What will these reserved Rust keywords do? Theory and speculation

1 April, 2025

I found this page which lists all reserved keywords for Rust: https://doc.rust-lang.org/reference/keywords.html

I did research and compiled a list of speculations / rfcs that use these keywords:

• priv: everything in Rust is already private by default¹ except trait items, enum variants, and fields in enum variants. Perhaps at some point these can be made opt-in private
• become: tail call optimization for recursive functions (https://github.com/rust-lang/rfcs/pull/3407)
• abstract, override, final: Was reserved for a possibility of adding "more efficient inheritance"²
• box: box patterns (https://github.com/rust-lang/rust/issues/29641)
• do: In the old days, do was a way to invoke a function that takes a closure as a parameter such as do task::span { println!("Hello, world!"); }.²
• macro: declarative macros 2.0 (https://github.com/rust-lang/rust/issues/39412)
• typeof: get the type of an expression so you can do let x: typeof("a") = "hello"
• unsized: syntax sugar for !Sized
• virtual: Rust used to have "virtual structs" which were implemented as a test, this lead to the virtual keyword. They were later removed, but the keyword was kept.
• yield and gen (weak): generator functions and blocks (https://github.com/rust-lang/rust/issues/117078)
• try: local blocks which the ? operator can return from without returning from the function (https://github.com/rust-lang/rust/issues/31436)

unsized only makes sense as sugar for !Sized but is this really necessary? Rust tries hard not to special case the standard library and just adding syntax for a built-in item seems like isn't necessary. I didn't find an RFC for it though so it could be used for more than that

do is used for the do yeet in https://github.com/rust-lang/rust/issues/96373 but the syntax will change when its stabilized so I don't count it