The Swift compiler on linux is built with an older stdlib then runs with the newer shipped stdlib about swift HOT 1 OPEN

You are right. We can't break ABI used by compiler. As stated in the commit 0988236#diff-4b74006b7c38d950627c4183690f1c6b5435d1b4da7a327fdf6ed9bcbb2cc33bR552-R555 , we intentionally did this to make macros work in Linux in the limited time frame.

I think the ideal solution is to do proper bootstrapping:

1. build a toolchain having RPATH to the runtime in the builder's toolchain
2. build a stdlib/runtime using the toolchain (1)
3. build a toolchain with the stdlib/runtime (2)

Until that, yeah, bundling the runtime from the builder toolchain might be a temporary solution. But I think we can defer that until an actual unavoidable ABI break occurs. And we should be able to detect that pretty obviously, like runtime crash.

from swift.