Make the Step implementations const.

Author: onestacked

library/core/src/iter/range.rs

@@ -20,7 +20,8 @@ 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")]
-pub trait Step: Clone + PartialOrd + Sized {
+#[const_trait]
+pub trait Step: ~const Clone + ~const 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`
@@ -234,7 +235,8 @@ macro_rules! step_integer_impls {
         $(
             #[allow(unreachable_patterns)]
             #[unstable(feature = "step_trait", reason = "recently redesigned", issue = "42168")]
-            impl Step for $u_narrower {
+            #[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+            impl const Step for $u_narrower {
                 step_identical_methods!();
 
                 #[inline]
@@ -266,7 +268,8 @@ macro_rules! step_integer_impls {
 
             #[allow(unreachable_patterns)]
             #[unstable(feature = "step_trait", reason = "recently redesigned", issue = "42168")]
-            impl Step for $i_narrower {
+            #[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+            impl const Step for $i_narrower {
                 step_identical_methods!();
 
                 #[inline]
@@ -330,7 +333,8 @@ macro_rules! step_integer_impls {
         $(
             #[allow(unreachable_patterns)]
             #[unstable(feature = "step_trait", reason = "recently redesigned", issue = "42168")]
-            impl Step for $u_wider {
+            #[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+            impl const Step for $u_wider {
                 step_identical_methods!();
 
                 #[inline]
@@ -355,7 +359,8 @@ macro_rules! step_integer_impls {
 
             #[allow(unreachable_patterns)]
             #[unstable(feature = "step_trait", reason = "recently redesigned", issue = "42168")]
-            impl Step for $i_wider {
+            #[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+            impl const Step for $i_wider {
                 step_identical_methods!();
 
                 #[inline]
@@ -405,7 +410,8 @@ step_integer_impls! {
 }
 
 #[unstable(feature = "step_trait", reason = "recently redesigned", issue = "42168")]
-impl Step for char {
+#[rustc_const_unstable(feature = "const_iter", issue = "92476")]
+impl const Step for char {
     #[inline]
     fn steps_between(&start: &char, &end: &char) -> Option<usize> {
         let start = start as u32;
@@ -423,6 +429,7 @@ impl 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)?;
@@ -439,6 +446,7 @@ impl 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)?;

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

@@ -86,4 +86,5 @@ 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]
-pub unsafe trait TrustedStep: Step {}
+#[const_trait]
+pub unsafe trait TrustedStep: ~const Step {}

library/core/src/lib.rs

@@ -126,6 +126,7 @@
 #![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)]