rust/tests
bors 21d94a3d2c Auto merge of #122055 - compiler-errors:stabilize-atb, r=oli-obk
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
2024-03-19 00:04:09 +00:00
..
assembly Rollup merge of #121953 - jhorstmann:assembly-tests-for-masked-simd-instructions, r=workingjubilee 2024-03-12 09:03:59 -07:00
auxiliary
codegen Remove an obsolete ignore-llvm-version 2024-03-17 10:52:00 -07:00
codegen-units [AUTO_GENERATED] Migrate compiletest to use ui_test-style //@ directives 2024-02-22 16:04:04 +00:00
coverage coverage: Enable branch coverage in the branch coverage tests 2024-03-14 17:19:06 +11:00
coverage-run-rustdoc [AUTO_GENERATED] Migrate compiletest to use ui_test-style //@ directives 2024-02-22 16:04:04 +00:00
debuginfo Auto merge of #121885 - reitermarkus:generic-nonzero-inner, r=oli-obk,wesleywiser 2024-03-17 02:27:52 +00:00
incremental Update test directives for wasm32-wasip1 2024-03-11 09:36:35 -07:00
mir-opt add_retag: ensure box-to-raw-ptr casts are preserved for Miri 2024-03-18 10:32:25 +01:00
pretty Re-bless tests/pretty 2024-02-22 16:04:05 +00:00
run-make Update the minimum external LLVM to 17 2024-03-17 10:11:04 -07:00
run-make-fulldeps Rename all ParseSess variables/fields/lifetimes as psess. 2024-03-05 08:11:45 +11:00
run-pass-valgrind Update test directives for wasm32-wasip1 2024-03-11 09:36:35 -07:00
rustdoc tests 2024-03-14 14:51:01 +01:00
rustdoc-gui [AUTO_GENERATED] Migrate compiletest to use ui_test-style //@ directives 2024-02-22 16:04:04 +00:00
rustdoc-js Rollup merge of #122247 - notriddle:notriddle/search-unbox-limit, r=GuillaumeGomez 2024-03-14 15:44:32 +01:00
rustdoc-js-std rustdoc-search: add search query syntax Fn(T) -> U 2024-03-11 22:27:22 -07:00
rustdoc-json [AUTO_GENERATED] Migrate compiletest to use ui_test-style //@ directives 2024-02-22 16:04:04 +00:00
rustdoc-ui Stabilize associated type bounds 2024-03-08 20:56:25 +00:00
ui Auto merge of #122055 - compiler-errors:stabilize-atb, r=oli-obk 2024-03-19 00:04:09 +00:00
ui-fulldeps Rollup merge of #122405 - celinval:smir-new-const, r=oli-obk 2024-03-14 11:09:58 +01:00
COMPILER_TESTS.md