It's barely faster. I would say it's more accurate to say it's as fast as C, which is still a great achievement.
replies(3):
I think there's lots of value in wrapping a raw/unsafe implementation with a rust API, but that's not quite what most people think of when writing code "in rust".
Unsafe Rust still has to conform to many of Rust’s rules. It is meaningfully different than C.
* defined in C, undefined in Rust
* undefined in C, undefined in Rust
* defined in Rust, undefined in C
* defined in Rust, defined in C
And there really aren't. The abbreviated/limited safety environment being exploited by this non-idiomatic Rust code seems to me to be basically isomorphic to the way you'd solve the problem in C.
Ah, so that was like, not in your comment, but in a parent.
> And there really aren't.
I mean, not all of the code is unsafe. From a cursory glance, there's surely way more here than I see in most Rust packages, but that doesn't mean that you get no advantages. I picked a random file, and chose some random code out of it, and see this:
pub fn copy<'a>(
dest: &mut MaybeUninit<DeflateStream<'a>>,
source: &mut DeflateStream<'a>,
) -> ReturnCode {
// SAFETY: source and dest are both mutable references, so guaranteed not to overlap.
// dest being a reference to maybe uninitialized memory makes a copy of 1 DeflateStream valid.
unsafe {
core::ptr::copy_nonoverlapping(source, dest.as_mut_ptr(), 1);
}
[1] FWIW, memcpy() arguments are declared restrict post-C99, the strict aliasing thing doesn't apply, for exactly the reason you're imagining.
Right, and in Rust, you don't have to do it yourself: the language does it for you. If the signature were in C, you'd have to analyze the callers to make sure that this property is upheld when invoked. In Rust, the compiler does that for you.
> the strict aliasing thing doesn't apply
Yes, this is the case in this specific instance due to it being literally memcpy, but if it were any other function with the same signature, the problem would exist. Again, I picked some code at random, I'm not saying this one specific instance is even the best one. The broader point of "Rust has a type system that lets you encode more invariants than C's" is still broadly true.
No it doesn't? That comment is expressing a human analysis. The compiler would allow you to stuff any pointer in that you want, even ones that overlap. You're right that some side effects of the runtime can be exploited to do that analysis. But that's true of C too! (Like, "these are two separate heap blocks", or "these are owned by two separate objects", etc...). Still human analysis.
Frankly you're overselling hard here. A human author can absolutely mess that analysis up, which is the whole reason Rust calls it "unsafe" to begin with.
I'm saying that even in a codebase with a lot of unsafe, the checks that are still performed have value.
In c++ i could do something like:
x_ptr = new object y_ptr = x_ptr
copy(x_ptr, y_ptr)
In safe rust there is no way to call the function in question if that sort of aliasing has happened. This means that if you get a bug from your copy, its in the copy method - the possibility it's been used inappropriately has been eliminated.
It reduces the search space for problems from: everywhere that created a pointer that is ultimately used in the copy, to: the copy function itself.
It reduces the number of programmers who have to keep the memory semantics of that copy in their head from "potentially everyone" to just "those who directly implement and check copy".
Pretending that has no value is absurd.