rust/src/librustc_error_codes/error_codes/E0743.md

The C-variadic type ... has been nested inside another type.

Erroneous code example:

#![feature(c_variadic)]

fn foo2(x: u8, y: &...) {} // error!

Only foreign functions can use the C-variadic type (...). In such functions, ... may only occur non-nested. That is, y: &'a ... is not allowed.

A C-variadic type is used to give an undefined number of parameters to a given function (like printf in C). The equivalent in Rust would be to use macros directly.