Add the minimum needed for a yield_expr feature

Author: eholk

compiler/rustc_ast_lowering/src/expr.rs

@@ -1690,6 +1690,16 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let yielded =
             opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
 
+        if !self.tcx.features().yield_expr() {
+            rustc_session::parse::feature_err(
+                &self.tcx.sess,
+                sym::yield_expr,
+                span,
+                fluent_generated::ast_lowering_yield,
+            )
+            .emit();
+        }
+
         let is_async_gen = match self.coroutine_kind {
             Some(hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Gen, _)) => false,
             Some(hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::AsyncGen, _)) => true,
@@ -1714,28 +1724,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     None,
                 );
             }
-            Some(hir::CoroutineKind::Coroutine(_)) => {
-                if !self.tcx.features().coroutines() {
-                    rustc_session::parse::feature_err(
-                        &self.tcx.sess,
-                        sym::coroutines,
-                        span,
-                        fluent_generated::ast_lowering_yield,
-                    )
-                    .emit();
-                }
-                false
-            }
+            Some(hir::CoroutineKind::Coroutine(_)) => false,
             None => {
-                if !self.tcx.features().coroutines() {
-                    rustc_session::parse::feature_err(
-                        &self.tcx.sess,
-                        sym::coroutines,
-                        span,
-                        fluent_generated::ast_lowering_yield,
-                    )
-                    .emit();
-                }
                 let suggestion = self.current_item.map(|s| s.shrink_to_lo());
                 self.dcx().emit_err(YieldInClosure { span, suggestion });
                 self.coroutine_kind = Some(hir::CoroutineKind::Coroutine(Movability::Movable));

compiler/rustc_feature/src/unstable.rs

@@ -669,6 +669,7 @@ declare_features! (
     (unstable, xop_target_feature, "1.81.0", Some(127208)),
     /// Allows `do yeet` expressions
     (unstable, yeet_expr, "1.62.0", Some(96373)),
+    (unstable, yield_expr, "CURRENT_RUSTC_VERSION", Some(43122)),
     // !!!!    !!!!    !!!!    !!!!   !!!!    !!!!    !!!!    !!!!    !!!!    !!!!    !!!!
     // Features are listed in alphabetical order. Tidy will fail if you don't keep it this way.
     // !!!!    !!!!    !!!!    !!!!   !!!!    !!!!    !!!!    !!!!    !!!!    !!!!    !!!!

tests/ui/feature-gates/feature-gate-yield-expr.rs

@@ -0,0 +1,9 @@
+//@ edition: 2024
+#![feature(coroutines, stmt_expr_attributes)]
+
+fn main() {
+    let _ = #[coroutine] || {
+        yield ();
+        //~^ ERROR yield syntax is experimental
+    };
+}

tests/ui/feature-gates/feature-gate-yield-expr.stderr

@@ -0,0 +1,13 @@
+error[E0658]: yield syntax is experimental
+  --> $DIR/feature-gate-yield-expr.rs:6:9
+   |
+LL |         yield ();
+   |         ^^^^^^^^
+   |
+   = note: see issue #43122 <https://github.com/rust-lang/rust/issues/43122> for more information
+   = help: add `#![feature(yield_expr)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.