String constructor can segfault

[vtjnash]

julia> String(convert(Ptr{UInt8}, 1))
Segmentation fault: 11

expected result: MethodError, "Ptr{UInt8}(1)", anything but crashing?

Julia constructors shouldn't segfault, only methods that can should have unsafe_ in the name

[nalimilan]

Cf. discussion following #16107 (comment). I also think methods playing with pointers should all be marked unsafe_*, but @stevengj has a different opinion.

[StefanKarpinski]

I don't really have a strong opinion here so whenever you guys decide what we should call this, I'll change it. If this is going to go out of the String constructor, then it should be part of the same function as pointer_to_string, which is a terrible name and also doesn't start with unsafe_.

[stevengj]

(If we rename pointer_to_string, we should also rename pointer_to_array.)

Maybe just unsafe_string(ptr, [len]) for the copying constructor (analogous to the old bytestring(ptr, len) function). It should be short, since this is used all over the place.

[nalimilan]

We could also have a more generic API, which would allow extensions for custom string types. Something like unsafe_constructor(String, ptr, [len]) and unsafe_constructor(Array, ptr, dims).