With the memory changes the UB surface of String(Vector{UInt8}) is larger about julia HOT 1 OPEN

I think it's pretty hard to accidentally cause UB with the current implementation, because as far as I can tell String(::Vector{UInt8}) will copy, unless the backing memory has been allocated using the internal Base.StringVector or Base.StringMemory methods.
Maybe in the future, String(::Vector{UInt8}) will be zero-copy for normal arrays in which case this is indeed a problem, but that is not now:

julia> v1 = zeros(UInt8, 3)
       v2 = view(v1.ref.mem, 1:3)
       s = String(v1)
       v2[1] = 0xff
       s
"\0\0\0"

Edit: I see that IOBuffer uses StringMemory internally...

from julia.