Use RefCell to prevent multiple mutable references of current value

[bvssvni]

As suggested on reddit, this enforces the safety at runtime and might be combined with the design such that the current value can only be borrowed once as mutable but multiple times as immutable.

[bvssvni]

Reddit comment: #3

[bvssvni]

If we put an extra field in the hash table, then we can tell whether it is borrowed mutable or immutable.

[bvssvni]

RefCell uses an uint with -1 as WRITING and 0 as UNUSED. The other values are used for immutable references.

http://doc.rust-lang.org/src/core/home/rustbuild/src/rust-buildbot/slave/nightly-linux/build/src/libcore/cell.rs.html#223-228

[bvssvni]

Perhaps best to use similar conventions as the standard library.

• Current::try_borrow
• Current::borrow
• Current::try_borrow_mut
• Current::borrow_mut

[bvssvni]

Suggestion for names: RefCurrent and RefMutCurrent

• Implement Drop for RefCurrent
• Implement Deref for RefCurrent
• Implement Show for RefCurrent
• Implement Drop for RefMutCurrent
• Implement Deref for RefMutCurrent
• Implement Show for RefMutCurrent

[bvssvni]

In order to avoid having to type "let foo: RefMutCurrent = Current::borrow_mut(scope)" we could do "let foo = Current::borrow_mut::::(scope);".

[bvssvni]

Alternative is to use immutable references and use RefCell as convention for mutable ones.

[bvssvni]

This will not be implemented in favor of using RefCell with immutable references.