Reduce encoded length of UTF-8 encoded Ruby string in C extension

Question

I'm writing a Ruby extension in C. It's a string processing module working on UTF-8 encoded strings only.

One method, full_width_to_ascii!, converts full width characters to ASCII equivalents (https://en.wikipedia.org/wiki/Halfwidth_and_fullwidth_forms). Essentially, it subtracts an offset of 0xfee0 from any full width characters. As the bang in the name implies, it works on the string in-place.

For example:

full_width_to_ascii!('Ａ Ｂ Ｃ')
=> 'A B C'

In UTF-8, all full-width characters are encoded in 3 bytes each, while ASCII characters are 1 byte each. So while the length of the resulting string in Unicode code points will always be the same, the new UTF-8 encoded length in bytes may be smaller.

I modify the encoded string data (retrieved using StringValueCStr()) and null-terminate it at the new end-point. Finally, I call the following function to reduce the encoded string length:

// from internal.h from MRI
#define STR_NOEMBED      FL_USER1
#define STR_SHARED       FL_USER2 /* = ELTS_SHARED */
#define STR_EMBED_P(str) (!FL_TEST_RAW((str), STR_NOEMBED))

void reduce_encoded_length(VALUE str, int length) {
  if (!STR_EMBED_P(str)) {
    RSTRING(str)->as.heap.len = length;
  } else {
    // see string.c, STR_SET_EMBED_LEN
    RBASIC(str)->flags &= ~RSTRING_EMBED_LEN_MASK;
    RBASIC(str)->flags |= (length) << RSTRING_EMBED_LEN_SHIFT;
  }
}

Looking at the MRI source, the implementation of strings is surprisingly complicated, but from my understanding so far, the function above should be safe for UTF-8 encoded strings.

Does this seem reasonable? Will it break horribly on older Ruby versions? Across platforms?

See also:

https://github.com/ruby/ruby/blob/32674b167bddc0d737c38f84722986b0f228b44b/string.c http://patshaughnessy.net/2012/1/4/never-create-ruby-strings-longer-than-23-characters