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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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10
compiler/rustc_ast_pretty/src/pprust/state/expr.rs
View file

@ -1,5 +1,6 @@
use crate::pp::Breaks::Inconsistent;
use crate::pprust::state::{AnnNode, PrintState, State, INDENT_UNIT};
use ast::ForLoopKind;
use itertools::{Itertools, Position};
use rustc_ast::ptr::P;
use rustc_ast::token;
@ -418,20 +419,23 @@ impl<'a> State<'a> {
self.space();
self.print_block_with_attrs(blk, attrs);
}
ast::ExprKind::ForLoop(pat, iter, blk, opt_label) => {
if let Some(label) = opt_label {
ast::ExprKind::ForLoop { pat, iter, body, label, kind } => {
if let Some(label) = label {
self.print_ident(label.ident);
self.word_space(":");
}
self.cbox(0);
self.ibox(0);
self.word_nbsp("for");
if kind == &ForLoopKind::ForAwait {
self.word_nbsp("await");
}
self.print_pat(pat);
self.space();
self.word_space("in");
self.print_expr_as_cond(iter);
self.space();
self.print_block_with_attrs(blk, attrs);
self.print_block_with_attrs(body, attrs);
}
ast::ExprKind::Loop(blk, opt_label, _) => {
if let Some(label) = opt_label {