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Comment by tayistay | Hacker News Reader

What if a compiler were to only allow an array access when it can prove that it's in bounds? Wherever it can't you'd have to wrap the array access in an if, or otherwise refactor your code to help the compiler. Then you'd have no panicking at least and more predictable performance.

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>What if a compiler were to only allow an array access when it can prove that it's in bounds?

Even very good static analysis tools have a hard time doing this. In a language like C++ this would effectively mean that very few index operations can be done naively and compile times are significantly increased. Performance is likely reduced as well over the trivial alternative of using a safe array.

With bounds checking by default, even if a compiler can't statically prove that an index is in bounds, if the index in practice is always in bounds, the compiler inlines the check/branch into the calling function, and you're not starved of branch prediction resources, the check will be "free" because the branch will always be predicted as taken.

That's how WUFFS (Wrangling Untrusted File Formats Safely) works:

https://github.com/google/wuffs#what-does-compile-time-check...

WUFFS is awesome. It won't even let you add two ints without proving they cannot overflow

Rust has a tool for that, it's iterators. It is only limited however.

also `slice.get()` will return on option

and using a range check manually before an index will normally optimize the internal bounds check away

there's a cheeky link to idris's vector type in the second paragraph: https://www.idris-lang.org/docs/idris2/current/base_docs/doc... which accomplishes just that

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