Drop unsafe code from next_u64

[Shnatsel]

The optimizer is smart enough to optimize the naive version into a single load. This produces identical assembly on x86_64 as the unsafe approach. Tested with the following code:

use rand;
use rand::Rng;
use rand_hc::Hc128Rng;
use rand::SeedableRng;

fn main() {
    let mut rng: Hc128Rng = Hc128Rng::from_entropy();
    let value: u64 = rng.gen();
    println!("{}", value);
}

The generated assembly for <rand_core::os::OsRng as rand_core::RngCore>::next_u64 is:

<rand_core::os::OsRng as rand_core::RngCore>::next_u64:
 push    rbx
 sub     rsp, 96
 mov     qword, ptr, [rsp, +, 8], 0
 lea     rdi, [rsp, +, 8]
 mov     esi, 8
 call    qword, ptr, [rip, +, _ZN9getrandom9getrandom17hf138ee78ea21d54aE@GOTPCREL]
 test    eax, eax
 jne     .LBB22_1
 mov     rax, qword, ptr, [rsp, +, 8]
 add     rsp, 96
 pop     rbx
 ret
.LBB22_1:
 mov     ebx, eax
 mov     edi, 4
 mov     esi, 4
 call    qword, ptr, [rip, +, __rust_alloc@GOTPCREL]
 test    rax, rax
 jne     .LBB22_4
 mov     edi, 4
 mov     esi, 4
 call    qword, ptr, [rip, +, _ZN5alloc5alloc18handle_alloc_error17h64858b6e648c0ffcE@GOTPCREL]
 ud2
.LBB22_4:
 mov     dword, ptr, [rax], ebx
 mov     qword, ptr, [rsp, +, 16], rax
 lea     rax, [rip, +, .Lanon.d19b81ac8d75321f86db47584407e1c3.7]
 mov     qword, ptr, [rsp, +, 24], rax
 lea     rax, [rsp, +, 16]
 mov     qword, ptr, [rsp, +, 32], rax
 mov     rax, qword, ptr, [rip, +, _ZN62_$LT$rand_core..error..Error$u20$as$u20$core..fmt..Display$GT$3fmt17h5b71b604d6d2a4b6E@GOTPCREL]
 mov     qword, ptr, [rsp, +, 40], rax
 lea     rax, [rip, +, .Lanon.d19b81ac8d75321f86db47584407e1c3.11]
 mov     qword, ptr, [rsp, +, 48], rax
 mov     qword, ptr, [rsp, +, 56], 1
 mov     qword, ptr, [rsp, +, 64], 0
 lea     rax, [rsp, +, 32]
 mov     qword, ptr, [rsp, +, 80], rax
 mov     qword, ptr, [rsp, +, 88], 1
 lea     rsi, [rip, +, .Lanon.d19b81ac8d75321f86db47584407e1c3.13]
 lea     rdi, [rsp, +, 48]
 call    qword, ptr, [rip, +, _ZN3std9panicking15begin_panic_fmt17h3f391831d1286decE@GOTPCREL]
 ud2
.LBB22_3:
 mov     rbx, rax
 lea     rdi, [rsp, +, 16]
 call    core::ptr::drop_in_place
 mov     rdi, rbx
 call    _Unwind_Resume
 ud2

[Shnatsel]

I have also confirmed that this particular path is exercised by my test code by inserting panic!() in the function and observing that my test code does indeed panic.

[Shnatsel]

Hmm, behavior changed, so I must have been looking at the wrong function.