is always destroyed by passing it to a function, but gets implicitly copied if (arg is copyable && arg is referenced below)

I am not sure what you are trying to say. This:

  bar(a) # => bar(copy(&a)) # create-and-pass copy
  baz(a) # last use, so dont bother copying

is not possible in Rust if a's type is not a copy type. a will be moved when calling bar, so trying to pass it to baz will result in a compiler error.

The story is quite simple: a value is always moved in a function call, unless it is a copy type (the type implements the Copy trait). When the type is a copy type, a bit-wise copy is made. References are not special: immutable references are copy types. Mutable references are not copy types (otherwise, they could be aliased).

You can find an overview of all copy types in the standard library in the implementations section of the Copy trait:

https://doc.rust-lang.org/beta/std/marker/trait.Copy.html#fo...