Auto merge of #116447 - oli-obk:gen_fn, r=compiler-errors

Implement `gen` blocks in the 2024 edition

Coroutines tracking issue https://github.com/rust-lang/rust/issues/43122
`gen` block tracking issue https://github.com/rust-lang/rust/issues/117078

This PR implements `gen` blocks that implement `Iterator`. Most of the logic with `async` blocks is shared, and thus I renamed various types that were referring to `async` specifically.

An example usage of `gen` blocks is

```rust
fn foo() -> impl Iterator<Item = i32> {
    gen {
        yield 42;
        for i in 5..18 {
            if i.is_even() { continue }
            yield i * 2;
        }
    }
}
```

The limitations (to be resolved) of the implementation are listed in the tracking issue

compiler/rustc_ast/src/ast.rs

@@ -1235,7 +1235,7 @@ impl Expr {
             ExprKind::Closure(..) => ExprPrecedence::Closure,
             ExprKind::Block(..) => ExprPrecedence::Block,
             ExprKind::TryBlock(..) => ExprPrecedence::TryBlock,
-            ExprKind::Async(..) => ExprPrecedence::Async,
+            ExprKind::Gen(..) => ExprPrecedence::Gen,
             ExprKind::Await(..) => ExprPrecedence::Await,
             ExprKind::Assign(..) => ExprPrecedence::Assign,
             ExprKind::AssignOp(..) => ExprPrecedence::AssignOp,
@@ -1405,11 +1405,9 @@ pub enum ExprKind {
     Closure(Box<Closure>),
     /// A block (`'label: { ... }`).
     Block(P<Block>, Option<Label>),
-    /// An async block (`async move { ... }`).
-    ///
-    /// The async block used to have a `NodeId`, which was removed in favor of
-    /// using the parent `NodeId` of the parent `Expr`.
-    Async(CaptureBy, P<Block>),
+    /// An `async` block (`async move { ... }`),
+    /// or a `gen` block (`gen move { ... }`)
+    Gen(CaptureBy, P<Block>, GenBlockKind),
     /// An await expression (`my_future.await`). Span is of await keyword.
     Await(P<Expr>, Span),
 
@@ -1499,6 +1497,28 @@ pub enum ExprKind {
     Err,
 }
 
+/// Used to differentiate between `async {}` blocks and `gen {}` blocks.
+#[derive(Clone, Encodable, Decodable, Debug, PartialEq, Eq)]
+pub enum GenBlockKind {
+    Async,
+    Gen,
+}
+
+impl fmt::Display for GenBlockKind {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.modifier().fmt(f)
+    }
+}
+
+impl GenBlockKind {
+    pub fn modifier(&self) -> &'static str {
+        match self {
+            GenBlockKind::Async => "async",
+            GenBlockKind::Gen => "gen",
+        }
+    }
+}
+
 /// The explicit `Self` type in a "qualified path". The actual
 /// path, including the trait and the associated item, is stored
 /// separately. `position` represents the index of the associated
@@ -2363,6 +2383,12 @@ pub enum Async {
     No,
 }
 
+#[derive(Copy, Clone, Encodable, Decodable, Debug)]
+pub enum Gen {
+    Yes { span: Span, closure_id: NodeId, return_impl_trait_id: NodeId },
+    No,
+}
+
 impl Async {
     pub fn is_async(self) -> bool {
         matches!(self, Async::Yes { .. })