This is a new proposal to introduce named arguments a.k.a. keyword arguments in Rust. While there have been multiple proposals for this (e.g. this one), this RFC uses a different syntax that doesn't conflict with existing and planned syntax such as type ascription.
EDIT: I changed the syntax from
EDIT 2: The RFC is now submitted, view it here. Note that I updated quite a few sections; argument names are no longer mandatory in function calls.