0xFF meaning s[i] is not the start of of UTF-8 sequence.
stride
fn
uint stride(const scope char[] s, size_t i) @safe @nogc pure nothrowstride() returns the length of a UTF-8 sequence starting at index i in string s.
Returns
The number of bytes in the UTF-8 sequence or
fn
uint stride(const scope wchar[] s, size_t i) @safe @nogc pure nothrowstride() returns the length of a UTF-16 sequence starting at index i in string s.
fn
uint stride(const scope dchar[] s, size_t i) @safe @nogc pure nothrowstride() returns the length of a UTF-32 sequence starting at index i in string s.
Returns
The return value will always be 1.