A way to dynamically replace specific Possibility with a noop without having to completely reevaluate the entire chain of Possibility

[Seelengrab]

This was brought up by @tecosaur. The general idea is that it's sometimes useful in manual exploration to temporarily "remove" a part of the statespace, so that certain generation can be more targeted towards specific errors.

I suggested the following:

mutable struct Toggleable{T} <: Possibility{T}
    const inner::Possibility{Some{T}}
    replacement::Option{T}
    Toggleable(pos::Possibility{T}; replacement::Option{T}=nothing)  where T = new{T}(map(Some, pos), active)
end

Data.produce!(tc::TestCase, t::Toggleable) = @something t.replacement Data.produce!(tc, tc.inner)

This is kind of similar to Data.Just, except that the just part only happens when replacement is actually set. One caveat with this approach is that it's kind of bad to change what a Possibility can produce mid-generation, so if possible I'd like to prevent that. In particular, it's an issue if one run happens to set replacement, which causes the test to pass. Supposition doesn't really have knowledge about this being set, so it can't unset it to get its proper shrinks back. Could be enough to sternly warn people not to do this, and/or could there be some way to detect this being set inside a @check? Alternatively, enforce that there is always a valid replacement present, and make the shrinker aware of having a choice which it uses (preferring inner if allowed).

That would look like so:

mutable struct Toggleable{T} <: Possibility{T}
    const inner::Possibility{T}
    const replacement::T
    sample::Bool
    Toggleable(pos::Possibility{T}, replacement::T; sample=true)  where T = new{T}(pos, replacement, sample)
end

function Data.produce!(tc::TestCase, t::Toggleable)
    sample = isone(Supposition.forced_choice!(tc, UInt(t.sample)))
    if sample
        Data.produce!(tc, t.inner)
    else
        t.replacement
    end
end

Which should allow the shrinker to choose which element to produce, shrinking towards the replacement.