effect overrides: Add notaskstate & refactor slightly

[Keno]

This adds an effect override for :notaskstate and shifts the
:total, meta-effect around slightly to prepare for any potential
future expansion. In particular :total now means the maximum
possible set of effects and should likely include any future additions.
The assume_effects macro learned to do negation, so we can now write
things like :total :!nothrow, which will be helpful writing tests
that care about one effect in particular, but are otherwise total.

The previous :total_may_throw, was renamed :foldable and includes
the effects required to allow compile-time constant folding. At this
point I don't anticipate the introduction of additional effects that
would affect constant-foldability, but if such an effect were introduced,
it would be added to :foldable. Note however, that :foldable
does not include :notaskstate (though as noted in the docstring,
because of the strong requirements of :consistent and :effect_free,
it is implied that anything annotated :notaskstate may be DCEd
and thus :notaskstate is implied). Nevertheless, :notaskstate is
not included in the :foldable override and future effect additions
may further separate :foldable from :total.

[aviatesk]

The :total_may_throw setting is backported to v1.8 and so this can be breaking if we don't backport it.

[aviatesk]

Suggested change

	val = true
	org_setting = setting
	while true
	if isa(setting, Expr) && setting.head === :call
	if setting.args[1] == :(!)
	val = !val
	setting = setting.args[2]
	end
	elseif isa(setting, QuoteNode) && isa(setting.value, Symbol)
	setting = setting.value
	else
	break
	end
	end
	org_setting = setting
	setting, val = compute_setting(setting)

with

function compute_setting(@nospecialize(setting), val::Bool=true)
    if isexpr(setting, :call) && settings.args[1] === :(!)
        return compute_setting(setting.args[2], !val)
    elseif isa(setting, QuoteNode)
        return compute_setting(setting.value, val)
    else
        return (setting, val)
    end
end

?

[Keno]

Sure. I'm assuming nobody will build exprs big enough that this bites us :)

[Keno]

The :total_may_throw setting is backported to v1.8 and so this can be breaking if we don't backport it.

I'll put up a PR to rename it to :foldable.

[Keno]

Hmm, I don't see it on 1.8 (https://github.com/JuliaLang/julia/blob/release-1.8/base/expr.jl#L538-L539). Is it pending in backport still?

[aviatesk]

Hmm, I don't see it on 1.8 (https://github.com/JuliaLang/julia/blob/release-1.8/base/expr.jl#L538-L539). Is it pending in backport still?

It is still on the backport branch:

julia/base/expr.jl

Line 583 in 8bca2f4

elseif setting === :total_may_throw

[Keno]

It is still on the backport branch:

Ok, then no packages could have possibly started using it, so we can just rename it to :foldable on the release branch.

[aviatesk]

LGTM.