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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. |
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| rustc | ||
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| rustc_arena | ||
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| rustc_attr | ||
| rustc_builtin_macros | ||
| rustc_codegen_cranelift | ||
| rustc_codegen_llvm | ||
| rustc_codegen_ssa | ||
| rustc_data_structures | ||
| rustc_driver | ||
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| rustc_errors | ||
| rustc_expand | ||
| rustc_feature | ||
| rustc_fs_util | ||
| rustc_graphviz | ||
| rustc_hir | ||
| rustc_hir_pretty | ||
| rustc_incremental | ||
| rustc_index | ||
| rustc_infer | ||
| rustc_interface | ||
| rustc_lexer | ||
| rustc_lint | ||
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| rustc_llvm | ||
| rustc_macros | ||
| rustc_metadata | ||
| rustc_middle | ||
| rustc_mir | ||
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| rustc_parse | ||
| rustc_parse_format | ||
| rustc_passes | ||
| rustc_plugin_impl | ||
| rustc_privacy | ||
| rustc_query_impl | ||
| rustc_query_system | ||
| rustc_resolve | ||
| rustc_save_analysis | ||
| rustc_serialize | ||
| rustc_session | ||
| rustc_span | ||
| rustc_symbol_mangling | ||
| rustc_target | ||
| rustc_trait_selection | ||
| rustc_traits | ||
| rustc_ty_utils | ||
| rustc_type_ir | ||
| rustc_typeck | ||