Clarify "immutable bytes" definition

[RalfJung]

Currently, we define "immutable bytes" as

All bytes inside a const item or within an implicitly const-promoted expression are immutable. The bytes owned by an immutable binding or immutable static are immutable, unless those bytes are part of an UnsafeCell.

However, there are some issue with this. First of all, a const item can only be used as a value, not as a place, so it doesn't really "contain" bytes that could be (im)mutable -- except if it is subject to lifetime extension, as in

const C: &Vec<i32> = &Vec::new();

Secondly, for static, the actual rule is that for bytes inside lifetime-extended values, they are all immutable. UnsafeCell makes no difference.

[chorman0773]

I have a few ideas about how to write it, for varying levels of rigor. I think that in terms of lifetime extension, const-promotion occurs for both static and const items, correct? I'd expect that the const-promotion rule would apply there as well.

[RalfJung]

Const promotion and lifetime extension are largely orthogonal mechanisms. I was talking about lifetime extension above. Promotion does not apply in that example as Vec has a destructor.

[chorman0773]

Ok, then I'd probably say

All bytes within a const-promoted expression, or a lifetime-extended expression in the initializer of a static or const item are immutable. All bytes owned (directly) by an immutable binding or immutable static are immutable, except bytes within an [core::cell::UnsafeCell].

[RalfJung]

Yeah, that sounds good.