21d94a3d2c
Stabilize associated type bounds (RFC 2289)
This PR stabilizes associated type bounds, which were laid out in [RFC 2289]. This gives us a shorthand to express nested type bounds that would otherwise need to be expressed with nested `impl Trait` or broken into several `where` clauses.
### What are we stabilizing?
We're stabilizing the associated item bounds syntax, which allows us to put bounds in associated type position within other bounds, i.e. `T: Trait<Assoc: Bounds...>`. See [RFC 2289] for motivation.
In all position, the associated type bound syntax expands into a set of two (or more) bounds, and never anything else (see "How does this differ[...]" section for more info).
Associated type bounds are stabilized in four positions:
* **`where` clauses (and APIT)** - This is equivalent to breaking up the bound into two (or more) `where` clauses. For example, `where T: Trait<Assoc: Bound>` is equivalent to `where T: Trait, <T as Trait>::Assoc: Bound`.
* **Supertraits** - Similar to above, `trait CopyIterator: Iterator<Item: Copy> {}`. This is almost equivalent to breaking up the bound into two (or more) `where` clauses; however, the bound on the associated item is implied whenever the trait is used. See #112573/#112629.
* **Associated type item bounds** - This allows constraining the *nested* rigid projections that are associated with a trait's associated types. e.g. `trait Trait { type Assoc: Trait2<Assoc2: Copy>; }`.
* **opaque item bounds (RPIT, TAIT)** - This allows constraining associated types that are associated with the opaque without having to *name* the opaque. For example, `impl Iterator<Item: Copy>` defines an iterator whose item is `Copy` without having to actually name that item bound.
The latter three are not expressible in surface Rust (though for associated type item bounds, this will change in #120752, which I don't believe should block this PR), so this does represent a slight expansion of what can be expressed in trait bounds.
### How does this differ from the RFC?
Compared to the RFC, the current implementation *always* desugars associated type bounds to sets of `ty::Clause`s internally. Specifically, it does *not* introduce a position-dependent desugaring as laid out in [RFC 2289], and in particular:
* It does *not* desugar to anonymous associated items in associated type item bounds.
* It does *not* desugar to nested RPITs in RPIT bounds, nor nested TAITs in TAIT bounds.
This position-dependent desugaring laid out in the RFC existed simply to side-step limitations of the trait solver, which have mostly been fixed in #120584. The desugaring laid out in the RFC also added unnecessary complication to the design of the feature, and introduces its own limitations to, for example:
* Conditionally lowering to nested `impl Trait` in certain positions such as RPIT and TAIT means that we inherit the limitations of RPIT/TAIT, namely lack of support for higher-ranked opaque inference. See this code example: https://github.com/rust-lang/rust/pull/120752#issuecomment-1979412531.
* Introducing anonymous associated types makes traits no longer object safe, since anonymous associated types are not nameable, and all associated types must be named in `dyn` types.
This last point motivates why this PR is *not* stabilizing support for associated type bounds in `dyn` types, e.g, `dyn Assoc<Item: Bound>`. Why? Because `dyn` types need to have *concrete* types for all associated items, this would necessitate a distinct lowering for associated type bounds, which seems both complicated and unnecessary compared to just requiring the user to write `impl Trait` themselves. See #120719.
### Implementation history:
Limited to the significant behavioral changes and fixes and relevant PRs, ping me if I left something out--
* #57428
* #108063
* #110512
* #112629
* #120719
* #120584
Closes #52662
[RFC 2289]: https://rust-lang.github.io/rfcs/2289-associated-type-bounds.html
288 lines
9.4 KiB
Rust
288 lines
9.4 KiB
Rust
//! # The Rust core allocation and collections library
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//!
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//! This library provides smart pointers and collections for managing
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//! heap-allocated values.
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//!
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//! This library, like core, normally doesn’t need to be used directly
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//! since its contents are re-exported in the [`std` crate](../std/index.html).
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//! Crates that use the `#![no_std]` attribute however will typically
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//! not depend on `std`, so they’d use this crate instead.
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//!
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//! ## Boxed values
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//!
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//! The [`Box`] type is a smart pointer type. There can only be one owner of a
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//! [`Box`], and the owner can decide to mutate the contents, which live on the
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//! heap.
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//!
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//! This type can be sent among threads efficiently as the size of a `Box` value
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//! is the same as that of a pointer. Tree-like data structures are often built
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//! with boxes because each node often has only one owner, the parent.
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//!
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//! ## Reference counted pointers
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//!
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//! The [`Rc`] type is a non-threadsafe reference-counted pointer type intended
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//! for sharing memory within a thread. An [`Rc`] pointer wraps a type, `T`, and
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//! only allows access to `&T`, a shared reference.
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//!
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//! This type is useful when inherited mutability (such as using [`Box`]) is too
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//! constraining for an application, and is often paired with the [`Cell`] or
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//! [`RefCell`] types in order to allow mutation.
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//!
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//! ## Atomically reference counted pointers
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//!
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//! The [`Arc`] type is the threadsafe equivalent of the [`Rc`] type. It
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//! provides all the same functionality of [`Rc`], except it requires that the
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//! contained type `T` is shareable. Additionally, [`Arc<T>`][`Arc`] is itself
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//! sendable while [`Rc<T>`][`Rc`] is not.
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//!
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//! This type allows for shared access to the contained data, and is often
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//! paired with synchronization primitives such as mutexes to allow mutation of
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//! shared resources.
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//!
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//! ## Collections
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//!
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//! Implementations of the most common general purpose data structures are
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//! defined in this library. They are re-exported through the
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//! [standard collections library](../std/collections/index.html).
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//!
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//! ## Heap interfaces
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//!
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//! The [`alloc`](alloc/index.html) module defines the low-level interface to the
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//! default global allocator. It is not compatible with the libc allocator API.
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//!
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//! [`Arc`]: sync
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//! [`Box`]: boxed
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//! [`Cell`]: core::cell
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//! [`Rc`]: rc
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//! [`RefCell`]: core::cell
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// To run alloc tests without x.py without ending up with two copies of alloc, Miri needs to be
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// able to "empty" this crate. See <https://github.com/rust-lang/miri-test-libstd/issues/4>.
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// rustc itself never sets the feature, so this line has no effect there.
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#![cfg(any(not(feature = "miri-test-libstd"), test, doctest))]
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//
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#![allow(unused_attributes)]
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#![stable(feature = "alloc", since = "1.36.0")]
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#![doc(
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html_playground_url = "https://play.rust-lang.org/",
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issue_tracker_base_url = "https://github.com/rust-lang/rust/issues/",
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test(no_crate_inject, attr(allow(unused_variables), deny(warnings)))
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)]
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#![doc(cfg_hide(
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not(test),
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not(any(test, bootstrap)),
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any(not(feature = "miri-test-libstd"), test, doctest),
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no_global_oom_handling,
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not(no_global_oom_handling),
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not(no_rc),
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not(no_sync),
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target_has_atomic = "ptr"
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))]
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#![doc(rust_logo)]
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#![feature(rustdoc_internals)]
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#![no_std]
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#![needs_allocator]
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// Lints:
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#![deny(unsafe_op_in_unsafe_fn)]
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#![deny(fuzzy_provenance_casts)]
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#![warn(deprecated_in_future)]
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#![warn(missing_debug_implementations)]
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#![warn(missing_docs)]
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#![allow(explicit_outlives_requirements)]
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#![warn(multiple_supertrait_upcastable)]
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#![allow(internal_features)]
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#![allow(rustdoc::redundant_explicit_links)]
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#![deny(ffi_unwind_calls)]
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//
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// Library features:
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// tidy-alphabetical-start
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#![cfg_attr(not(no_global_oom_handling), feature(const_alloc_error))]
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#![cfg_attr(not(no_global_oom_handling), feature(const_btree_len))]
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#![cfg_attr(test, feature(is_sorted))]
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#![cfg_attr(test, feature(new_uninit))]
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#![feature(alloc_layout_extra)]
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#![feature(allocator_api)]
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#![feature(array_chunks)]
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#![feature(array_into_iter_constructors)]
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#![feature(array_windows)]
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#![feature(ascii_char)]
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#![feature(assert_matches)]
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#![feature(async_fn_traits)]
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#![feature(async_iterator)]
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#![feature(coerce_unsized)]
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#![feature(const_align_of_val)]
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#![feature(const_box)]
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#![feature(const_cow_is_borrowed)]
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#![feature(const_eval_select)]
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#![feature(const_maybe_uninit_as_mut_ptr)]
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#![feature(const_maybe_uninit_write)]
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#![feature(const_pin)]
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#![feature(const_refs_to_cell)]
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#![feature(const_size_of_val)]
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#![feature(const_waker)]
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#![feature(core_intrinsics)]
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#![feature(deprecated_suggestion)]
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#![feature(dispatch_from_dyn)]
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#![feature(error_generic_member_access)]
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#![feature(error_in_core)]
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#![feature(exact_size_is_empty)]
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#![feature(extend_one)]
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#![feature(fmt_internals)]
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#![feature(fn_traits)]
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#![feature(generic_nonzero)]
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#![feature(hasher_prefixfree_extras)]
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#![feature(hint_assert_unchecked)]
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#![feature(inline_const)]
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#![feature(inplace_iteration)]
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#![feature(iter_advance_by)]
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#![feature(iter_next_chunk)]
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#![feature(iter_repeat_n)]
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#![feature(layout_for_ptr)]
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#![feature(local_waker)]
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#![feature(maybe_uninit_slice)]
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#![feature(maybe_uninit_uninit_array)]
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#![feature(maybe_uninit_uninit_array_transpose)]
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#![feature(non_null_convenience)]
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#![feature(panic_internals)]
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#![feature(pattern)]
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#![feature(ptr_internals)]
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#![feature(ptr_metadata)]
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#![feature(ptr_sub_ptr)]
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#![feature(receiver_trait)]
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#![feature(set_ptr_value)]
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#![feature(sized_type_properties)]
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#![feature(slice_from_ptr_range)]
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#![feature(slice_index_methods)]
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#![feature(slice_ptr_get)]
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#![feature(slice_ptr_len)]
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#![feature(slice_range)]
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#![feature(std_internals)]
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#![feature(str_internals)]
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#![feature(strict_provenance)]
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#![feature(trusted_fused)]
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#![feature(trusted_len)]
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#![feature(trusted_random_access)]
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#![feature(try_trait_v2)]
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#![feature(try_with_capacity)]
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#![feature(tuple_trait)]
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#![feature(unchecked_math)]
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#![feature(unicode_internals)]
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#![feature(unsize)]
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#![feature(utf8_chunks)]
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// tidy-alphabetical-end
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//
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// Language features:
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// tidy-alphabetical-start
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#![cfg_attr(bootstrap, feature(associated_type_bounds))]
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#![cfg_attr(not(test), feature(coroutine_trait))]
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#![cfg_attr(test, feature(panic_update_hook))]
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#![cfg_attr(test, feature(test))]
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#![feature(allocator_internals)]
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#![feature(allow_internal_unstable)]
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#![feature(c_unwind)]
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#![feature(cfg_sanitize)]
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#![feature(const_mut_refs)]
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#![feature(const_precise_live_drops)]
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#![feature(const_ptr_write)]
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#![feature(const_trait_impl)]
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#![feature(const_try)]
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#![feature(decl_macro)]
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#![feature(dropck_eyepatch)]
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#![feature(exclusive_range_pattern)]
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#![feature(fundamental)]
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#![feature(hashmap_internals)]
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#![feature(lang_items)]
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#![feature(min_specialization)]
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#![feature(multiple_supertrait_upcastable)]
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#![feature(negative_impls)]
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#![feature(never_type)]
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#![feature(pointer_is_aligned)]
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#![feature(rustc_allow_const_fn_unstable)]
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#![feature(rustc_attrs)]
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#![feature(slice_internals)]
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#![feature(staged_api)]
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#![feature(stmt_expr_attributes)]
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#![feature(unboxed_closures)]
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#![feature(unsized_fn_params)]
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#![feature(with_negative_coherence)]
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// tidy-alphabetical-end
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//
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// Rustdoc features:
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#![feature(doc_cfg)]
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#![feature(doc_cfg_hide)]
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// Technically, this is a bug in rustdoc: rustdoc sees the documentation on `#[lang = slice_alloc]`
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// blocks is for `&[T]`, which also has documentation using this feature in `core`, and gets mad
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// that the feature-gate isn't enabled. Ideally, it wouldn't check for the feature gate for docs
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// from other crates, but since this can only appear for lang items, it doesn't seem worth fixing.
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#![feature(intra_doc_pointers)]
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// Allow testing this library
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#[cfg(test)]
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#[macro_use]
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extern crate std;
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#[cfg(test)]
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extern crate test;
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#[cfg(test)]
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mod testing;
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// Module with internal macros used by other modules (needs to be included before other modules).
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#[macro_use]
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mod macros;
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mod raw_vec;
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// Heaps provided for low-level allocation strategies
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pub mod alloc;
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// Primitive types using the heaps above
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// Need to conditionally define the mod from `boxed.rs` to avoid
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// duplicating the lang-items when building in test cfg; but also need
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// to allow code to have `use boxed::Box;` declarations.
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#[cfg(not(test))]
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pub mod boxed;
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#[cfg(test)]
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mod boxed {
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pub use std::boxed::Box;
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}
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pub mod borrow;
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pub mod collections;
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#[cfg(all(not(no_rc), not(no_sync), not(no_global_oom_handling)))]
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pub mod ffi;
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pub mod fmt;
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#[cfg(not(no_rc))]
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pub mod rc;
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pub mod slice;
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pub mod str;
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pub mod string;
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#[cfg(all(not(no_rc), not(no_sync), target_has_atomic = "ptr"))]
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pub mod sync;
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#[cfg(all(not(no_global_oom_handling), not(no_rc), not(no_sync)))]
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pub mod task;
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#[cfg(test)]
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mod tests;
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pub mod vec;
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#[doc(hidden)]
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#[unstable(feature = "liballoc_internals", issue = "none", reason = "implementation detail")]
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pub mod __export {
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pub use core::format_args;
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}
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#[cfg(test)]
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#[allow(dead_code)] // Not used in all configurations
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pub(crate) mod test_helpers {
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/// Copied from `std::test_helpers::test_rng`, since these tests rely on the
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/// seed not being the same for every RNG invocation too.
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pub(crate) fn test_rng() -> rand_xorshift::XorShiftRng {
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use std::hash::{BuildHasher, Hash, Hasher};
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let mut hasher = std::hash::RandomState::new().build_hasher();
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std::panic::Location::caller().hash(&mut hasher);
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let hc64 = hasher.finish();
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let seed_vec =
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hc64.to_le_bytes().into_iter().chain(0u8..8).collect::<crate::vec::Vec<u8>>();
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let seed: [u8; 16] = seed_vec.as_slice().try_into().unwrap();
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rand::SeedableRng::from_seed(seed)
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}
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}
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