From edf65e735cd871d01149131f5d050293a9f1037c Mon Sep 17 00:00:00 2001
From: Eric Holk <ericholk@microsoft.com>
Date: Wed, 12 Mar 2025 15:59:28 -0700
Subject: [PATCH] Add support for postfix yield expressions

We had a discussion[1] today about whether postfix yield would make sense.
It's easy enough to support both in the parser, so we might as well have
both and see how people use it while the feature is experimental.

[1]: https://rust-lang.zulipchat.com/#narrow/channel/481571-t-lang.2Fgen/topic/postfix-yield/with/505231568
---
 compiler/rustc_parse/src/parser/expr.rs |  7 +++++
 tests/ui/coroutine/postfix-yield.rs     | 34 +++++++++++++++++++++++++
 2 files changed, 41 insertions(+)
 create mode 100644 tests/ui/coroutine/postfix-yield.rs

diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index 9b2d562a69ec..1df6283af264 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -1310,6 +1310,13 @@ impl<'a> Parser<'a> {
             return self.parse_match_block(lo, match_span, self_arg, MatchKind::Postfix);
         }
 
+        // Post-fix yield
+        if self.eat_keyword(exp!(Yield)) {
+            let yield_span = self.prev_token.span;
+            self.psess.gated_spans.gate(sym::yield_expr, yield_span);
+            return Ok(self.mk_expr(yield_span, ExprKind::Yield(Some(self_arg))));
+        }
+
         let fn_span_lo = self.token.span;
         let mut seg = self.parse_path_segment(PathStyle::Expr, None)?;
         self.check_trailing_angle_brackets(&seg, &[exp!(OpenParen)]);
diff --git a/tests/ui/coroutine/postfix-yield.rs b/tests/ui/coroutine/postfix-yield.rs
new file mode 100644
index 000000000000..77b1c2a19d0a
--- /dev/null
+++ b/tests/ui/coroutine/postfix-yield.rs
@@ -0,0 +1,34 @@
+// This demonstrates a proposed alternate or additional option of having yield in postfix position.
+
+//@ run-pass
+//@ edition: 2024
+
+#![feature(gen_blocks, coroutines, coroutine_trait, yield_expr)]
+
+use std::ops::{Coroutine, CoroutineState};
+use std::pin::pin;
+
+fn main() {
+    // generators (i.e. yield doesn't return anything useful) 
+    let mut gn = gen {
+        yield 1;
+        2.yield;
+    };
+
+    assert_eq!(gn.next(), Some(1));
+    assert_eq!(gn.next(), Some(2));
+    assert_eq!(gn.next(), None);
+
+    //coroutines (i.e. yield returns something useful)
+    let mut coro = pin!(
+        #[coroutine]
+        |_: i32| {
+            let x = yield 1;
+            yield x + 2;
+        }
+    );
+
+    assert_eq!(coro.as_mut().resume(0), CoroutineState::Yielded(1));
+    assert_eq!(coro.as_mut().resume(2), CoroutineState::Yielded(4));
+    assert_eq!(coro.as_mut().resume(3), CoroutineState::Complete(()));
+}