Feature request: flag to print all input files

I'm writing a drop-in replacement/wrapper for rustfmt, and I need to be able to extract the list of all files that were passed. This is hard to do in a version-stable way: I could extract all positional arguments by filtering out all conditional arguments, but future updates to rustfmt might add more optional arguments with an unknown syntax that would break this logic.

The simplest solution would be to have a flag on rustfmt (e.g. --passthrough-filenames) that simply prints the provided list of input files without doing anything else.

Start by tracing rustfmt's command-line argument handling and how positional input filenames are collected. Implement the requested --passthrough-filenames behavior so it prints all provided input files without formatting, then verify it with representative positional and optional arguments.