unused `exclude` in mypy.ini

mypy's config file 'mypy.ini supports an exclude option, which is a regex that determines which files to exclude from analysis. This provides a low-friction path to introducing static analysis to an existing (untyped) Python codebase.

However as the codebase evolves & more type safety is built in, there's currently no easy way to determine which (if any) files are unnecessarily excluded from analysis (i.e. files that could be included in analysis without triggering any errors or warnings). I wonder if we could add a flag to mypy to warn of files that can safely be included in analysis?

A point of reference here is the warn_unused_ignores option, which flags up # type: ignore comments in source code that can safely be removed.

Some potential approaches:

• Run analysis over all files (as if the exclude option didn't exist), then test the name of every file

A few questions to answer first:

• Does the exclude option double as a performance optimization?