Function subtle::black_box

pub(crate) fn black_box<T: Copy>(input: T) -> T

This function is a best-effort attempt to prevent the compiler from knowing anything about the value of the returned u8, other than its type.

Because we want to support stable Rust, we don’t have access to inline assembly or test::black_box, so we use the fact that volatile values will never be elided to register values.

Note: Rust’s notion of “volatile” is subject to change over time. While this code may break in a non-destructive way in the future, “constant-time” code is a continually moving target, and this is better than doing nothing.