What changes would Cow :: clone need to return shallow copies?

Question

I have a question related to Rust issue # 34284 . I understand why this is not possible in the current rust, but I'm curious what change (break) is needed to avoid highlighting in Cow::clone .

I suspect that this will require some kind of specialization for life. Something like (pseudo-rust):

 impl<'a, B: ?Sized> Clone for Cow<'a, B> where B: ToOwned { fn clone(&'b self) -> Cow<'a, B> { if /*lifetimes 'a and 'b allow for optimization*/ { return /*shallow copy*/ } else { return /*as it is today*/ } } } 

RFC 1210 states that life-cycle specialization is not possible:

We cannot, because when the compiler is sent to the actual code ("trans"), life information has been erased

but later he says that

Unfortunately, we cannot easily rule out unwanted time-dependent specializations, because they can be "hidden" behind innocent looks.

Does this mean that:

• Optimization of Cow::clone impossible without changing the compiler, so that information about the lifetime is stored longer
• could only be optimized for language-defined lifetimes ( 'static ) if RFC 1210 allowed this?