error: expected `{`, found identifier `foo`
  --> $DIR/issue-39848.rs:3:21
   |
LL |         if $tgt.has_$field() {}
   |                     ^^^^^^ expected `{`
...
LL |     get_opt!(bar, foo);
   |     ------------------ in this macro invocation
   |
note: the `if` expression is missing a block after this condition
  --> $DIR/issue-39848.rs:3:12
   |
LL |         if $tgt.has_$field() {}
   |            ^^^^^^^^^
...
LL |     get_opt!(bar, foo);
   |     ------------------ in this macro invocation
   = note: this error originates in the macro `get_opt` (in Nightly builds, run with -Z macro-backtrace for more info)
help: try placing this code inside a block
   |
LL |         if $tgt.has_{ $field() } {}
   |                     +          +

error: aborting due to 1 previous error