Building with stdlib assertions enabled. Why?

As @dabrahams mentions, internal checks have been very useful for detecting bugs as they can be a lot more exhaustive. But I agree that stdlib-assertions is primarily useful for internal testing of the standard library itself, but should be disabled by default in build-script.

I think it's vital to have a run in CI with these assertions, but you're right that they reduce testing of the SIL optimizer, especially constant-folding. As for which is the default for PR-testing, I'm not sure. Ideally we'd do both in parallel, but I understand CI can be pretty slow and constrained.

edit PSA: these are described in the programmer's manual