No warning on returning pointer to stack

[ellistg]

Was writing some benchmarks the other day and made the mistake of returning a pointer to the stack, what I did specifically was create an array or random u8's and attempted to return str[0..];, I can see now that this is incorrect but seems like something that the compiler/debug mode should warn or error.

Currently it returns the value but if you attempt to access it will just be random data. I don't know how possible this is and am fairly new to low level programming so it may just be something that is not worth having, but thought it was worth mentioning.

[Vexu]

Duplicate of #2646.