Add offset_of! macro (RFC 3308) Implements https://github.com/rust-lang/rfcs/pull/3308 (tracking issue #106655) by adding the built in macro `core::mem::offset_of`. Two of the future possibilities are also implemented: * Nested field accesses (without array indexing) * DST support (for `Sized` fields) I wrote this a few months ago, before the RFC merged. Now that it's merged, I decided to rebase and finish it. cc `@thomcc` (RFC author)