I'm fairly certain that shadowing a keyword with a macro is forbidden by the standard (or invokes UB?) but this will still work for most compilers, provided LET is defined reasonably. Hm, something like

     #define LET(name, X) \
       for (int let_once_=1, name=(X); \
            let_once_; \
            let_once_=0)

may be reasonable, though tokenpasting in __LINE__ for good measure might be necessary for nesting.

Shadowing keywords is perfectly valid, although I believe you can't shadow keywords in standard headers so that compiler writers don't need to worry about it. It gives rise to some "useful" C/C++ features that should never be used, like if you want to access private members in foo.cpp, do

     #define private public 
     #include "foo.cpp"
     #undef private

I can imagine some insidious menace adding this to the top of a file:

    #define public protected

I once someone jokingly do (the second only always works in C, not C++)

    #define else
    #define struct union

Evil!