Function with_call_scratch 

pub fn with_call_scratch<R>(f: impl FnOnce(&mut CallScratch) -> R) -> R

Borrow this thread’s cleared CallScratch for the length of one call.

The buffers keep their capacity between calls, so a host adapter that wraps its call in this stops allocating once the app is warm. A body that calls back into another function finds the shared buffers taken and gets fresh ones, so a nested call cannot disturb the one it is inside.