completion-in-region-function

completion-in-region-function is a variable defined in `minibuffer.el'.
Its value is

completion--in-region

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

Documentation:
Function to perform the job of `completion-in-region'.
The function is called with 4 arguments: START END COLLECTION PREDICATE.
The arguments and expected return value are as specified for
`completion-in-region'.