isearch-search-fun-function

isearch-search-fun-function is a variable defined in `isearch.el'.
Its value is

isearch-search-fun-default

• This variable may be risky if used as a file-local variable.

Documentation:
Non-default value overrides the behavior of `isearch-search-fun-default'.
This variable's value should be a function, which will be called
with no arguments, and should return a function that takes three
arguments: STRING, BOUND, and NOERROR.

This returned function will be used by `isearch-search-string' to
search for the first occurrence of STRING or its translation.