Mark black_box function as unsafe

[kitcatier]

Hello, I found a soundness issue in this crate.

bencher/lib.rs

Lines 590 to 596 in 8afeeb1

	pub fn black_box<T>(dummy: T) -> T {
	unsafe {
	let ret = ptr::read_volatile(&dummy);
	forget(dummy);
	ret
	}
	}

https://doc.rust-lang.org/std/ptr/fn.read_volatile.html
The unsafe function called needs to ensure that the parameter must be :

• src must be valid for reads.

• src must be properly aligned.

• src must point to a properly initialized value of type T.

and the developer who calls the black_box function may not notice this safety requirement.
Marking them unsafe also means that callers must make sure they know what they're doing.

[bluss]

please explain how black_box has different requirements than on a hypothetical function black_box2:

pub fn black_box2<T>(dummy: T) -> T {
}

I believe a user has to follow the same rules for calling black_box as black_box2, thus there is nothing that needs to be changed. Otherwise, please point out the details