Hello Kris

It wouldn't help to relocate the build folder elsewhere, because the decision to recompile is based on the comparison of the source and object file's creation and modification times. As unity rewrites the source files, they get a new creation/modification time which is in all cases more recent than the last build's corresponding object files. The build system sees it and decides to recompile the file.

There is however a caching system underneath the compiler, called ccache (for compiler cache) ; if it detects that the checksum of the contents of the source file plus all of its include is identical to the checksum that was saved the last time that source file was built, the object file is not recompiled but instead pulled from the compiler cache.

It's faster than recompiling, but still, checksumming all that content with all headers included takes some time. There's no ideal solution to this problem unfortunately. What could be done here would be totally on Unity's side.

For example, they could keep some track of the checksum of the file(s) generated by the editor, and in case this checksum doesn't change compared to the last time it was created, set the file's creation and modification times to the date and time it was first created. Thus, any build system would deduce that it doesn't need to be recompiled.

I like that kind of outsmarting stuff :) The sad thing is the thought that it could be such a simple thing to implement on Unity's side that you're working out 10x the force it would require them to achieve the same goal.

Anyway, about your observations:

These duplicate entries in HEADER_SEARCH_PATH are benign. They'll simply be passed as duplicated -I<include_path> argument to the compiler. It would certainly become a problem if you ended up passing hundreds of them, but at that point I suppose your project would have changed significantly enough for you to have triggered a total rebuild already, resetting the counters.

These duplicate entries in OTHER_LDFLAGS are benign too, they'll be passed verbatim to the linker. -lc++ simply tell the linker to link against "libc++.dylib". It won't link against the same libraries multiple times, it'll just discard the repeated arguments.

The post-build check I advise you to make is simply to remove any two identical consecutive lines in the pbxproj file. Problem solved :)

And to handle clashing flags (example 2), you could extend that logic by attempting to split each line as "key = value;", and remove any two consecutive lines that begin with an identical "key = " prefix.

As for where they come from, Unity has their own library for constructing/appending to pbxproj files. These files follow the "ASCII plist" format invented by NeXT in the 1990s. "(" denotes the start of an array value and "{" a dictionary. It simply looks like Unity's library is not able to handle duplicates in arrays and dictionaries.

Hope it helps :)