Isn't the build cache tied to a specific compiler version? As soon as the compiler changes just a bit, every package would need to be rebuilt.

Can you elaborate on how the build cache could be used by cmd/coordinator?

Many CLs don't affect the compiler. (std changes, doc updates, etc)

And today we effectively build+test everything twice: trybots and then again for post-submit builds.

Would both the trybots and builders use caching? I presume that trybots are the ones where total run-time matters the most.

I agree that this would be a good step forward. One possible disadvantage I can see is that we would test the compiler less often. It's easier to catch flakes and very unlikely crashes in the compiler if every builder compiles the entirety of Go at every run. I wonder if it would be a problem if we lost that, though.

I don't recall seeing much flakiness in the compiler itself, nor the assembler nor linker. I think it's OK to run them less often.

If we have spare capacity and want to use it to check for flakiness, we can always issue extra builds with clean caches (or with GODEBUG=gocacheverify=1).