error[E0507]: cannot move out of `a`, a captured variable in an `FnMut` closure
  --> tests/ui/move-captured-variable.rs:12:9
   |
10 |       let a = String::new();
   |           - captured outer variable
11 |       foo(|| {
   |           -- captured by this `FnMut` closure
12 | /         stream_block! { //~ ERROR cannot move out of `a`, a captured variable in an `FnMut` closure
13 | |             yield a
   | |                   -
   | |                   |
   | |                   variable moved due to use in coroutine
   | |                   move occurs because `a` has type `String`, which does not implement the `Copy` trait
14 | |         };
   | |_________^ `a` is moved here
   |
   = note: this error originates in the macro `stream_block` (in Nightly builds, run with -Z macro-backtrace for more info)