by-ref closure capture semantics are a backwards compatibility hazard

[thestinger]

Unboxed closures (#8622) would capture by-value, and there would be no need to add syntax for a by-reference capture. Rust's references are first-class values, unlike C++, so it's trivial to capture a reference by-value.

This would create an unboxed closure (f) with the same size as b:

let b = 5;
let f = |a| a + b;

A by-reference capture would need to be explicit:

let b = &mut 5;
let f = |a| *b = a;

I think we should consider changing the current rules, to permit dropping in unboxed closures as a backwards compatible enhancement in the future.

[pnkfelix]

We do need to address the problem described here; it is not yet clear whether the suggested strategy is what we will follow, but we need to do something.

Assigning 1.0, P-backcompat-lang.