Consider the following code:

s = "123"
v = Vector{UInt8}(s)

Now changing v mutates s.
Since strings should be immutable I would it find it more natural to:

• define reinterpret(UInt8, s::String) that would perform such in-place conversion (as this is what one would expect from reinterpret);
• make convert(Vector{UInt8}, s::String) allocate a new vector (so that users do not accidentally mutate strings).

Additionally this would be consistent with the general behavior that Vector{UInt8}(s) for AbstractString other than String would create a copy.

A non-breaking alternative would be to add an information to the documentation that this conversion is in-place.