Is something like `realloc_zeroed` and `grow_in_place_zeroed` useful?

[TimDiekmann]

When growing an allocation, is there a reason why there is no method that guarantees that the new memory is zeroed?

[petertodd]

+1

Implementation-wise for large reallocations there's definitely cases where allocators can efficiently do things like mmap additional (zeroed) pages to the end of an allocation. Also can have a default implementation that just forwards to realloc() so impls don't need to worry about them if they don't want too.

[gnzlbg]

I think these two can be useful. Whether these are worth having in an MVP, I don't know, we can always add them later.

[TimDiekmann]

AllocRef has alloc_zeroed, but no realloc_zeroed and grow_in_place_zeroed. Do we want to add those?

Shall we introduce a NonZeroLayout and use this in the alloc trait(s)?

Currently, zeroed allocation is pretty error prone and even experienced users may miss this: rust-lang/rust#63291 (comment). On the other hand, we would restrict implementations which are fine to get zeroed layout like a bump allocator.

	Add them now	Not for an MVP	No need
@Amanieu	👍
@Ericson2314
@glandium
@gnzlbg
@Lokathor		😕
@scottjmaddox	👍
@TimDiekmann	👍
@Wodann	👍

Please vote with

• 👍 merge: Add them now
• 😕 postpone: I want to have them, but not for an MVP
• 👎 close: We don't need those (why?)

[Lokathor]

This seems like maybe it's about two things.

Is this about having newly allocated memory blocks already be zeroed?
Or is this about being able to pass a zero sized allocation request into the allocator?

[TimDiekmann]

It's just like alloc_zeroed: The newly allocated memory is guaranteed to be zeroed. A default implementation would look like alloc_zeroed.

[Lokathor]

Ah, that's very good, but i agree that it's not really necessary for the minimum "get something working" stage. Voted.

[scottjmaddox]

It would be better to have these before stabilizing AllocRef.

[TimDiekmann]

I think stabilizing the whole thing will take at least one year

[Lokathor]

I also wouldn't actually stabilize AllocRef without this

But it's low priority to just get people using AllocRef and get people hammering on usage problems in Nightly and so on.