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

compiler/rustc_ast/src/lib.rs

@@ -17,6 +17,8 @@
 #![feature(iter_zip)]
 #![feature(label_break_value)]
 #![feature(nll)]
+#![feature(min_specialization)]
+#![feature(trusted_step)]
 #![recursion_limit = "256"]
 
 #[macro_use]

compiler/rustc_hir/src/lib.rs

@@ -7,6 +7,8 @@
 #![cfg_attr(bootstrap, feature(extended_key_value_attributes))]
 #![feature(in_band_lifetimes)]
 #![feature(once_cell)]
+#![feature(min_specialization)]
+#![feature(trusted_step)]
 #![recursion_limit = "256"]
 
 #[macro_use]

compiler/rustc_index/src/lib.rs

@@ -6,6 +6,7 @@
 #![feature(unboxed_closures)]
 #![feature(test)]
 #![feature(fn_traits)]
+#![feature(trusted_step)]
 
 pub mod bit_set;
 pub mod vec;

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 ----
 
@@ -184,7 +184,7 @@ macro_rules! newtype_index {
             }
         }
 
-        unsafe impl ::std::iter::Step for $type {
+        impl ::std::iter::Step for $type {
             #[inline]
             fn steps_between(start: &Self, end: &Self) -> Option<usize> {
                 <usize as ::std::iter::Step>::steps_between(
@@ -204,6 +204,9 @@ macro_rules! newtype_index {
             }
         }
 
+        // Safety: The implementation of `Step` upholds all invariants.
+        unsafe impl ::std::iter::TrustedStep for $type {}
+
         impl From<$type> for u32 {
             #[inline]
             fn from(v: $type) -> u32 {

compiler/rustc_infer/src/lib.rs

@@ -22,6 +22,8 @@
 #![feature(never_type)]
 #![feature(in_band_lifetimes)]
 #![feature(control_flow_enum)]
+#![feature(min_specialization)]
+#![feature(trusted_step)]
 #![recursion_limit = "512"] // For rustdoc
 
 #[macro_use]

compiler/rustc_middle/src/lib.rs

@@ -50,6 +50,7 @@
 #![feature(associated_type_defaults)]
 #![feature(iter_zip)]
 #![feature(thread_local_const_init)]
+#![feature(trusted_step)]
 #![recursion_limit = "512"]
 
 #[macro_use]

compiler/rustc_mir/src/lib.rs

@@ -31,6 +31,7 @@ Rust MIR: a lowered representation of Rust.
 #![feature(option_get_or_insert_default)]
 #![feature(once_cell)]
 #![feature(control_flow_enum)]
+#![feature(trusted_step)]
 #![recursion_limit = "256"]
 
 #[macro_use]

compiler/rustc_mir_build/src/lib.rs

@@ -10,6 +10,8 @@
 #![feature(bool_to_option)]
 #![feature(iter_zip)]
 #![feature(once_cell)]
+#![feature(min_specialization)]
+#![feature(trusted_step)]
 #![recursion_limit = "256"]
 
 #[macro_use]

compiler/rustc_passes/src/lib.rs

@@ -10,6 +10,8 @@
 #![feature(in_band_lifetimes)]
 #![feature(iter_zip)]
 #![feature(nll)]
+#![feature(min_specialization)]
+#![feature(trusted_step)]
 #![recursion_limit = "256"]
 
 #[macro_use]

compiler/rustc_query_system/src/lib.rs

@@ -6,6 +6,7 @@
 #![feature(iter_zip)]
 #![feature(min_specialization)]
 #![feature(stmt_expr_attributes)]
+#![feature(trusted_step)]
 
 #[macro_use]
 extern crate tracing;