Proposed backward-compatible syntax for owning and output references

[NXTangl]

Since move is already a keyword, I propose the following:

&move T indicates a move reference to T. A move reference implicitly moves out of whatever it borrows, and dereferencing it consumes the reference, yielding a value of type T, but it may also be borrowed as &T and &mut T. &move T is NOT copy, and does not allow reborrowing like &mut T does. The borrowed variable is considered dropped at the end of the lifetime of the &move T by borrowck. Drop::drop() will be implicitly called.

&move mut T indicases an output reference to T. An output reference is considered an uninitialized variable by the borrowchecker. Only an uninitialized variable can be borrowed as &move mut, and it is considered initialized by borrowck after the end of the lifetime of the &move mut. Once initialized, it is considered identical to &mut T.

&move T is the owning reference that a lot of people have been wanting for a while, and &move mut T is my idea for a generalized placement new syntax, which might be necessary when/if we support unmovable types. An alternative to &move mut T would be a combination of placement syntax and guaranteed return-value optimization for functions returning unmovable types; I suppose the latter would be necessary anyway for them to work at all.

Thoughts? Reasons why this wouldn't actually parse? Alternative syntax?

[sfackler]

&move has been discussed fairly extensively: see #998 and #1646