Revert #104100, Allow using Range as an Iterator in const contexts.

library/core/src/iter/range.rs

@@ -1,5 +1,4 @@
 use crate::convert::TryFrom;
-use crate::marker::Destruct;
 use crate::mem;
 use crate::num::NonZeroUsize;
 use crate::ops::{self, Try};
@@ -22,8 +21,7 @@ unsafe_impl_trusted_step![char i8 i16 i32 i64 i128 isize u8 u16 u32 u64 u128 usi
 /// The *successor* operation moves towards values that compare greater.
 /// The *predecessor* operation moves towards values that compare lesser.
 #[unstable(feature = "step_trait", reason = "recently redesigned", issue = "42168")]
-#[const_trait]
-pub trait Step: ~const Clone + ~const PartialOrd + Sized {
+pub trait Step: Clone + PartialOrd + Sized {
     /// Returns the number of *successor* steps required to get from `start` to `end`.
     ///
     /// Returns `None` if the number of steps would overflow `usize`
@@ -237,8 +235,7 @@ macro_rules! step_integer_impls {
         $(
             #[allow(unreachable_patterns)]
             #[unstable(feature = "step_trait", reason = "recently redesigned", issue = "42168")]
-            #[rustc_const_unstable(feature = "const_iter", issue = "92476")]
-            impl const Step for $u_narrower {
+            impl Step for $u_narrower {
                 step_identical_methods!();
 
                 #[inline]
@@ -270,8 +267,7 @@ macro_rules! step_integer_impls {
 
             #[allow(unreachable_patterns)]
             #[unstable(feature = "step_trait", reason = "recently redesigned", issue = "42168")]
-            #[rustc_const_unstable(feature = "const_iter", issue = "92476")]
-            impl const Step for $i_narrower {
+            impl Step for $i_narrower {
                 step_identical_methods!();
 
                 #[inline]
@@ -335,8 +331,7 @@ macro_rules! step_integer_impls {
         $(
             #[allow(unreachable_patterns)]
             #[unstable(feature = "step_trait", reason = "recently redesigned", issue = "42168")]
-            #[rustc_const_unstable(feature = "const_iter", issue = "92476")]
-            impl const Step for $u_wider {
+            impl Step for $u_wider {
                 step_identical_methods!();
 
                 #[inline]
@@ -361,8 +356,7 @@ macro_rules! step_integer_impls {
 
             #[allow(unreachable_patterns)]
             #[unstable(feature = "step_trait", reason = "recently redesigned", issue = "42168")]
-            #[rustc_const_unstable(feature = "const_iter", issue = "92476")]
-            impl const Step for $i_wider {
+            impl Step for $i_wider {
                 step_identical_methods!();
 
                 #[inline]
@@ -412,8 +406,7 @@ step_integer_impls! {
 }
 
 #[unstable(feature = "step_trait", reason = "recently redesigned", issue = "42168")]
-#[rustc_const_unstable(feature = "const_iter", issue = "92476")]
-impl const Step for char {
+impl Step for char {
     #[inline]
     fn steps_between(&start: &char, &end: &char) -> Option<usize> {
         let start = start as u32;
@@ -431,7 +424,6 @@ impl const Step for char {
     }
 
     #[inline]
-    #[rustc_allow_const_fn_unstable(const_try)]
     fn forward_checked(start: char, count: usize) -> Option<char> {
         let start = start as u32;
         let mut res = Step::forward_checked(start, count)?;
@@ -448,7 +440,6 @@ impl const Step for char {
     }
 
     #[inline]
-    #[rustc_allow_const_fn_unstable(const_try)]
     fn backward_checked(start: char, count: usize) -> Option<char> {
         let start = start as u32;
         let mut res = Step::backward_checked(start, count)?;
@@ -524,7 +515,6 @@ macro_rules! range_incl_exact_iter_impl {
 }
 
 /// Specialization implementations for `Range`.
-#[const_trait]
 trait RangeIteratorImpl {
     type Item;
 
@@ -539,7 +529,7 @@ trait RangeIteratorImpl {
     fn spec_advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize>;
 }
 
-impl<A: ~const Step + ~const Destruct> const RangeIteratorImpl for ops::Range<A> {
+impl<A: Step> RangeIteratorImpl for ops::Range<A> {
     type Item = A;
 
     #[inline]
@@ -625,7 +615,7 @@ impl<A: ~const Step + ~const Destruct> const RangeIteratorImpl for ops::Range<A>
     }
 }
 
-impl<T: ~const TrustedStep + ~const Destruct> const RangeIteratorImpl for ops::Range<T> {
+impl<T: TrustedStep> RangeIteratorImpl for ops::Range<T> {
     #[inline]
     fn spec_next(&mut self) -> Option<T> {
         if self.start < self.end {
@@ -713,8 +703,7 @@ impl<T: ~const TrustedStep + ~const Destruct> const RangeIteratorImpl for ops::R
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_const_unstable(feature = "const_iter", issue = "92476")]
-impl<A: ~const Step + ~const Destruct> const Iterator for ops::Range<A> {
+impl<A: Step> Iterator for ops::Range<A> {
     type Item = A;
 
     #[inline]
@@ -824,8 +813,7 @@ range_incl_exact_iter_impl! {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_const_unstable(feature = "const_iter", issue = "92476")]
-impl<A: ~const Step + ~const Destruct> const DoubleEndedIterator for ops::Range<A> {
+impl<A: Step> DoubleEndedIterator for ops::Range<A> {
     #[inline]
     fn next_back(&mut self) -> Option<A> {
         self.spec_next_back()

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

@@ -1,4 +1,3 @@
-use crate::marker::Destruct;
 use crate::num::NonZeroUsize;
 use crate::ops::{ControlFlow, Try};
 
@@ -39,7 +38,6 @@ use crate::ops::{ControlFlow, Try};
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "DoubleEndedIterator")]
-#[const_trait]
 pub trait DoubleEndedIterator: Iterator {
     /// Removes and returns an element from the end of the iterator.
     ///
@@ -136,10 +134,7 @@ 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<(), NonZeroUsize>
-    where
-        Self::Item: ~const Destruct,
-    {
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         for i in 0..n {
             if self.next_back().is_none() {
                 // SAFETY: `i` is always less than `n`.
@@ -192,7 +187,6 @@ 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> {
         if self.advance_back_by(n).is_err() {
             return None;
@@ -232,7 +226,6 @@ 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,
@@ -304,7 +297,6 @@ 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,
@@ -360,7 +352,6 @@ 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

@@ -1,6 +1,5 @@
 use crate::array;
 use crate::cmp::{self, Ordering};
-use crate::marker::Destruct;
 use crate::num::NonZeroUsize;
 use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try};
 
@@ -340,10 +339,8 @@ pub trait Iterator {
     /// ```
     #[inline]
     #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
-    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize>
-    where
-        Self::Item: ~const Destruct,
-    {
+    #[rustc_do_not_const_check]
+    fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         for i in 0..n {
             if self.next().is_none() {
                 // SAFETY: `i` is always less than `n`.
@@ -394,10 +391,8 @@ pub trait Iterator {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    fn nth(&mut self, n: usize) -> Option<Self::Item>
-    where
-        Self::Item: ~const Destruct,
-    {
+    #[rustc_do_not_const_check]
+    fn nth(&mut self, n: usize) -> Option<Self::Item> {
         self.advance_by(n).ok()?;
         self.next()
     }

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

@@ -86,5 +86,4 @@ pub unsafe trait InPlaceIterable: Iterator {}
 /// for details. Consumers are free to rely on the invariants in unsafe code.
 #[unstable(feature = "trusted_step", issue = "85731")]
 #[rustc_specialization_trait]
-#[const_trait]
-pub unsafe trait TrustedStep: ~const Step {}
+pub unsafe trait TrustedStep: Step {}

library/core/src/lib.rs

@@ -123,11 +123,9 @@
 #![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_ipv4)]
 #![feature(const_ipv6)]
-#![feature(const_iter)]
 #![feature(const_likely)]
 #![feature(const_maybe_uninit_uninit_array)]
 #![feature(const_maybe_uninit_as_mut_ptr)]

library/core/tests/iter/mod.rs

@@ -20,8 +20,6 @@ mod range;
 mod sources;
 mod traits;
 
-mod consts;
-
 use core::cell::Cell;
 use core::convert::TryFrom;
 use core::iter::*;

library/core/tests/lib.rs

@@ -12,11 +12,8 @@
 #![feature(const_caller_location)]
 #![feature(const_cell_into_inner)]
 #![feature(const_convert)]
-#![feature(const_for)]
 #![feature(const_hash)]
 #![feature(const_heap)]
-#![feature(const_intoiterator_identity)]
-#![feature(const_iter)]
 #![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(const_maybe_uninit_assume_init_read)]
 #![feature(const_nonnull_new)]

tests/ui/consts/const-fn-error.rs

@@ -7,6 +7,7 @@ const fn f(x: usize) -> usize {
         //~| ERROR `for` is not allowed in a `const fn`
         //~| ERROR mutable references are not allowed in constant functions
         //~| ERROR cannot call non-const fn
+        //~| ERROR the trait bound
         sum += i;
     }
     sum

tests/ui/consts/const-fn-error.stderr

@@ -5,7 +5,7 @@ LL | /     for i in 0..x {
 LL | |
 LL | |
 LL | |
-LL | |
+...  |
 LL | |         sum += i;
 LL | |     }
    | |_____^
@@ -33,6 +33,19 @@ LL |     for i in 0..x {
    = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information
    = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
+error[E0277]: the trait bound `std::ops::Range<usize>: Iterator` is not satisfied
+  --> $DIR/const-fn-error.rs:5:14
+   |
+LL |     for i in 0..x {
+   |              ^^^^ `std::ops::Range<usize>` is not an iterator
+   |
+   = help: the trait `~const Iterator` is not implemented for `std::ops::Range<usize>`
+note: the trait `Iterator` is implemented for `std::ops::Range<usize>`, but that implementation is not `const`
+  --> $DIR/const-fn-error.rs:5:14
+   |
+LL |     for i in 0..x {
+   |              ^^^^
+
 error[E0015]: cannot call non-const fn `<std::ops::Range<usize> as Iterator>::next` in constant functions
   --> $DIR/const-fn-error.rs:5:14
    |
@@ -42,7 +55,7 @@ LL |     for i in 0..x {
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
    = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
 
-error: aborting due to 4 previous errors
+error: aborting due to 5 previous errors
 
-Some errors have detailed explanations: E0015, E0658.
+Some errors have detailed explanations: E0015, E0277, E0658.
 For more information about an error, try `rustc --explain E0015`.

tests/ui/consts/const-for.rs

@@ -5,6 +5,7 @@ const _: () = {
     for _ in 0..5 {}
     //~^ error: cannot call
     //~| error: cannot convert
+    //~| error: the trait bound
 };
 
 fn main() {}

tests/ui/consts/const-for.stderr

@@ -9,6 +9,19 @@ note: impl defined here, but it is not `const`
    = note: calls in constants are limited to constant functions, tuple structs and tuple variants
    = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
 
+error[E0277]: the trait bound `std::ops::Range<i32>: Iterator` is not satisfied
+  --> $DIR/const-for.rs:5:14
+   |
+LL |     for _ in 0..5 {}
+   |              ^^^^ `std::ops::Range<i32>` is not an iterator
+   |
+   = help: the trait `~const Iterator` is not implemented for `std::ops::Range<i32>`
+note: the trait `Iterator` is implemented for `std::ops::Range<i32>`, but that implementation is not `const`
+  --> $DIR/const-for.rs:5:14
+   |
+LL |     for _ in 0..5 {}
+   |              ^^^^
+
 error[E0015]: cannot call non-const fn `<std::ops::Range<i32> as Iterator>::next` in constants
   --> $DIR/const-for.rs:5:14
    |
@@ -18,6 +31,7 @@ LL |     for _ in 0..5 {}
    = note: calls in constants are limited to constant functions, tuple structs and tuple variants
    = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0015`.
+Some errors have detailed explanations: E0015, E0277.
+For more information about an error, try `rustc --explain E0015`.

tests/ui/typeck/typeck_type_placeholder_item.rs

@@ -228,4 +228,5 @@ fn evens_squared(n: usize) -> _ {
 
 const _: _ = (1..10).filter(|x| x % 2 == 0).map(|x| x * x);
 //~^ ERROR the trait bound
+//~| ERROR the trait bound
 //~| ERROR the placeholder