Ninja's "-d explain" is next to useless in a large build: It says what's dirty but it's hard to relate which dirty input caused which output to be re-built. At the beginning of your build you get pages & pages of "these things are dirty" and then it starts the actual build. Also, this "explain" output is printed before ninja has realised that a lot of those things aren't actually dirty (thanks to "restat").

I've been using this in my build system to make the explain output slightly more useful: https://github.com/ninja-build/ninja/pull/2067

It's just a quick hack that took me half a day (most of which was remembering how to C++ after 8 years...) but I think it makes "-d explain" much more useful.

Really, the "explain" implementation could do with an overhaul. I'd love ninja to serialise the build graph to disk (or maybe just the subset of the graph that actually needed rebuilding). This could be augmented with the reason why each rule was run, and possibly even with the command output for each rule. Then the existing ninja tools like "graph", "browse", and "query" could optionally use this graph instead of the "build.ninja" file, so that you can inspect what happened the last time you ran "ninja".

Realistically I'll never get around to implementing this, and my pull request above gets me close enough.