[swift-evolution] String(validatingUTF8:) doesn't

[Drew Crawford]

I have just now for the second time root-caused yet another "nasty nasty UB bug" from yet another developer who got cut on the very sharp edge of this API:

    var result = [40,50,60] as [Int8]
    return String(validatingUTF8: result)

This poorly-named String constructor does not take a Swift array of UTF8 bytes, it takes an UnsafePointer to a C string.  When that C string is not null-terminated (as shown here), UB ensues.

I believe *at least* we need a sane name for this constructor like String(validatingUTF8CString:) that vaguely suggests what the programmer can do to avoid UB.

I further believe that this API is just plain bad, but swift-dev disagrees and so in the interests of doing something to stop the bleeding I propose we rename.

Drew
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