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I don't think the annotation itself is actually the point. The point is being able to find all sources of unsafety and reason about them. Annotations do that, b

by azakai 4y ago

I don't think the annotation itself is actually the point. The point is being able to find all sources of unsafety and reason about them. Annotations do that, but other things can too. Imagine if you could run a tool on a codebase, and it statically found all the unsafe locations, and in idiomatic code there were very few of those, and you could reason about them. That would be equivalent to writing "unsafe" in the code, except you need to run the tool to "see" the annotations. That would be as good as annotations - it could prevent the same number of bugs. (Though I guess an argument could be, maybe some people forget to run the tool; fair enough.) No such tool can exist for C or C++. But such a tool could exist for a new language like Zig and Carbon, if they design themselves in certain ways, at least for spatial memory safety. Concretely, unsafe things like raw pointers in Zig have different types than safe slices, so a tool can actually find them; and unsafe things are (AFAIK) not idiomatic either, so they'd be rare.

burntsushi 4y ago
I think we'll just have to agree to disagree. I'm not certain you're wrong, so we'll have to see how it shakes out. And I love what the Zig project is doing, so I wouldn't at all be surprised if they find some other way here that works well in practice. Otherwise, I do think you underestimate the differences between an annotation like 'unsafe' and what you can find easily only through a static analysis tool. The key bit of the 'unsafe' annotation is the concept of "safe API" that it inspires, and also gives one a vocabulary for talking about things like 'soundness.' For example, you can mark Rust functions as 'unsafe' and then document the preconditions for avoiding UB. If you didn't mark that function as 'unsafe', then we would call it unsound because there exists an input that could cause UB, but it does not necessarily result in UB for every input. The annotation gives people a way to talk about and crystalize precisely what it means for an API to be safe for all uses. It funnels everything about memory safety down into that one concept and makes reasoning about it much much easier. Without an annotation like that, it's cumbersome or downright difficult to even talk or communicate about soundness.

by azakai 4y ago

burntsushi 4y ago

I think we'll just have to agree to disagree. I'm not certain you're wrong, so we'll have to see how it shakes out. And I love what the Zig project is doing, so I wouldn't at all be surprised if they find some other way here that works well in practice. Otherwise, I do think you underestimate the differences between an annotation like 'unsafe' and what you can find easily only through a static analysis tool. The key bit of the 'unsafe' annotation is the concept of "safe API" that it inspires, and also gives one a vocabulary for talking about things like 'soundness.' For example, you can mark Rust functions as 'unsafe' and then document the preconditions for avoiding UB. If you didn't mark that function as 'unsafe', then we would call it unsound because there exists an input that could cause UB, but it does not necessarily result in UB for every input. The annotation gives people a way to talk about and crystalize precisely what it means for an API to be safe for all uses. It funnels everything about memory safety down into that one concept and makes reasoning about it much much easier. Without an annotation like that, it's cumbersome or downright difficult to even talk or communicate about soundness.