Memory protection keys

This seems extremely useful for partitioning untrusted code in a shared process, eg a web browser that doesn't want to take the process-per-domain hit. And for isolating components like the GC and JIT. It's not a complete protection from malicious attack, of course, since if you have somewhat-controlled execution of code within one partition, you may be able to change the key register, but it seems like a pretty good additional barrier to preventing that access in the first place.

I would love to be able to efficiently ensure that no stray writes corrupt GC bookkeeping data. Any memory corruption anywhere tends to crash in the GC because it scans through and chases pointers with lots and lots of memory. This would make it so the mutator would crash immediately at the right place, making errors far far more obvious and likely to be fixed at the root.