SE-0519: Borrow and Inout types for safe, first-class references

Evolution Proposal Reviews
82

Hi all,

The LSG is accepting this proposal in principle, but is inclined to change the proposed type names based on discussion here. We want to leave a little more time for feedback about the naming before finalizing it.

The LSG intends to change the names of these types to Ref and MutableRef, respectively. There are a number of factors that went into this change:

  • Ref better matches the way we are talking about these as "references", and we found that trying to explain them as "borrows" (which is odd as a known) or "borrow types" (which is odd in our value-centric lifetime system) was awkward.
  • LSG feels that the conceptual overlap with "reference types" (like classes) is generally a helpful association. Reference types are referenced, not copied. You can create a Ref<T> to store a (conceptual) reference to a T. The fact that there's an optimization where we copy small values rather than store them indirectly doesn't invalidate that the type operates like a reference; it's just an implementation detail.
  • These follow the MutableXYZ pattern that is well-established in the standard library. That pattern does not work as well with other alternatives discussed (like Borrow, where "mutable borrow" is a phrase we've been trying to avoid).
  • We could spell out Ref as Reference instead, but this is much longer (especially for MutableReference), and fundamental types in the standard library do often have shorthand names (Int, Bool, etc.).

We'd love to hear your thoughts on this direction.

Doug, Review Manager

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