Comment by gpm

Safety is a feature of a system - yes. It's also a property of what it's against. A computer could be safe against being hacked but still be dangerously easy to drop on someones toe and break it.

Safety [against something] is also a feature of components - a system made up of only safe components [against a thing] is safe [against the same thing... I'm going to stop this qualification now for brevity]. A system containing unsafe components may or may not be safe but at least you know what components usage you need to look at carefully.

If your linker is safe, linking code will never result in the thing it is safe against. Ever. This is a useful property even if running the linked thing is not safe because it means:

1. When things go wrong in strange ways, you have strict bounds guiding you in figuring out what went wrong.

2. You can build reliable systems that do part of the job, and only have to sandbox the other half of the job. Compiling in a CI system will (if the compiler was entirely safe) be safe. You can do it with secrets present against malicious code. Running tests will have to be sandboxed (assuming running tests isn't safe). This could for instance enable safely sharing significantly more artifacts for incremental builds in CI.

Unfortunately very few compilers are really safe against anything (though I do wonder how I could break my toe on one). Rustc for instance has a giant C++ half called llvm that isn't really hardened at all. We get away with this by just not trusting the compiler when run against potentially malicious code.