ValueTask is more of an optimization for scenarios where the Future (or Task<T>) is often intended to be ready immediately (synchronously). For those cases its wasteful to first allocate a continuation on the heap, and then not to use it - because the code after the await can directly be executed.

The introduction of ValueTask allowed C# code to only perform dynamic allocations whenever the task definitely needed to be yielded - opposed to one allocation for every Task<T>. However it doesn't allow for guaranteed avoidance of allocations - like Rusts model can do.

However on the positive side removing the allocations for those cases is probably good enough since the other yields are of low-frequency (when waiting for IO). And Rust code currently requires allocations like Box<dyn Future> in order to make certain things work (like type-erasure and dynamically dispatched interfaces) that are not necessarily required in .NET in the non-yielded case.

From an ergonomic perspective I definitely prefer .NETs easy-by-default model which is still highly optimizable up to the point of avoiding all allocations. But I understand why this wouldn't have worked for Rust and it's constraints (no GC, no classes, no runtime).