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by-ref closure capture semantics are a backwards compatibility hazard #12831
Description
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.
Activity
pnkfelix commentedon Mar 13, 2014
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.
librustc: Copy or move upvars by value.
auto merge of #15929 : pcwalton/rust/by-ref-closures, r=alexcrichton
librustc: Require the `ref` keyword to get by-reference closure
nikomatsakis commentedon Jan 8, 2015
This is done.
Auto merge of rust-lang#12831 - fasterthanlime:proc-macro-test-in-tmp…