Zlib-rs is faster than C

I found out I already know Rust:

        unsafe {
            let x_tmp0 = _mm_clmulepi64_si128(xmm_crc0, crc_fold, 0x10);
            xmm_crc0 = _mm_clmulepi64_si128(xmm_crc0, crc_fold, 0x01);
            xmm_crc1 = _mm_xor_si128(xmm_crc1, x_tmp0);
            xmm_crc1 = _mm_xor_si128(xmm_crc1, xmm_crc0);

Kidding aside, I thought the purpose of Rust was for safety but the keyword unsafe is sprinkled liberally throughout this library. At what point does it really stop mattering if this is C or Rust?

Presumably with inline assembly both languages can emit what is effectively the same machine code. Is the Rust compiler a better optimizing compiler than C compilers?

Using unsafe blocks in Rust is confusing when you first see it. The idea is that you have to opt-out of compiler safety guarantees for specific sections of code, but they’re clearly marked by the unsafe block.

In good practice it’s used judiciously in a codebase where it makes sense. Those sections receive extra attention and analysis by the developers.

Of course you can find sloppy codebases where people reach for unsafe as a way to get around Rust instead of writing code the Rust way, but that’s not the intent.

You can also find die-hard Rust users who think unsafe should never be used and make a point to avoid libraries that use it, but that’s excessive.

Isn't it the case that once you use unsafe even a single time, you lose all of Rust's nice guarantees? As far as I'm aware, inside the unsafe block you can do whatever you want which means all of the nice memory-safety properties of the language go away.

It's like letting a wet dog (who'd just been swimming in a nearby swamp) run loose inside your hermetically sealed cleanroom.

It seems like you've got it backwards. Even unsafe rust is still more strict than C. Here's what the book has to say (https://doc.rust-lang.org/book/ch20-01-unsafe-rust.html)

"You can take five actions in unsafe Rust that you can’t in safe Rust, which we call unsafe superpowers. Those superpowers include the ability to:

    Dereference a raw pointer
    Call an unsafe function or method
    Access or modify a mutable static variable
    Implement an unsafe trait
    Access fields of a union

It’s important to understand that unsafe doesn’t turn off the borrow checker or disable any other of Rust’s safety checks: if you use a reference in unsafe code, it will still be checked. The unsafe keyword only gives you access to these five features that are then not checked by the compiler for memory safety. You’ll still get some degree of safety inside of an unsafe block.

In addition, unsafe does not mean the code inside the block is necessarily dangerous or that it will definitely have memory safety problems: the intent is that as the programmer, you’ll ensure the code inside an unsafe block will access memory in a valid way.

People are fallible, and mistakes will happen, but by requiring these five unsafe operations to be inside blocks annotated with unsafe you’ll know that any errors related to memory safety must be within an unsafe block. Keep unsafe blocks small; you’ll be thankful later when you investigate memory bugs."

Would someone with more experience be able to explain to me why can't these operations be "safe"? What is blocking rust from producing the same machine code in a "safe" way?