Are the `memcpy`s still done with Rust 2018/2021 without using this crate?

[jaques-sam]

Are the `memcpy`s still done with Rust 2018/2021 without using this crate?

[kvark]

Not seeing the copy any more on Rust playground:

#[inline(never)]
fn foo() -> Box<Foo> {
    Box::new(Foo::Small(4)) // this has 1 memcopy
    //Box::alloc().init(Foo::Small(4)) // this has 0 memcopies
}

LLVM:

; playground::foo
; Function Attrs: noinline nounwind nonlazybind uwtable
define internal fastcc noalias nonnull align 4 %Foo* @_ZN10playground3foo17h5e126e6c87b576a8E() unnamed_addr #2 personality i32 (i32, i32, i64, %"unwind::libunwind::_Unwind_Exception"*, %"unwind::libunwind::_Unwind_Context"*)* @rust_eh_personality {
start:
  %0 = tail call dereferenceable_or_null(404) i8* @__rust_alloc(i64 404, i64 4) #11, !noalias !7
  %1 = icmp eq i8* %0, null
  br i1 %1, label %bb3.i.i, label %"_ZN5alloc5boxed12Box$LT$T$GT$3new17hab47e76bc0e39a63E.exit"

bb3.i.i:                                          ; preds = %start
; call alloc::alloc::handle_alloc_error
  tail call void @_ZN5alloc5alloc18handle_alloc_error17h0ece3c434a8e0eb4E(i64 404, i64 4) #12, !noalias !7
  unreachable

"_ZN5alloc5boxed12Box$LT$T$GT$3new17hab47e76bc0e39a63E.exit": ; preds = %start
  %2 = bitcast i8* %0 to %Foo*
  store i8 0, i8* %0, align 4
  %_1.sroa.4.0..sroa_raw_idx = getelementptr inbounds i8, i8* %0, i64 1
  store i8 4, i8* %_1.sroa.4.0..sroa_raw_idx, align 1
  ret %Foo* %2
}

Asm:

playground::foo:
	pushq	%rax
	movl	$404, %edi
	movl	$4, %esi
	callq	*__rust_alloc@GOTPCREL(%rip)
	testq	%rax, %rax
	je	.LBB2_1
	movw	$1024, (%rax)
	popq	%rcx
	retq

Added the "[deprecated]" to crate description now.