The Objective-C convention is actually fine with you stack-allocating all block arguments; it's considered the callee's responsibility to Block_copy them. cc @rjmccall

https://clang.llvm.org/docs/BlockLanguageSpec.html
https://clang.llvm.org/docs/AutomaticReferenceCounting.html#blocks

Correct. We don't do this only because we're being cautious about lifetimes. If we had a reliable way of detecting that the copy is unnecessary, it would be a significant win.

Clang has actually implemented an optimization where it eliminates the copying logic from `noescape` blocks entirely, essentially generating them as global blocks that happen to be allocated on the stack.