Comment by naruhodo

The rabbit hole I just went down is called C/C++ Statement Expressions [1] which are a GCC extension:

    #define FAST_STATIC(T) \
      *({ \
          \                                // statements separated by semicolons
          reinterpret_cast<T *>(ph.buf); \ // the value of the macro as a statement
       })

The reinterpret_cast<T*>(...) statement is a conventional C++ Expression Statement, but when enclosed in ({}), GCC considers the whole kit and kaboodle a Statement Expression that propagates a value.

There is no C equivalent, but in in C++, since C++11 you can achieve the same effect with lambdas:

    auto value = [](){ return 12345; }();

As noted in the linked SO discussion, this is analogous to a JS Immediately-Invoked Function Expression (IIFE).

[1] https://stackoverflow.com/questions/76890861/what-is-called-...