Yield is an expression form, not a statement.

src/librustc_mir/borrow_check/mod.rs

@@ -1424,7 +1424,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     }
 
     /// Reports an error if this is a borrow of local data.
-    /// This is called for all Yield statements on movable generators
+    /// This is called for all Yield expressions on movable generators
     fn check_for_local_borrow(&mut self, borrow: &BorrowData<'tcx>, yield_span: Span) {
         debug!("check_for_local_borrow({:?})", borrow);
 

src/librustc_mir/borrow_check/path_utils.rs

@@ -131,7 +131,7 @@ pub(super) fn is_active<'tcx>(
 }
 
 /// Determines if a given borrow is borrowing local data
-/// This is called for all Yield statements on movable generators
+/// This is called for all Yield expressions on movable generators
 pub(super) fn borrow_of_local_data(place: &Place<'_>) -> bool {
     match place.base {
         PlaceBase::Static(_) => false,

src/librustc_typeck/check/expr.rs

@@ -1775,7 +1775,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     self.tcx.sess,
                     expr.span,
                     E0627,
-                    "yield statement outside of generator literal"
+                    "yield expression outside of generator literal"
                 )
                 .emit();
             }

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

@@ -1,6 +1,6 @@
 fn main() {
     yield true; //~ ERROR yield syntax is experimental
-                //~^ ERROR yield statement outside of generator literal
+                //~^ ERROR yield expression outside of generator literal
 }
 
 #[cfg(FALSE)]

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

@@ -25,12 +25,13 @@ 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
+error[E0627]: yield expression outside of generator literal
   --> $DIR/feature-gate-generators.rs:2:5
    |
 LL |     yield true;
    |     ^^^^^^^^^^
 
 error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0658`.
+Some errors have detailed explanations: E0627, E0658.
+For more information about an error, try `rustc --explain E0627`.

src/test/ui/generator/yield-in-const.rs

@@ -1,6 +1,6 @@
 #![feature(generators)]
 
 const A: u8 = { yield 3u8; 3u8};
-//~^ ERROR yield statement outside
+//~^ ERROR yield expression outside
 
 fn main() {}

src/test/ui/generator/yield-in-const.stderr

@@ -1,8 +1,9 @@
-error[E0627]: yield statement outside of generator literal
+error[E0627]: yield expression outside of generator literal
   --> $DIR/yield-in-const.rs:3:17
    |
 LL | const A: u8 = { yield 3u8; 3u8};
    |                 ^^^^^^^^^
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0627`.

src/test/ui/generator/yield-in-function.rs

@@ -1,4 +1,4 @@
 #![feature(generators)]
 
 fn main() { yield; }
-//~^ ERROR yield statement outside
+//~^ ERROR yield expression outside

src/test/ui/generator/yield-in-function.stderr

@@ -1,8 +1,9 @@
-error[E0627]: yield statement outside of generator literal
+error[E0627]: yield expression outside of generator literal
   --> $DIR/yield-in-function.rs:3:13
    |
 LL | fn main() { yield; }
    |             ^^^^^
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0627`.

src/test/ui/generator/yield-in-static.rs

@@ -1,6 +1,6 @@
 #![feature(generators)]
 
 static B: u8 = { yield 3u8; 3u8};
-//~^ ERROR yield statement outside
+//~^ ERROR yield expression outside
 
 fn main() {}

src/test/ui/generator/yield-in-static.stderr

@@ -1,8 +1,9 @@
-error[E0627]: yield statement outside of generator literal
+error[E0627]: yield expression outside of generator literal
   --> $DIR/yield-in-static.rs:3:18
    |
 LL | static B: u8 = { yield 3u8; 3u8};
    |                  ^^^^^^^^^
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0627`.