I agree. After compiling it is even quite likely that the compiler does not care you gave a name to a return value (assuming you let it infer the variable type)

I agree. After compiling it is even quite likely that the compiler does not care you gave a name to a return value (assuming you let it infer the variable type). What you will see in practice is that the intermediate is explicitly “materialized” (e.g. into a list), because the author wanted to inspect it in the debugger. That will have some cost, mostly in the form of avoidable allocations.