# ThinBox::new\_unsize should not allocate **URL:** \ **Category:** Uncategorized\ **Created:** [January 21, 2023, 12:43am UTC](https://internals.rust-lang.org/t/thinbox-new-unsize-should-not-allocate/18180 "2023-01-21T00:43:06Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![stepancheg](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/stepancheg/32/63_2.png) [@stepancheg](https://internals.rust-lang.org/u/stepancheg)\ **Post date:** [January 21, 2023, 12:43am UTC](https://internals.rust-lang.org/t/thinbox-new-unsize-should-not-allocate/18180/1 "2023-01-21T00:43:06Z")
Rust issue tracker suggested I should post feature request here. The problem is: ```rust ThinBox::<[u32]>::new_unsize([0; 0]); ThinBox::::new_unsize([0; 0]); ``` both allocate 8 bytes in the heap to store the pointer. However, there's no need. Ideally, `new_unsize` function should statically allocate pointer metadata, and return pointer to it. That's it. I tried to implement it, and encountered this problem. I can create pointer metadata for `T` like this: ```rust pub fn new_unsize(value: T) -> Self where T: Unsize, { if mem::size_of_value(&value) == 0 { // Here I create pointer metadata, with only type, // no value, this is const expression const { ptr::metadata(NonNull::::dangling().as_ptr() as *mut Dyn) } // But there is no way to tell the rust compiler to put it into a static variable // to return it from this function. } ... } ``` Or an I missing something? # Why it is important Because, `Box<[String]>` is cheap for empty strings, but `ThinBox<[String]>` is not. It can be partially worked around by storing it an option, like: ```rust struct MyThinSlice(Option>); // None means empty ``` But this makes `Option>` as big as `Option>` making `ThinBox` less efficient than just `Box`. CC @yaahc ---
**Author:** ![cuviper](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/cuviper/32/1897_2.png) [@cuviper](https://internals.rust-lang.org/u/cuviper)\ **Post date:** [January 21, 2023, 3:32am UTC](https://internals.rust-lang.org/t/thinbox-new-unsize-should-not-allocate/18180/2 "2023-01-21T03:32:37Z")
> [@stepancheg](#): > > Ideally, `new_unsize` function should statically allocate pointer metadata, and return pointer to it. Note that the pointer should be directly on the value, with the metadata at a negative offset. That will be at the end of your "allocation" for a ZST, but they can also require greater alignment, so your static may need some padding. ---
**Author:** ![stepancheg](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/stepancheg/32/63_2.png) [@stepancheg](https://internals.rust-lang.org/u/stepancheg)\ **Post date:** [January 21, 2023, 6:15am UTC](https://internals.rust-lang.org/t/thinbox-new-unsize-should-not-allocate/18180/3 "2023-01-21T06:15:42Z")
> [@cuviper](#): > > your static may need some padding Right. It could be something like: ```rust #[repr(C)] struct StaticZst { metadata: ::Metadata, data: [T; 0], } static ZST>: StaticZst = StaticZst { metadata: ptr::metadata(NonNull::::dangling().as_ptr() as *Dyn), data: [], }; ``` Everything is doable except generic statics. ---
**Author:** ![CAD97](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/cad97/32/3460_2.png) [@CAD97](https://internals.rust-lang.org/u/CAD97)\ **Post date:** [January 21, 2023, 9:32am UTC](https://internals.rust-lang.org/t/thinbox-new-unsize-should-not-allocate/18180/4 "2023-01-21T09:32:19Z")
No generic statics required: ```rust const fn make>() -> &'static StaticZst { const { &StaticZst { metadata: ptr::metadata(NonNull::::dangling().as_ptr() as *mut Dyn), data: [], } } } ``` except that still doesn't work because ```plaintext error[E0492]: constants cannot refer to interior mutable data --> src/lib.rs:17:9 | 17 | / &StaticZst { 18 | | metadata: ptr::metadata(NonNull::::dangling().as_ptr() as *mut Dyn), 19 | | data: [], 20 | | } | | _________ ^ this borrow of an interior mutable value may end up in the final value ``` ... so yeah, putting it in a `static` would allow the `?Freeze` value. But we don't need the uniqueness guarantee of `static` (which at best requires linker shenanigans for generic statics), so what we really want is a `Freeze` bound on the pointee metadata. (It holds for the only current pointee kinds, with `()`, `usize` (slices), and `DynMetadata`.) (I'm kind of surprised that this just works; for some reason I thought inline const wasn't allowed to use outer generics; perhaps from using the stable approximation using a `const` item which does have that restriction.) The other alternative is just to `#[repr(align(128))]` and only do the zero-alloc opt for ZST where that alignment is sufficient. ---
**Author:** ![CAD97](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/cad97/32/3460_2.png) [@CAD97](https://internals.rust-lang.org/u/CAD97)\ **Post date:** [January 21, 2023, 9:41am UTC](https://internals.rust-lang.org/t/thinbox-new-unsize-should-not-allocate/18180/5 "2023-01-21T09:41:10Z")
This is also making the assumption that unsizing from a dangling pointer will get you the same result as unsizing the concrete value. This is true for all current pointee kinds and might be necessarily true for any future custom DST using, but also might not necessarily be true. If you want to implement this, the way to go about it is probably by utilizing specialization, so it's tied to the pointee kind. And in that case, the inline const will work just fine since the metadata is known not to contain `UnsafeCell`. ---
**Author:** ![steffahn](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/steffahn/32/13288_2.png) [@steffahn](https://internals.rust-lang.org/u/steffahn)\ **Post date:** [January 21, 2023, 10:54am UTC](https://internals.rust-lang.org/t/thinbox-new-unsize-should-not-allocate/18180/6 "2023-01-21T10:54:20Z")
> [@CAD97](#): > > perhaps from using the stable approximation using a `const` item which does have that restriction The more appropriate stable approximation is using an associated const on a type; those can be generic. ```rust struct HelperStruct<…generics…>; impl<…generics…> HelperStruct<…generics…> { const TheConstant = /* expression here! */; } ``` ---
**Author:** ![steffahn](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/steffahn/32/13288_2.png) [@steffahn](https://internals.rust-lang.org/u/steffahn)\ **Post date:** [January 21, 2023, 11:08am UTC](https://internals.rust-lang.org/t/thinbox-new-unsize-should-not-allocate/18180/7 "2023-01-21T11:08:50Z")
> [@stepancheg](#): > > Ideally, `new_unsize` function should statically allocate pointer metadata, and return pointer to it. That's it. Another adjustment would be in the drop implementation. Presumably introducing a new branching (in the optimized code) that will need to check the zero-sizedness at run-time. Since this only negatively affects code that subsequently deallocates, this single additional check is probably negligible. ---
**Author:** ![stepancheg](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/stepancheg/32/63_2.png) [@stepancheg](https://internals.rust-lang.org/u/stepancheg)\ **Post date:** [January 22, 2023, 11:14am UTC](https://internals.rust-lang.org/t/thinbox-new-unsize-should-not-allocate/18180/8 "2023-01-22T11:14:56Z")
TL;DR: it may work if we add `+ Freeze + 'static` on `Pointee::Metadata`. OK, I think I got it. `Pointee::Metadata` needs to be marked `Freeze`. (`Freeze` needs to be made public, but that's probably OK). Even if I do ```rust unsafe impl Freeze for StaticZst<...> {} ``` I failed to tell rust to allocate it statically (it tells something about `Dyn` need to be `'static`). But that's good enough for now. `::Metadata` also needs to have `'static` bound, otherwise it doesn't work. And since we cannot have `StaticZst` struct with empty `[T; 0]` field in it, we place metadata in a struct which is `repr(align(128))`, and do this optimizations only for such types. I got some code which compiles (and even alloc tests pass). ---
**Author:** ![system](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/system/32/14092_2.png) [@system](https://internals.rust-lang.org/u/system)\ **Post date:** [April 22, 2023, 11:15am UTC](https://internals.rust-lang.org/t/thinbox-new-unsize-should-not-allocate/18180/9 "2023-04-22T11:15:28Z")
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