Temporarily binding a (mutable) value to another type?

Using Swift
1

i feel like this should really work:

public
protocol BSONEncoder
{
    static
    var type:BSON { get }

    static
    func move(_:consuming BSON.Output<[UInt8]>) -> Self

    consuming
    func move() -> BSON.Output<[UInt8]>
}

alas, i seem to be running into a compiler bug:

extension BSON.Output<[UInt8]>
{
    public
    subscript<Encoder>(as _:Encoder.Type) -> Encoder 
        where Encoder:BSONEncoder
    {
        _read
        {
            let encoder:Encoder = .move(self)
            yield encoder
        }
        _modify
        {
            var encoder:Encoder = .move(consume self)
            defer { self = (consume encoder).move() }
            yield &encoder
        }
    }
}

/swift/swift-mongodb/Sources/BSONStreaming/BSON.Output.swift:243:37: 
error: 'consume' applied to value that the compiler does not support. 
This is a compiler bug. Please file a bug with a small example of 
the bug

            defer { self = (consume encoder).move() }
                                    ^

error: fatalError

any better ways to move a buffer back to its original location in a defer statement, without unnecessarily refcounting?

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