Preserve yield position during pretty printing

compiler/rustc_ast/src/ast.rs

@@ -1657,7 +1657,7 @@ pub enum ExprKind {
     Try(P<Expr>),
 
     /// A `yield`, with an optional value to be yielded.
-    Yield(Option<P<Expr>>),
+    Yield(Option<P<Expr>>, YieldKind),
 
     /// A `do yeet` (aka `throw`/`fail`/`bail`/`raise`/whatever),
     /// with an optional value to be returned.
@@ -1903,6 +1903,15 @@ pub enum MatchKind {
     Postfix,
 }
 
+/// The kind of yield expression
+#[derive(Clone, Copy, Encodable, Decodable, Debug, PartialEq)]
+pub enum YieldKind {
+    /// yield expr { ... }
+    Prefix,
+    /// expr.yield { ... }
+    Postfix,
+}
+
 /// A literal in a meta item.
 #[derive(Clone, Encodable, Decodable, Debug, HashStable_Generic)]
 pub struct MetaItemLit {