Can you build in some kind of "unsafe release" mode, so that every array bound check that were asked in the code are skipped ? If not, would it be an interesting feature ?
No. Such a thing could only remove some kinds of checks; for example, if you see that code sample later in the thread with a manual check, it wouldn't know that's what you're doing.
In general, we don't want to make it easy to turn checks off. They get removed if the compiler can prove they're not needed; if they're there, they're almost always for good reason.
You could do something with rusts feature system and macros. In essence you'd have a macro that would run a different line of code if your feature is enabled versus disabled, so you could use the unbounded lookup on the array.
That said, this would be a user implementation and wouldn't be likely to be provided by the standard Library
In general, we don't want to make it easy to turn checks off. They get removed if the compiler can prove they're not needed; if they're there, they're almost always for good reason.