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

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`

compiler/rustc_ast/src/ast.rs

@@ -1249,7 +1249,7 @@ impl Expr {
             ExprKind::Let(..) => ExprPrecedence::Let,
             ExprKind::If(..) => ExprPrecedence::If,
             ExprKind::While(..) => ExprPrecedence::While,
-            ExprKind::ForLoop(..) => ExprPrecedence::ForLoop,
+            ExprKind::ForLoop { .. } => ExprPrecedence::ForLoop,
             ExprKind::Loop(..) => ExprPrecedence::Loop,
             ExprKind::Match(..) => ExprPrecedence::Match,
             ExprKind::Closure(..) => ExprPrecedence::Closure,
@@ -1411,10 +1411,10 @@ pub enum ExprKind {
     While(P<Expr>, P<Block>, Option<Label>),
     /// A `for` loop, with an optional label.
     ///
-    /// `'label: for pat in expr { block }`
+    /// `'label: for await? pat in iter { block }`
     ///
     /// This is desugared to a combination of `loop` and `match` expressions.
-    ForLoop(P<Pat>, P<Expr>, P<Block>, Option<Label>),
+    ForLoop { pat: P<Pat>, iter: P<Expr>, body: P<Block>, label: Option<Label>, kind: ForLoopKind },
     /// Conditionless loop (can be exited with `break`, `continue`, or `return`).
     ///
     /// `'label: loop { block }`
@@ -1517,6 +1517,13 @@ pub enum ExprKind {
     Err,
 }
 
+/// Used to differentiate between `for` loops and `for await` loops.
+#[derive(Clone, Copy, Encodable, Decodable, Debug, PartialEq, Eq)]
+pub enum ForLoopKind {
+    For,
+    ForAwait,
+}
+
 /// Used to differentiate between `async {}` blocks and `gen {}` blocks.
 #[derive(Clone, Encodable, Decodable, Debug, PartialEq, Eq)]
 pub enum GenBlockKind {