C:

  int foo(int *x, int *y) {
    *x = 0;
    *y = 1;
    return *x;
  }

  foo(int*, int*): # @foo(int*, int*)
    mov dword ptr [rdi], 0
    mov dword ptr [rsi], 1
    mov eax, dword ptr [rdi]
    ret

  fn foo(x: &mut i32, y: &mut i32) -> i32 {
    *x = 0;
    *y = 1;
    *x
  }

  example::foo:
    mov dword ptr [rdi], 0
    mov dword ptr [rsi], 1
    xor eax, eax
    ret

Does LLVM implement optimization that only Rust can use, or at least those that cannot be used from C/C++?

LLVM is adding some semantics specific to non-C or C++ languages. I’m on my phone so I can’t link you, but they’re adding an intrinsic related to infinite loops because languages like Rust have different semantics here.

  int foo(int *restrict x, int *restrict y) {
    *x = 0;
    *y = 1;
    return *x;
  }

  foo:
    movl	$0, (%rdi)
    movl	$1, (%rsi)
    xorl	%eax, %eax
    ret