rust - Copy/move semantics documentation of &T/&mut T types itself

Question

I'm looking for the document about copy/move semantics of reference and mutable reference types.

The following code snippet shows immutable references (& T) implement the Copy trait and mutable references (&mut T) do not.

struct T;
fn copyable<U>(_: U) where U: Copy {}

fn main() {
    let a = &T;
    copyable(a);  // OK

    let b = &mut T;
    copyable(b);
    // error: the trait `core::marker::Copy` is not implemented for the type `&mut T`
}

But I can't find the description of this behavior. Someone know some (un)official documents? (or am I wrong?)

Answer 1

Rust's std::marker::Copy trait reference says (thanks to @Chris Emerson):

When can my type not be Copy?
Some types can't be copied safely. For example, copying &mut T would create an aliased mutable reference, and copying String would result in two attempts to free the same buffer.
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