_chunks.rs

@@ -26,7 +26,7 @@ where
     I: Iterator,
 {
     #[track_caller]
-    pub(in crate::iter) fn new(iter: I) -> Self {
+    pub(in crate::iter) const fn new(iter: I) -> Self {
         assert!(N != 0, "chunk size must be non-zero");
         Self { iter, remainder: None }
     }

library/core/src/iter/adapters/chain.rs

@@ -33,7 +33,7 @@ pub struct Chain<A, B> {
     b: Option<B>,
 }
 impl<A, B> Chain<A, B> {
-    pub(in super::super) fn new(a: A, b: B) -> Chain<A, B> {
+    pub(in super::super) const fn new(a: A, b: B) -> Chain<A, B> {
         Chain { a: Some(a), b: Some(b) }
     }
 }

library/core/src/iter/adapters/cloned.rs

@@ -20,7 +20,7 @@ pub struct Cloned<I> {
 }
 
 impl<I> Cloned<I> {
-    pub(in crate::iter) fn new(it: I) -> Cloned<I> {
+    pub(in crate::iter) const fn new(it: I) -> Cloned<I> {
         Cloned { it }
     }
 }

library/core/src/iter/adapters/copied.rs

@@ -21,7 +21,7 @@ pub struct Copied<I> {
 }
 
 impl<I> Copied<I> {
-    pub(in crate::iter) fn new(it: I) -> Copied<I> {
+    pub(in crate::iter) const fn new(it: I) -> Copied<I> {
         Copied { it }
     }
 

library/core/src/iter/adapters/cycle.rs

@@ -17,7 +17,8 @@ pub struct Cycle<I> {
     iter: I,
 }
 
-impl<I: Clone> Cycle<I> {
+#[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+const impl<I: [const] Clone> Cycle<I> {
     pub(in crate::iter) fn new(iter: I) -> Cycle<I> {
         Cycle { orig: iter.clone(), iter }
     }

library/core/src/iter/adapters/enumerate.rs

@@ -20,7 +20,7 @@ pub struct Enumerate<I> {
     count: usize,
 }
 impl<I> Enumerate<I> {
-    pub(in crate::iter) fn new(iter: I) -> Enumerate<I> {
+    pub(in crate::iter) const fn new(iter: I) -> Enumerate<I> {
         Enumerate { iter, count: 0 }
     }
 

library/core/src/iter/adapters/filter.rs

@@ -24,7 +24,7 @@ pub struct Filter<I, P> {
     predicate: P,
 }
 impl<I, P> Filter<I, P> {
-    pub(in crate::iter) fn new(iter: I, predicate: P) -> Filter<I, P> {
+    pub(in crate::iter) const fn new(iter: I, predicate: P) -> Filter<I, P> {
         Filter { iter, predicate }
     }
 }

library/core/src/iter/adapters/filter_map.rs

@@ -20,7 +20,7 @@ pub struct FilterMap<I, F> {
     f: F,
 }
 impl<I, F> FilterMap<I, F> {
-    pub(in crate::iter) fn new(iter: I, f: F) -> FilterMap<I, F> {
+    pub(in crate::iter) const fn new(iter: I, f: F) -> FilterMap<I, F> {
         FilterMap { iter, f }
     }
 }

library/core/src/iter/adapters/flatten.rs

@@ -185,7 +185,8 @@ pub struct Flatten<I: Iterator<Item: IntoIterator>> {
     inner: FlattenCompat<I, <I::Item as IntoIterator>::IntoIter>,
 }
 
-impl<I: Iterator<Item: IntoIterator>> Flatten<I> {
+#[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+const impl<I: [const] Iterator<Item: IntoIterator>> Flatten<I> {
     pub(in super::super) fn new(iter: I) -> Flatten<I> {
         Flatten { inner: FlattenCompat::new(iter) }
     }
@@ -358,9 +359,11 @@ struct FlattenCompat<I, U> {
     frontiter: Option<U>,
     backiter: Option<U>,
 }
-impl<I, U> FlattenCompat<I, U>
+
+#[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+const impl<I, U> FlattenCompat<I, U>
 where
-    I: Iterator,
+    I: [const] Iterator,
 {
     /// Adapts an iterator by flattening it, for use in `flatten()` and `flat_map()`.
     fn new(iter: I) -> FlattenCompat<I, U> {