The code intentionally uses `y` to access stuff that it also accesses through `x`. Declaring `x` and `y` to be `restrict` was a promise that it wouldn't do that.

> Does the standard not allow you to pass any pointers which could be aliased based on arbitrary pointer arithmetic

With the right hackish arithmetic, all pointers could be aliased with pointer arithmetic in C. Using `restrict` says your code won't do that so the compiler can optimize more.

> It seems very hard in general to know if two mutable references point to items in the same allocation.

Yeah, and in the limit it's impossible for C. It's not the compiler's job to figure it out, it's the code's job to uphold the promise.

It's different with Rust because you've got mutable/exclusive or immutable/shared promises from the borrow checker. And Rust doesn't let you dereference raw pointers outside of an unsafe block.

It seems to me that mislabeling your pointers `restrict` in C is akin to doing reckless operations inside of `unsafe` blocks in Rust. In both cases, you told the compiler "trust me, I know what I'm doing".