Rollup merge of #70812 - rossmacarthur:fix/unit-called-nil, r=LukasKalbertodt
Rollup merge of #70812 - rossmacarthur:fix/unit-called-nil, r=LukasKalbertodt

Do not use "nil" to refer to `()`

"nil" is not used in the [book](https://doc.rust-lang.org/book) or in the [standard library](https://doc.rust-lang.org/std) anywhere else. Because "nil" is often used in programming languages to refer to "None" or "null" I think it could be a little confusing for newcomers to see this type referred to as "nil".

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust