SE-0377: borrow and take parameter ownership modifiers

[Joe_Groff]

@dabrahams

I’ll just repeat something I’ve tried to say before: we should find our way to a future where “borrow” doesn’t need to be written at all. For copyable types there’s no difference between borrowing and pass-by-copying-a-value, and for noncopyable types the latter doesn’t exist. When do you ever want to pass by implicit copy? I say never.

borrow will still be the default in almost all cases, so you shouldn't have to write it much. The alternative to borrow isn't pass-by-copying, it's pass-by-ownership-transfer (what we call take in the proposal). Whether the caller makes a copy or not is an orthogonal decision made by the caller, and there are reasons we'd do so for both borrowed and taken parameters—to avoid aliasing writes to shared mutable state in the former case, or to preserve the lifetime of a value the caller either still needs to keep around for itself, or doesn't have ownership of in the first place, in the latter. Stay tuned for future proposals if you want to hear about controlling or preventing implicit copies.