Comment by Joker_vD

I am pretty certain the following is a valid "for"-loop translation:

    block
        ;; code for "i = 0"
        loop
            ;; code for "i < 5"
            i32.eqz
            br_if 1
        
            i32.const 1
            loop
                if
                    ;; code for "i = i + 1"
                    br 2
                else
                end
        
                ;; code for "j = j * 2 + 1"

                i32.const 0
            end
        end
    end

It doesn't require cloning the lexer so probably would still fit in 500 lines? But yeah, in normal assembly it's way easier, even in one-pass:

        ;; code for "i = 0"
    .loop_test:
        ;; code for "i < 5"
        jz  .loop_end
        jmp .loop_body
    .loop_incr:
        ;; code for "i = i + 1"
        jmp .loop_test
    .loop_body:
        ;; code for "j = j * 2 + 1"
        jmp .loop_incr
    .loop_end:

Of course, normally you'd want to re-arrange things like so:

        ;; code for "i = 0"
        jmp .loop_test
    .loop_body:
        ;; code for "j = j * 2 + 1"
    .loop_incr:
        ;; code for "i = i + 1"
    .loop_test:
        ;; code for "i < 5"
        jnz .loop_body
    .loop_end:

I propose the better loop syntax for languages with one-pass implementations, then: "for (i = 0) { j = j * 2 + 1; } (i = i + 1; i < 5);" :)