> The Rust Nomicon actually documents these constraints for each 'unsafe' operation. Code that can ensure that these constraints hold can be regarded as 'safe' and rely on an unsafe{ } block. Code that can't, should be marked unsafe and document its own constraints in turn.
>
> If you're relying on internal details, you're most likely doing it wrong, even by the standards of 'unsafe'. There are ways to nail down these details where required, at which point they're not even "internal" details anymore, but this has to be opted-into explicitly, for sensible reasons.

It's not that simple, what I'm getting at is that 'unsafe' code is still required to meet all the constraints that 'safe' code has to meet, even though those constraints are not fully documented (Because in general, in 'safe' code there are lots of things that are impossible to do but not explicitly UB), and they have changed them in significant ways in the past. You can see an incomplete list of them here[0]. There has been work to nail these details down for years, with the most recent being this one[1], but in general I would argue there hasn't been much progress since I first looked into it years ago now. Fun fact, the first Rust issue I remember reading that touched on this (and evolved into basically everything going on today) is this[4] one, which was made exactly 4 years ago today (And I mean exact!).

The 'Safety' sections for 'unsafe' operations are actually just extra requirements for those particular API on top that you also have to keep in addition to whatever safe Rust constraints apply. And I would argue in general they're not actually exhaustive, and they're mostly just a "best guess" - it's not a breaking change to introduce or realize there are more constraints. Ex. A little more then a year ago, they added the info that a slice can't be larger than `isize::MAX`[2], and a few months ago was this[3] one, where they added that the pointer length must be from a single allocation.

My point is that when you're doing something like writing an OS kernel, you end up running into these types of issues and corner-cases because Ex. They determine what your allocator is allowed to produce, or if some particular pointer can be validly put into a slice.

> No, "other code" should not be marked unsafe because this would mean that relying on slices is inherently unsafe. Which is silly; the whole point of a "slice" type, contrasted with its "raw parts", is in the guarantees it provides. This is why slice::from_raw_parts is unsafe: not because if what it does, but what it states about the result.

Sure, my point is that this is at best unclear, and I would argue wildly misunderstood. We've basically made two definitions of 'unsafe' - one for code that has to perform one of the five 'unsafe' operations (Which is a clearly defined 'yes' or 'no', and also the one described by the Rust docs), and one for code that can potentially break some type of invariant the will potentially cause UB somewhere else in the program (even if the code in question is completely safe and can't on its own cause any problems).

The problem is that Rust only guarantees the type of safety that it does if you do the second, but yet lots of people (I would almost argue most...) assume the first is sufficient and 'unsafe' by itself ensures all your memory bugs are in `unsafe` code. The Rust docs even describes safe vs. unsafe as two completely different languages (which they are) which brings into question why you would write `unsafe` code when you don't even need any of the `unsafe` operations. You arguably want a separate keyword for this - one that marks a function `unsafe`, but doesn't actually allow you to perform any of the `unsafe` operations in it.

[0] https://doc.rust-lang.org/nomicon/what-unsafe-does.html
[1] https://github.com/rust-lang/unsafe-code-guidelines
[2] https://github.com/rust-lang/rust/commit/1975b8d21b21d5ede54...
[3] https://github.com/rust-lang/rust/commit/1a254e4f434e0cbb9ff...
[4] https://github.com/rust-lang/rfcs/issues/1447