[icu-design] ICU API proposal: Add const to UReplaceableCallbacks* params in utrans API

[Peter E. <pe...@ap...>]

Dear ICU team & users,

I would like to propose the following API change for: ICU 61
Please provide feedback by: next Tuesday, 2018-02-13
Designated API reviewer: Andy
Ticket: http://bugs.icu-project.org/trac/ticket/12907 <http://bugs.icu-project.org/trac/ticket/12907> 

utrans.h includes the following functions:

void
utrans_trans(const UTransliterator* trans,
             UReplaceable* rep,
             UReplaceableCallbacks* repFunc,
             int32_t start,
             int32_t* limit,
             UErrorCode* status);

void
utrans_transIncremental(const UTransliterator* trans,
                        UReplaceable* rep,
                        UReplaceableCallbacks* repFunc,
                        UTransPosition* pos,
                        UErrorCode* status);

The proposal is to change "UReplaceableCallbacks* repFunc" in each of these to "const UReplaceableCallbacks* repFunc"; repFunc is a pointer to a caller-supplied list of function pointers that manipulate the text being transliterated, and the utrans functions do not (and should not) change those pointers. Adding const should be a safe change and will make these functions easier to use in certain contexts.

- Peter E