`Data.Text.isSingleton` accesses out of bounds memory for zero length input

[sol]

While reading some unrelated code I got distracted by this:

text/src/Data/Text.hs

Lines 634 to 638 in 1be3f68

	-- | /O(1)/ Tests whether a 'Text' contains exactly one character.
	isSingleton :: Text -> Bool
	isSingleton (Text arr off len) =
	len == utf8LengthByLeader (A.unsafeIndex arr off)
	{-# INLINE isSingleton #-}

(introduced in acf18b6)

Now it's currently only used in places where the input can never be zero length + it's not exported. I still see the risk that someone will use this without being aware of the unsafe nature at some point (or even worse, export it).

So maybe:

• rename this to unsafeIsSingleton?
• remove the Haddocks to emphasize that this is an internal function
• maybe even add a comment that states that the input must be non-zero length, but then again it's pretty obvious from the implementation, so maybe skip that

[Bodigrim]

You prescribe me more diligence than is due ;) It never crossed my mind that Data.Text.isSingleton isn't used externally, I just introduced a segfault without realising it. I think a proper fix would be to check len /= 0 && len == utf8LengthByLeader ....

[Bodigrim]

@sol thanks for catching it!