we could provide hooks (via user defined function delegates) in the compiler when some action happen, eg: opening a file to be compiled, parsing an import, processing an expression etc.

This would allow for example writing user defined callbacks to interactively visualize on a webserver the number of [files,functions,symbols,LOC, mixin expansions, CTFE execution] being compiled in realtime, warn when a file about to be compiled doesn't match some regular expression, and many other use cases (see video)