I am trying to alter the standard logic used by fish to find filename completions. In particular, I want fish not to consider any filename that ends in a tilde (~) character, as these are emacs backup files and are not interesting.
I had assumed that the list of possible completions would be provided by a fish function, which I could then edit to remove the ones ending in tildes. But I cannot find the function. I have looked in the documentation at fishshell.com, and I have also tried functions | grep complete. What function should I be editing?
Unlike most completions file name completions is not implemented as a function; it's baked into the C++ code. See the completer_t::complete_param_expand() method in src/complete.cpp. If you can make a cogent argument for how this type of customization would be implemented I'd encourage you to open an issue.
P.S., Note that functions will not show private functions unless you invoke it as functions -a. I mention this because many completion functions are marked private by beginning their name with an underscore.