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Comment by bonzini | Hacker News Reader

The alternative was to do the same and also not merge what they had. Stuff like custom allocator support is not optional.

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I was confused when reading this because I was pretty sure that using other allocators had been supported for a while in Rust. From refreshing myself on the details, it seems that replacing the default allocator is stable (https://doc.rust-lang.org/std/alloc/trait.GlobalAlloc.html), but the API for arbitrary allocators (which includes stuff like being able to do "zero-size" allocations) is not yet stable (https://doc.rust-lang.org/std/alloc/trait.Allocator.html). I guess if there were ever a project that needed fine-grained control over how allocators be work, it would be the kernel.

There's also https://docs.rs/allocator-api2/latest/allocator_api2/ -- I end up using this more often than I end up using custom allocators, simply because it has a stable version of the currently-unstable functions for constructing uninitialized Vec<MaybeUninit<u8>>s.

More importantly allocator_api adds try_new which means you can handle allocation errors.

Ah, that is important. I didn't even notice that wasn't possible in the GlobalAlloc API, but you're definitely right that it's not.

GlobalAlloc can be used for fallible allocations: its functions just return a null pointer on failure, as with malloc() and realloc() in C. The main limitations are around the safe heap data structures in the standard library, which don't stably expose any fallible APIs except for Vec::try_reserve().

Hmm, I'd expect that would mean that it's possible to add those APIs today then rather than requiring the `Allocator` trait. Is the idea that the Allocator a parameter (maybe a generic one) when calling `try_new`, so they don't want to stabilize anything now?

The Allocator type is an unstable parameter on the heap type; Vec<T> is unstably Vec<T, A>, Arc<T> is unstably Arc<T, A>, and so on. (The allocator "A" defaults to Global, which is an Allocator that forwards to the registered GlobalAlloc.) I think the Linux kernel also wants an Allocator trait for other reasons than fallibility, such as allocating different kinds of objects on different heaps.

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