Feature gate 'yield ?' pre-expansion.

src/libsyntax/feature_gate.rs

@@ -2088,11 +2088,6 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                                        "type ascription is experimental");
                 }
             }
-            ast::ExprKind::Yield(..) => {
-                gate_feature_post!(&self, generators,
-                                  e.span,
-                                  "yield syntax is experimental");
-            }
             ast::ExprKind::TryBlock(_) => {
                 gate_feature_post!(&self, try_blocks, e.span, "`try` expression is experimental");
             }
@@ -2464,6 +2459,13 @@ pub fn check_crate(krate: &ast::Crate,
         "async closures are unstable"
     ));
 
+    for_each_in_lock(&sess.yield_spans, |span| gate_feature!(
+        &ctx,
+        generators,
+        *span,
+        "yield syntax is experimental"
+    ));
+
     let visitor = &mut PostExpansionVisitor {
         context: &ctx,
         builtin_attributes: &*BUILTIN_ATTRIBUTE_MAP,

src/libsyntax/parse/mod.rs

@@ -63,6 +63,8 @@ pub struct ParseSess {
     pub let_chains_spans: Lock<Vec<Span>>,
     // Places where `async || ..` exprs were used and should be feature gated.
     pub async_closure_spans: Lock<Vec<Span>>,
+    // Places where `yield e?` exprs were used and should be feature gated.
+    pub yield_spans: Lock<Vec<Span>>,
     pub injected_crate_name: Once<Symbol>,
 }
 
@@ -92,6 +94,7 @@ impl ParseSess {
             param_attr_spans: Lock::new(Vec::new()),
             let_chains_spans: Lock::new(Vec::new()),
             async_closure_spans: Lock::new(Vec::new()),
+            yield_spans: Lock::new(Vec::new()),
             injected_crate_name: Once::new(),
         }
     }

src/libsyntax/parse/parser/expr.rs

@@ -997,6 +997,9 @@ impl<'a> Parser<'a> {
                     } else {
                         ex = ExprKind::Yield(None);
                     }
+
+                    let span = lo.to(hi);
+                    self.sess.yield_spans.borrow_mut().push(span);
                 } else if self.eat_keyword(kw::Let) {
                     return self.parse_let_expr(attrs);
                 } else if is_span_rust_2018 && self.eat_keyword(kw::Await) {

src/test/ui/feature-gates/feature-gate-generators.rs

@@ -2,3 +2,9 @@ fn main() {
     yield true; //~ ERROR yield syntax is experimental
                 //~^ ERROR yield statement outside of generator literal
 }
+
+#[cfg(FALSE)]
+fn foo() {
+    yield; //~ ERROR yield syntax is experimental
+    yield 0; //~ ERROR yield syntax is experimental
+}

src/test/ui/feature-gates/feature-gate-generators.stderr

@@ -7,12 +7,30 @@ LL |     yield true;
    = note: for more information, see https://github.com/rust-lang/rust/issues/43122
    = help: add `#![feature(generators)]` to the crate attributes to enable
 
+error[E0658]: yield syntax is experimental
+  --> $DIR/feature-gate-generators.rs:8:5
+   |
+LL |     yield;
+   |     ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/43122
+   = help: add `#![feature(generators)]` to the crate attributes to enable
+
+error[E0658]: yield syntax is experimental
+  --> $DIR/feature-gate-generators.rs:9:5
+   |
+LL |     yield 0;
+   |     ^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/43122
+   = help: add `#![feature(generators)]` to the crate attributes to enable
+
 error[E0627]: yield statement outside of generator literal
   --> $DIR/feature-gate-generators.rs:2:5
    |
 LL |     yield true;
    |     ^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0658`.