Avoid closure creation in hot code

[nicolasstucki]

Unfortunately the inlining solution of #3267 fails here as the closure is impure an hence evaluated before the inlined block of code.

[odersky]

In what sense is the closure impure?

[nicolasstucki]

After some investigation I discovered the issue, it has nothing to do with pureness.

The problem is that DottyPredef.assert is only inlined when the code is compiled by dotty-bootstrapped (and not by dotty-nonbootstrapped). As dotty-bootstrapped is compiled with dotty-nonbootstrapped it does not get the asserts inlined. We would need a dotty-bootstrapped-bootstrapped to have this working.

I would suggest adding special casing this assert and adding the optimization by hand as it is in hot code. We should discuss what to do with the other ones later.

[smarter]

Or you could compile the bootstrapped compiler with the non-bootstrapped compiler as usual but with the bootstrapped library on the Scala classpath