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Auto merge of #118847 - eholk:for-await, r=compiler-errors

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Add support for `for await` loops

This adds support for `for await` loops. This includes parsing, desugaring in AST->HIR lowering, and adding some support functions to the library.

Given a loop like:
```rust
for await i in iter {
    ...
}
```
this is desugared to something like:
```rust
let mut iter = iter.into_async_iter();
while let Some(i) = loop {
    match core::pin::Pin::new(&mut iter).poll_next(cx) {
        Poll::Ready(i) => break i,
        Poll::Pending => yield,
    }
} {
    ...
}
```

This PR also adds a basic `IntoAsyncIterator` trait. This is partly for symmetry with the way `Iterator` and `IntoIterator` work. The other reason is that for async iterators it's helpful to have a place apart from the data structure being iterated over to store state. `IntoAsyncIterator` gives us a good place to do this.

I've gated this feature behind `async_for_loop` and opened #118898 as the feature tracking issue.

r? `@compiler-errors`
This commit is contained in:
bors 2023-12-22 14:17:10 +00:00
commit 208dd2032b
34 changed files with 367 additions and 79 deletions
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2
compiler/rustc_hir/src/lang_items.rs
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@ -307,6 +307,8 @@ language_item_table! {
Context, sym::Context, context, Target::Struct, GenericRequirement::None;
FuturePoll, sym::poll, future_poll_fn, Target::Method(MethodKind::Trait { body: false }), GenericRequirement::None;
AsyncIteratorPollNext, sym::async_iterator_poll_next, async_iterator_poll_next, Target::Method(MethodKind::Trait { body: false }), GenericRequirement::Exact(0);
Option, sym::Option, option_type, Target::Enum, GenericRequirement::None;
OptionSome, sym::Some, option_some_variant, Target::Variant, GenericRequirement::None;
OptionNone, sym::None, option_none_variant, Target::Variant, GenericRequirement::None;