Function transcode_into 

pub fn transcode_into<F: Encoding, T: Encoding>(
    src: &[F::Unit],
    dst: &mut [T::Unit],
    case: AsciiCase,
) -> (usize, usize)

Transcodes as much of src into dst as fits, returning (units_read, units_written). Never splits a codepoint: encoding stops at the last codepoint whose output fits.

dst past the written count may be clobbered with scratch by the SIMD fast path.

When F and T are the same encoding and case is Preserve, this degrades to a bulk copy of min(src.len(), dst.len()) units and can split a trailing codepoint, mirroring the original’s memcpy shortcut.