rust-lang/rfcs — pull request #127 by nikomatsakis (2014-06-17 23:32:59+00:00)
Title: Opt-in builtin traits, take 2: default and negative impls.

The high-level idea is to add language features that simultaneously
achieve three goals:

1. move `Send` and `Share` out of the language entirely and into the
standard library, providing mechanisms for end users to easily
implement and use similar "marker" traits of their own devising;
2. make "normal" Rust types sendable and sharable by default, without
the need for explicit opt-in; and,
3. continue to require "unsafe" Rust types (those that manipulate
unsafe pointers or implement special abstractions) to "opt-in" to
sendability and sharability with an unsafe declaration.