Dependent blocks

[timholy]

Revise's "top-level block" design does not allow it to do the right thing when flag is changed below:

flag = false
if flag
    f() = 1
else
    f() = 2
end

Revise will happily update flag but the consequences will not propagate to f, as f is defined in a different :block expression.

In case anyone thinks this is easy, consider

flag = false
if flag
    f() = 1
else
    f() = 2
end

flag = true
ccall(:initialize_a_c_library_that_will_crash_julia_if_initialized_twice, Any, (CInt,), Cint(0))
push!(container, "I really didn't want two copies of this string in `container`")
if flag
    g() = 1
else
    g() = 2
end

Now just update the second time flag is set and imagine trying to come up with the right definition of both f and g without crashing Julia. And make it work if the definitions of g are in a different file from this one, etc.

The whole backedges.jl infrastructure lays the groundwork for this, but there is still a lot to do. And it's also possible that this shouldn't be fixed at all; imagine moving the push! command inside the flag block; should it be run again or not?