error: expected expression, found `let` statement
  --> $DIR/macro-expanded.rs:7:20
   |
LL |     ($e:expr) => { let Some(x) = $e }
   |                    ^^^
...
LL |         () if m!(Some(5)) => {}
   |               ----------- in this macro invocation
   |
   = note: only supported directly in conditions of `if` and `while` expressions
   = note: this error originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 1 previous error