Constant propogation through slurping

[bramtayl]

Recurring through tuples with first and Base.tail is incredibly useful but tail doesn't do constant propagation. I tracked the issue down to argtail in Base, which needs a pure annotation to do constant propagation correctly.

Base.@pure argtail(x, rest...) = rest

In fact, constant propagation never seems to survive slurping without pure annotations. This seems like a useful feature to have.

[JeffBezanson]

It would be useful to post a complete test case here.

[bramtayl]

Base.@pure argtail(x, rest...) = rest
tail(x) = argtail(x...)

getindex_unrolled(into::Tuple{}, switch::Tuple{}) = ()
getindex_unrolled(into, switch::Tuple{}) = ()
getindex_unrolled(into::Tuple{}, switch) = ()

function getindex_unrolled(into, switch)
    next = getindex_unrolled(tail(into), tail(switch))
    if first(switch)
        (first(into), next...)
    else
        next
    end
end

test() = getindex_unrolled((1, "b", 3.0), (true, false, true))
Test.@inferred test()

This only works if argtail is marked as pure.

[bramtayl]

Solved by #26826?

[vtjnash]

No, that PR doesn't include detection that all(isa(Const), result.vargs) implies args[nargs] = Const(tuple((x.var for x in result.vargs)...))

[vtjnash]

Closed by #29264.