Unify feature flags as `step_trait`

While stdlib implementations of the unchecked methods require unchecked
math, there is no reason to gate it behind this for external users. The
reasoning for a separate `step_trait_ext` feature is unclear, and as
such has been merged as well.

compiler/rustc_index/src/vec.rs

@@ -65,7 +65,7 @@ impl Idx for u32 {
 /// `u32::MAX`. You can also customize things like the `Debug` impl,
 /// what traits are derived, and so forth via the macro.
 #[macro_export]
-#[allow_internal_unstable(step_trait, step_trait_ext, rustc_attrs)]
+#[allow_internal_unstable(step_trait, rustc_attrs)]
 macro_rules! newtype_index {
     // ---- public rules ----
 

library/core/src/iter/range.rs

@@ -57,7 +57,6 @@ pub trait Step: Clone + PartialOrd + Sized {
     ///
     /// * `Step::forward_checked(a, n) == (0..n).try_fold(a, |x, _| Step::forward_checked(&x, 1))`
     ///   * Corollary: `Step::forward_checked(&a, 0) == Some(a)`
-    #[unstable(feature = "step_trait_ext", reason = "recently added", issue = "42168")]
     fn forward_checked(start: Self, count: usize) -> Option<Self>;
 
     /// Returns the value that would be obtained by taking the *successor*
@@ -83,7 +82,6 @@ pub trait Step: Clone + PartialOrd + Sized {
     ///   * Corollary: `Step::forward(a, 0) == a`
     /// * `Step::forward(a, n) >= a`
     /// * `Step::backward(Step::forward(a, n), n) == a`
-    #[unstable(feature = "step_trait_ext", reason = "recently added", issue = "42168")]
     fn forward(start: Self, count: usize) -> Self {
         Step::forward_checked(start, count).expect("overflow in `Step::forward`")
     }
@@ -108,7 +106,6 @@ pub trait Step: Clone + PartialOrd + Sized {
     /// For any `a` and `n`, where no overflow occurs:
     ///
     /// * `Step::forward_unchecked(a, n)` is equivalent to `Step::forward(a, n)`
-    #[unstable(feature = "step_trait_ext", reason = "recently added", issue = "42168")]
     unsafe fn forward_unchecked(start: Self, count: usize) -> Self {
         Step::forward(start, count)
     }
@@ -129,7 +126,6 @@ pub trait Step: Clone + PartialOrd + Sized {
     ///
     /// * `Step::backward_checked(a, n) == (0..n).try_fold(a, |x, _| Step::backward_checked(&x, 1))`
     ///   * Corollary: `Step::backward_checked(&a, 0) == Some(a)`
-    #[unstable(feature = "step_trait_ext", reason = "recently added", issue = "42168")]
     fn backward_checked(start: Self, count: usize) -> Option<Self>;
 
     /// Returns the value that would be obtained by taking the *predecessor*
@@ -155,7 +151,6 @@ pub trait Step: Clone + PartialOrd + Sized {
     ///   * Corollary: `Step::backward(a, 0) == a`
     /// * `Step::backward(a, n) <= a`
     /// * `Step::forward(Step::backward(a, n), n) == a`
-    #[unstable(feature = "step_trait_ext", reason = "recently added", issue = "42168")]
     fn backward(start: Self, count: usize) -> Self {
         Step::backward_checked(start, count).expect("overflow in `Step::backward`")
     }
@@ -180,7 +175,6 @@ pub trait Step: Clone + PartialOrd + Sized {
     /// For any `a` and `n`, where no overflow occurs:
     ///
     /// * `Step::backward_unchecked(a, n)` is equivalent to `Step::backward(a, n)`
-    #[unstable(feature = "step_trait_ext", reason = "recently added", issue = "42168")]
     unsafe fn backward_unchecked(start: Self, count: usize) -> Self {
         Step::backward(start, count)
     }

library/core/tests/lib.rs

@@ -40,7 +40,6 @@
 #![feature(maybe_uninit_write_slice)]
 #![feature(min_specialization)]
 #![feature(step_trait)]
-#![feature(step_trait_ext)]
 #![feature(str_internals)]
 #![feature(test)]
 #![feature(trusted_len)]

src/test/ui/impl-trait/example-calendar.rs

@@ -3,7 +3,6 @@
 
 #![feature(fn_traits,
            step_trait,
-           step_trait_ext,
            unboxed_closures,
 )]