smaller refcounts RFC

active/0000-smaller-refcounts.md

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+- Start Date: 2014-03-28
+- RFC PR #: 
+- Rust Issue #: 
+
+# Summary
+
+I propose removing support for weak pointers from `Rc<T>`, and adding it to a
+separate type `WRc<T>`.
+
+# Motivation
+
+`Rc<T>` currently uses two words for reference counting, to support an
+infrequently used feature - weak pointers. A search of the rust and servo
+codebases show zero uses of weak pointers, and the vast majority of reference
+counted code does not create cycles. However, outside of the spirit of not
+paying for what you don't use, every single `Rc<T>` keeps an extra word,
+the weak count, than it needs to.
+
+Hopefully this will help decrease the memory usage of rustc after all of the
+managed boxes get turned into `Rc<T>`s.
+
+# Detailed design
+
+I just want to rename `Rc<T>` to `WRc<T>`, and create a similar datastructure
+with its old name `Rc<T>` which just does not have a weak count. It will
+support all of the same operations except for downgrading.
+
+A similar transformation will also be done on `ARc<T>`, with a renaming to
+`AWRc<T>`.
+
+# Alternatives
+
+Of course, we could do nothing. That's fine, but using all this memory for no
+good reason is kind of unfortunate.
+
+# Unresolved questions
+
+Bikeshedding over the naming.