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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
93 lines
2.4 KiB
Rust
93 lines
2.4 KiB
Rust
#![cfg_attr(bootstrap, feature(associated_type_bounds))]
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#![feature(allocator_api)]
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#![feature(alloc_layout_extra)]
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#![feature(iter_array_chunks)]
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#![feature(assert_matches)]
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#![feature(btree_extract_if)]
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#![feature(cow_is_borrowed)]
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#![feature(const_cow_is_borrowed)]
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#![feature(const_heap)]
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#![feature(const_mut_refs)]
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#![feature(const_slice_from_raw_parts_mut)]
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#![feature(const_ptr_write)]
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#![feature(const_try)]
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#![feature(core_intrinsics)]
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#![feature(extract_if)]
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#![feature(exact_size_is_empty)]
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#![feature(generic_nonzero)]
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#![feature(linked_list_cursors)]
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#![feature(map_try_insert)]
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#![feature(new_uninit)]
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#![feature(pattern)]
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#![feature(trusted_len)]
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#![feature(try_reserve_kind)]
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#![feature(try_with_capacity)]
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#![feature(unboxed_closures)]
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#![feature(binary_heap_into_iter_sorted)]
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#![feature(binary_heap_drain_sorted)]
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#![feature(slice_ptr_get)]
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#![feature(binary_heap_as_slice)]
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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(round_char_boundary)]
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#![feature(slice_partition_dedup)]
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#![feature(string_remove_matches)]
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#![feature(const_btree_len)]
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#![feature(const_trait_impl)]
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#![feature(const_str_from_utf8)]
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#![feature(panic_update_hook)]
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#![feature(pointer_is_aligned)]
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#![feature(slice_flatten)]
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#![feature(thin_box)]
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#![feature(strict_provenance)]
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#![feature(drain_keep_rest)]
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#![feature(local_waker)]
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#![allow(internal_features)]
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#![deny(fuzzy_provenance_casts)]
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#![deny(unsafe_op_in_unsafe_fn)]
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use std::hash::{DefaultHasher, Hash, Hasher};
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mod arc;
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mod autotraits;
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mod borrow;
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mod boxed;
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mod btree_set_hash;
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mod c_str;
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mod const_fns;
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mod cow_str;
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mod fmt;
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mod heap;
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mod linked_list;
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mod rc;
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mod slice;
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mod str;
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mod string;
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mod task;
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mod thin_box;
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mod vec;
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mod vec_deque;
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fn hash<T: Hash>(t: &T) -> u64 {
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let mut s = DefaultHasher::new();
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t.hash(&mut s);
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s.finish()
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}
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// FIXME: Instantiated functions with i128 in the signature is not supported in Emscripten.
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// See https://github.com/kripken/emscripten-fastcomp/issues/169
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#[cfg(not(target_os = "emscripten"))]
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#[test]
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fn test_boxed_hasher() {
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let ordinary_hash = hash(&5u32);
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let mut hasher_1 = Box::new(DefaultHasher::new());
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5u32.hash(&mut hasher_1);
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assert_eq!(ordinary_hash, hasher_1.finish());
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let mut hasher_2 = Box::new(DefaultHasher::new()) as Box<dyn Hasher>;
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5u32.hash(&mut hasher_2);
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assert_eq!(ordinary_hash, hasher_2.finish());
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}
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