With the memory changes the UB surface of String(Vector{UInt8}) is larger

[gbaraldi]

Observing/modifying the data of a Vector{UInt8} after one has called String(vec) on it has always been UB, this was usually fine because it's quite hard on a pre-Memory{T} world to have two arrays alias eachother while not being ===, this is no longer true with Memory, where one can still keep that Memory around and have it be shared by the string, which can make it possible to mutate it, causing UB.

One option woul to have String(vec) now make a copy and create something like unsafe_take! or similar like suggested in #54424 which makes it clear that this is a potentially unsafe function.

[jakobnissen]

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...