Comment by bialpio

> Because in current code, it was ~all includes last time I checked.

That's another matter - just because forward-declares are allowed, doesn't mean they are mandated, but in my experience the reviewers were paying attention to that pretty well.

Counter-exeamples to "~all includes": https://source.chromium.org/chromium/chromium/src/+/main:thi..., https://source.chromium.org/chromium/chromium/src/+/main:thi..., https://source.chromium.org/chromium/chromium/src/+/main:thi....

I picked couple random headers from the directory where I've contributed the most to blink, and from what I'm seeing, most of the classes that could be forward-declared, were. I have not looked at .cc files given that those tend to need to see the declaration (except when it's unused, but then why have a forward-decl at all?) or the compiler would complain about access into incomplete type.

> Well, I remember one - very biased - example where I had a look at a class that was especially expensive to compile, like 40 seconds (on a Ryzen 7950X) and maybe 2 GB of RAM. It had under 200 LOC and didn't seem to do anything that's typically expensive to compile... except for the stuff it included.

Maybe the stuff was actually being compiled because of some member in a class (so it was actually expensive to compile). Or maybe you stumbled upon a place where folks weren't paying attention. Hard to say without a concrete example. The "compile firewall" was added pretty recently I think, but I don't know if it's going to block anything from landing.

Edit: formatting (switched bulleted list into comma-separated because clearly I don't know how to format it).