It should be possible to remove this part of the lib (and its external dependencies) using a CMake option.

Numeric limits returns 0...

Possible implementation:

template<typename T>
struct Pinned : T {
    using T::T;

    Pinned(Pinned const&) = delete;
    Pinned(Pinned&&) = delete;
    Pinned& operator=(Pinned const&) = delete;
    Pinned& operator=(Pinned&&) = delete;
    ~Pinned() = default;
};

see: https://godbolt.org/z/a8j55KE4x

Also add static_asserts that ensure we do not violate strict aliasing rules (meaning: types like utf8proc_uint8_t have to be type aliases to types with aliasing rule exceptions, presumably char, unsigned char, signed char, and std::byte, but verify these rules beforehand).

Some thing like this should work:

auto const str = "test"_utf8;
auto it = str.cbegin();
it = it + 1;
it += 1;
it = it - 1;
it -= 1;
EXPECT_EQ(*it, 't'_utf8);

Comparing the raw bytes is enough here.