Mark DoubleEndedIterator as #[const_trait] using rustc_do_not_const_check, implement const Iterator and DoubleEndedIterator for Range.

Author: onestacked

library/core/src/iter/range.rs

@@ -1,5 +1,6 @@
 use crate::char;
 use crate::convert::TryFrom;
+use crate::marker::Destruct;
 use crate::mem;
 use crate::ops::{self, Try};
 
@@ -523,6 +524,7 @@ macro_rules! range_incl_exact_iter_impl {
 }
 
 /// Specialization implementations for `Range`.
+#[const_trait]
 trait RangeIteratorImpl {
     type Item;
 
@@ -537,7 +539,7 @@ trait RangeIteratorImpl {
     fn spec_advance_back_by(&mut self, n: usize) -> Result<(), usize>;
 }
 
-impl<A: Step> RangeIteratorImpl for ops::Range<A> {
+impl<A: ~const Step + ~const Destruct> const RangeIteratorImpl for ops::Range<A> {
     type Item = A;
 
     #[inline]
@@ -623,7 +625,7 @@ impl<A: Step> RangeIteratorImpl for ops::Range<A> {
     }
 }
 
-impl<T: TrustedStep> RangeIteratorImpl for ops::Range<T> {
+impl<T: ~const TrustedStep + ~const Destruct> const RangeIteratorImpl for ops::Range<T> {
     #[inline]
     fn spec_next(&mut self) -> Option<T> {
         if self.start < self.end {
@@ -711,6 +713,70 @@ impl<T: TrustedStep> RangeIteratorImpl for ops::Range<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+#[cfg(not(bootstrap))]
+impl<A: ~const Step + ~const Destruct> const Iterator for ops::Range<A> {
+    type Item = A;
+
+    #[inline]
+    fn next(&mut self) -> Option<A> {
+        self.spec_next()
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        if self.start < self.end {
+            let hint = Step::steps_between(&self.start, &self.end);
+            (hint.unwrap_or(usize::MAX), hint)
+        } else {
+            (0, Some(0))
+        }
+    }
+
+    #[inline]
+    fn nth(&mut self, n: usize) -> Option<A> {
+        self.spec_nth(n)
+    }
+
+    #[inline]
+    fn last(mut self) -> Option<A> {
+        self.next_back()
+    }
+
+    #[inline]
+    fn min(mut self) -> Option<A> {
+        self.next()
+    }
+
+    #[inline]
+    fn max(mut self) -> Option<A> {
+        self.next_back()
+    }
+
+    #[inline]
+    fn is_sorted(self) -> bool {
+        true
+    }
+
+    #[inline]
+    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+        self.spec_advance_by(n)
+    }
+
+    #[inline]
+    unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item
+    where
+        Self: TrustedRandomAccessNoCoerce,
+    {
+        // SAFETY: The TrustedRandomAccess contract requires that callers only pass an index
+        // that is in bounds.
+        // Additionally Self: TrustedRandomAccess is only implemented for Copy types
+        // which means even repeated reads of the same index would be safe.
+        unsafe { Step::forward_unchecked(self.start.clone(), idx) }
+    }
+}
+#[stable(feature = "rust1", since = "1.0.0")]
+#[cfg(bootstrap)]
 impl<A: Step> Iterator for ops::Range<A> {
     type Item = A;
 
@@ -821,6 +887,27 @@ range_incl_exact_iter_impl! {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+#[cfg(not(bootstrap))]
+impl<A: ~const Step + ~const Destruct> const DoubleEndedIterator for ops::Range<A> {
+    #[inline]
+    fn next_back(&mut self) -> Option<A> {
+        self.spec_next_back()
+    }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<A> {
+        self.spec_nth_back(n)
+    }
+
+    #[inline]
+    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+        self.spec_advance_back_by(n)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+#[cfg(bootstrap)]
 impl<A: Step> DoubleEndedIterator for ops::Range<A> {
     #[inline]
     fn next_back(&mut self) -> Option<A> {

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

@@ -1,3 +1,4 @@
+use crate::marker::Destruct;
 use crate::ops::{ControlFlow, Try};
 
 /// An iterator able to yield elements from both ends.
@@ -37,6 +38,7 @@ use crate::ops::{ControlFlow, Try};
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "DoubleEndedIterator")]
+#[cfg_attr(not(bootstrap), const_trait)]
 pub trait DoubleEndedIterator: Iterator {
     /// Removes and returns an element from the end of the iterator.
     ///
@@ -131,7 +133,10 @@ pub trait DoubleEndedIterator: Iterator {
     /// [`Err(k)`]: Err
     #[inline]
     #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_back_by(&mut self, n: usize) -> Result<(), usize>
+    where
+        Self::Item: ~const Destruct,
+    {
         for i in 0..n {
             self.next_back().ok_or(i)?;
         }
@@ -181,6 +186,7 @@ pub trait DoubleEndedIterator: Iterator {
     /// ```
     #[inline]
     #[stable(feature = "iter_nth_back", since = "1.37.0")]
+    #[rustc_do_not_const_check]
     fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
         self.advance_back_by(n).ok()?;
         self.next_back()
@@ -218,6 +224,7 @@ pub trait DoubleEndedIterator: Iterator {
     /// ```
     #[inline]
     #[stable(feature = "iterator_try_fold", since = "1.27.0")]
+    #[rustc_do_not_const_check]
     fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
     where
         Self: Sized,
@@ -289,6 +296,7 @@ pub trait DoubleEndedIterator: Iterator {
     #[doc(alias = "foldr")]
     #[inline]
     #[stable(feature = "iter_rfold", since = "1.27.0")]
+    #[rustc_do_not_const_check]
     fn rfold<B, F>(mut self, init: B, mut f: F) -> B
     where
         Self: Sized,
@@ -344,6 +352,7 @@ pub trait DoubleEndedIterator: Iterator {
     /// ```
     #[inline]
     #[stable(feature = "iter_rfind", since = "1.27.0")]
+    #[rustc_do_not_const_check]
     fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
     where
         Self: Sized,

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

@@ -1,5 +1,6 @@
 use crate::array;
 use crate::cmp::{self, Ordering};
+use crate::marker::Destruct;
 use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try};
 
 use super::super::try_process;
@@ -336,8 +337,10 @@ pub trait Iterator {
     /// ```
     #[inline]
     #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
-    #[rustc_do_not_const_check]
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+    fn advance_by(&mut self, n: usize) -> Result<(), usize>
+    where
+        Self::Item: ~const Destruct,
+    {
         for i in 0..n {
             self.next().ok_or(i)?;
         }
@@ -385,8 +388,10 @@ pub trait Iterator {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_do_not_const_check]
-    fn nth(&mut self, n: usize) -> Option<Self::Item> {
+    fn nth(&mut self, n: usize) -> Option<Self::Item>
+    where
+        Self::Item: ~const Destruct,
+    {
         self.advance_by(n).ok()?;
         self.next()
     }

library/core/src/lib.rs

@@ -123,6 +123,7 @@
 #![feature(const_index_range_slice_index)]
 #![feature(const_inherent_unchecked_arith)]
 #![feature(const_int_unchecked_arith)]
+#![feature(const_intoiterator_identity)]
 #![feature(const_intrinsic_forget)]
 #![feature(const_iter)]
 #![feature(const_likely)]

tests/ui/consts/const_range_iterator.rs

@@ -0,0 +1,19 @@
+// check-pass
+#![feature(const_for)]
+#![feature(const_mut_refs)]
+#![feature(const_trait_impl)]
+#![feature(const_iter)]
+#![feature(const_intoiterator_identity)]
+#![feature(inline_const)]
+
+fn main() {
+    const {
+        let mut arr = [0; 3];
+        for i in 0..arr.len() {
+            arr[i] = i;
+        }
+        assert!(arr[0] == 0);
+        assert!(arr[1] == 1);
+        assert!(arr[2] == 2);
+    }
+}