The following test:

$ cat foo-bar.h

struct {
   int x;
   int y;
} foo;

Generates layout annotations with invalid names:

$ jextract -d classes foo-bar.h

$ javap -v classes/foo_bar_h\$anon\$foo_bar_h\$0.class | grep \(anon
  #38 = Utf8 [i32(x)i32(y)](anon$foo-bar_h$0)
      value="[i32(x)i32(y)](anon$foo-bar_h$0)"

See the "-" character, which is not allowed in the production for layout name.