[ninja author] I did some thinking about this problem and eventually revisited with what I think is a pretty neat solution. I wrote about it here: https://neugierig.org/software/blog/2022/03/n2.html

Imagine if filesystems had exposed the file hash next to its mtime.

	▲	oftenwrong 2 hours ago \| parent [-]
		I might be missing your sarcasm, but this is a common approach for large scale builds. Virtual filesystems are used to provide a pre-computed tree hash as a xattr. In a more typical case, you can read the git tree hash.