make range indexing into a String produce a SubString (consistent with the generic fallback)

[KristofferC]

Ref #59828 (comment).

It doesn't make sense to me if the generic fallback is to produce a sub string that String should not follow this.

But I guess this will lead to a lot of code that returns Union{String, SubString{String}}. But it will allocate less. Don't know if it is worth it or not but thought I would put up a PR at least.

cc @jakobnissen

[jakobnissen]

Having indexing into immutable strings return a view is reasonable, but this is going to be massively breaking. We can't actually change the type of string slicing. There is no way this doesn't result in hundreds of packages failing.

To bad. I do agree it was a mistake to have string indexing copy.

[KristofferC]

@nanosoldier runtests()

[nanosoldier]

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[aplavin]

FWIW, getindex on other immutable types returns the same type: at least Tuple, NamedTuple, StaticArrays. Neither returns a different type of SubXXX.

[KristofferC]

Those don't need an allocation so there is no need for a cure concept there.