Struct Stream 

struct Stream {
    buffer: Buffer,
    water_mark: u32,
}

One layout’s buffer and how much of it is in use.

Fields

buffer: Buffer

The buffer every mesh of this layout shares.

water_mark: u32

The highest element index the buffer has to cover, rounded up to the pool’s growth step.

A layout only needs to cover the elements something actually allocated in it, not the allocator’s whole capacity: a layout used by one small mesh should not pay for a capacity a busier layout drove up.