People ask for complex syntax for indenifier mixins and it will probably be a big mess to find a way to make it work.

What if instead you just add two mixin keywords that tell the parser how it will work and it will grab the bytes and append the raw string of your code a mixin statement

mixinheader(...) expects a {} block after it; which the parser will ignore/store for later
mixinassign(...) expects a =...; "block" after it; ditto

mixinheader("void foo()"){ bar;} gets lowered to mixin("void foo()"~"){ bar;}")