As you note, the compiler doesn't provide any guarantees about things like this today. However, there's a reasonable optimization in here that we are missing: don't call _Block_copy for a block created from a closure known to have no captures. (Even Objective-C will do a heap allocation for a block that has captures; you can't get around it.)

cc @eeckstein

Just so we are on the same page, I expect the following conditions to hold:

1. The caller of internalRenderBlock makes a copy of the block that I return.
2. Any variables that I capture explicitly (see below) will get captured during the _Block_copy invoked by the caller of internalRenderBlock.
3. The block that gets returned should be equivalent to whatever closure #​1 is emitted by internalRenderBlock.
4. More specifically, when the audio system invokes the block that I return, the first instruction executed should be whatever code corresponds to the 1st line of the returned closure.

Regarding the capture: What I have in mind is something like this:

// The "math parts" of the audio unit
final class DSPKernel {
  // Assume this contains:
  //   * Statically-dispatched, thread-safe parameter updaters
  //   * A statically-dispatched buffer reader/writer function
}

final class MyAudioUnit: AUAudioUnit {
  var someDSPKernel: DSPKernel
  
  var internalRenderBlock: AUInternalRenderBlock {
    return { 
        [kernel = self.someDSPKernel] // Don't capture self
        (param1, param2, ...) in
      kernel.readAndWriteBuffers(param1, param2, ...)
    }
  }
}

Given my current understanding of Swift, the kernel capture above should act as any other pointer would be passed along with a block. Once the block is constructed, and returned in the getter above, I would expect:the reference is bound to the block object, and when the block is called there should be no further allocations (i.e. _swift_Alloc) required inside the closure #​1. (Provided, of course, that I take care not to allocate anything in readAndWriteBuffers and any code invoked within.)

Please let me know if I am missing (or appear to be misunderstanding) anything. Thanks!