Auto merge of rust-lang#83772 - jhpratt:revamp-step-trait, r=Mark-Simulacrum

Make `Step` trait safe to implement

This PR makes a few modifications to the `Step` trait that I believe better position it for stabilization in the short term. In particular,

1. `unsafe trait TrustedStep` is introduced, indicating that the implementation of `Step` for a given type upholds all stated invariants (which have remained unchanged). This is gated behind a new `trusted_step` feature, as stabilization is realistically blocked on min_specialization.
2. The `Step` trait is internally specialized on the `TrustedStep` trait, which avoids a serious performance regression.
3. `TrustedLen` is implemented for `T: TrustedStep` as the latter's invariants subsume the former's.
4. The `Step` trait is no longer `unsafe`, as the invariants must not be relied upon by unsafe code (unless the type implements `TrustedStep`).
5. `TrustedStep` is implemented for all types that implement `Step` in the standard library and compiler.
6. The `step_trait_ext` feature is merged into the `step_trait` feature. I was unable to find any reasoning for the features being split; the `_unchecked` methods need not necessarily be stabilized at the same time, but I think it is useful to have them under the same feature flag.

All existing implementations of `Step` will be broken, as it is not possible to `unsafe impl` a safe trait. Given this trait only exists on nightly, I feel this breakage is acceptable. The blanket `impl<T: Step> TrustedLen for T` will likely cause some minor breakage, but this should be covered by the equivalent impl for `TrustedStep`.

Hopefully these changes are sufficient to place `Step` in decent position for stabilization, which would allow user-defined types to be used with `a..b` syntax.

Author: bors

core/src/iter/mod.rs

@@ -384,6 +384,8 @@ pub use self::traits::FusedIterator;
 pub use self::traits::InPlaceIterable;
 #[unstable(feature = "trusted_len", issue = "37572")]
 pub use self::traits::TrustedLen;
+#[unstable(feature = "trusted_step", issue = "85731")]
+pub use self::traits::TrustedStep;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::traits::{
     DoubleEndedIterator, ExactSizeIterator, Extend, FromIterator, IntoIterator, Product, Sum,