The __shared parameter annotation means that a value type parameter can be passed by reference.

For value types, this enables us to elide a copy before we make the call and instead pass a reference pointing right at the memory we allocated. SIL calls this convention in_guaranteed for (indirect reference with guaranteed lifetime). It's currently the way we pass self in non-mutating functions.

It was introduced by the Ownership Manifesto.

Here is the difference between the ownership annotations:

• inout: mutating pointer-like value
• __shared: non-mutating pointer-like value
• __owned: an explicit way of writing the default

Here is a summary of the manifesto: Swift Ownership Manifesto TL;DR.