Add support for Yield

src/expr.rs

@@ -181,6 +181,17 @@ pub fn format_expr(
                 )
             }
         }
+        ast::ExprKind::Yield(ref opt_expr) => {
+            if let Some(ref expr) = *opt_expr {
+                rewrite_unary_prefix(context, "yield ", &**expr, shape)
+            } else {
+                wrap_str(
+                    "yield".to_string(),
+                    context.config.max_width(),
+                    shape,
+                )
+            }
+        }
         ast::ExprKind::Closure(capture, ref fn_decl, ref body, _) => {
             rewrite_closure(capture, fn_decl, body, expr.span, context, shape)
         }