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A language like Rust aims for "zero-cost abstractions", which means the features and behaviors of the language must be evaluated in the context of the implement

• Waterluvian 7y ago
  Let me attempt to unpack this into language I think I understand: It's up to Rust's compiler to verify all the contracts made, because in order for the binary to be "zero-cost", none of those checks are being done at runtime. If you looked at the output assembly, you would see what looks like very carefully written code that shows no explicit signs of protecting itself. Ie. there's no swaths of boilerplate assembly doing borrow checking, out of bounds checking, etc.
  • steveklabnik 7y ago
    You're not wrong, exactly, but I'm not 100% sure this is right. The way I would put it is this: Rust has made certain commitments about performance. This means that language changes have to be made in the context of how they are implemented, because releasing a language feature that causes a significant performance degradation would make it not a good fit by definition.
  • eximius 7y ago
    That might be a stronger way of putting it than I would. The canonical example is iterator chains with complex logic compiling down into vectorised and unrolled loops. Powerful logical abstractions are used by the compiler to generate code that does what it says to do without the runtime cost of closures or whatever other logical but not mechanically necessary things you have. Iterators can't go out of bounds, so the compiler can elide those checks. There are still some runtime costs at the intersection of safety, ergonomics, and performance. Bounds checking, overflow checking. But they have escape hatches and are relatively rare in the language. Most things do compile out.