RFC: Capture upvars by value unless the ref keyword is used

Author: pcwalton

active/0000-capture-by-value.md

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+- Start Date: 2014-07-02
+- RFC PR #: (leave this empty)
+- Rust Issue #: 12831
+
+# Summary
+
+Closures should capture their upvars by value unless the `ref` keyword is used.
+
+# Motivation
+
+For unboxed closures, we will need to syntactically distinguish between captures by value and captures by reference.
+
+# Detailed design
+
+This is a small part of #114, split off to separate it from the rest of the discussion going on in that RFC.
+
+Closures should capture their upvars (closed-over variables) by value unless the `ref` keyword precedes the opening `|` of the argument list. Thus `|x| x + 2` will capture `x` by value (and thus, if `x` is not `Copy`, it will move `x` into the closure), but `ref |x| x + 2` will capture `x` by reference.
+
+In an unboxed-closures world, the immutability/mutability of the borrow (as the case may be) is inferred from the type of the closure: `Fn` captures by immutable reference, while `FnMut` captures by mutable reference. In a boxed-closures world, the borrows are always mutable.
+
+# Drawbacks
+
+It may be that `ref` is unwanted complexity; it only changes the semantics of 10%-20% of closures, after all. This does not add any core functionality to the language, as a reference can always be made explicitly and then captured. However, there are a *lot* of closures, and the workaround to capture a reference by value is painful.
+
+# Alternatives
+
+As above, the impact of not doing this is that reference semantics would have to be achieved. However, the diff against current Rust was thousands of lines of pretty ugly code.
+
+Another alternative would be to annotate each individual upvar with its capture semantics, like capture clauses in C++11. This proposal does not preclude adding that functionality should it be deemed useful in the future. Note that C++11 provides a syntax for capturing all upvars by reference, exactly as this proposal does.
+
+# Unresolved questions
+
+None.