Rust fails to optimize away useless unwrap check

[upsuper]

See the following code:

pub struct ListNode {
    next: Option<Box<ListNode>>
}

pub fn foo(mut head: Box<ListNode>) -> Box<ListNode> {
    let mut cur = &mut head;
    while let Some(next) = std::mem::replace(&mut cur.next, None) {
        cur.next = Some(next);
        cur = cur.next.as_mut().unwrap();
    }
    head
}

Apparently the unwrap should never panic given the assignment in the previous line. However, based on the assembly output, that check isn't optimized away.

(The while let statement seems to be generating useless function calls for drop as well, btw.)

[RustyYato]

Minimal example

This no-op

pub fn foo(x: String) -> String {
    Some(x).unwrap()
}

generates

core::ptr::real_drop_in_place:
	movq	%rdi, %rax
	movq	(%rdi), %rdi
	testq	%rdi, %rdi
	je	.LBB0_2
	movq	8(%rax), %rsi
	testq	%rsi, %rsi
	je	.LBB0_2
	movl	$1, %edx
	jmpq	*__rust_dealloc@GOTPCREL(%rip)

.LBB0_2:
	retq

playground::foo:
	pushq	%rbx
	subq	$32, %rsp
	movups	(%rsi), %xmm0
	movaps	%xmm0, (%rsp)
	movq	16(%rsi), %rcx
	movq	%rcx, 16(%rsp)
	cmpq	$0, (%rsp)
	je	.LBB1_1
	movq	%rdi, %rax
	movq	16(%rsp), %rcx
	movq	%rcx, 16(%rdi)
	movaps	(%rsp), %xmm0
	movups	%xmm0, (%rdi)
	addq	$32, %rsp
	popq	%rbx
	retq

.LBB1_1:
	leaq	.Lanon.9b27569f3e5c884c6d6bffb1a8af3c8b.2(%rip), %rdi
	callq	*core::panicking::panic@GOTPCREL(%rip)
	ud2
	movq	%rax, %rbx
	movq	%rsp, %rdi
	callq	core::ptr::real_drop_in_place
	movq	%rbx, %rdi
	callq	_Unwind_Resume@PLT
	ud2

.Lanon.9b27569f3e5c884c6d6bffb1a8af3c8b.0:
	.ascii	"called `Option::unwrap()` on a `None` value"

.Lanon.9b27569f3e5c884c6d6bffb1a8af3c8b.1:
	.ascii	"src/libcore/option.rs"

.Lanon.9b27569f3e5c884c6d6bffb1a8af3c8b.2:
	.quad	.Lanon.9b27569f3e5c884c6d6bffb1a8af3c8b.0
	.asciz	"+\000\000\000\000\000\000"
	.quad	.Lanon.9b27569f3e5c884c6d6bffb1a8af3c8b.1
	.asciz	"\025\000\000\000\000\000\000\000[\001\000\000\025\000\000"

On playground for the latest nightly (2019-07-14 83e4eed)

[upsuper]

That reminds me that this issue may be from the enum size optimization. When we don't have a separate field for the enum tag (in case like Option<Box<_>>), compiler doesn't make use of non-null guarantee when optimizing it.

To prove that, an even simpler testcase is:

use std::num::NonZeroUsize;

pub fn foo(x: NonZeroUsize) -> NonZeroUsize {
    Some(x).unwrap()
}

which produces the following assembly:

Details

playground::foo:
	pushq	%rax
	testq	%rdi, %rdi
	je	.LBB0_1
	movq	%rdi, %rax
	popq	%rcx
	retq

.LBB0_1:
	leaq	.Lanon.c21f24ad05c622049a6df4006ef31e03.2(%rip), %rdi
	callq	*core::panicking::panic@GOTPCREL(%rip)
	ud2

.Lanon.c21f24ad05c622049a6df4006ef31e03.0:
	.ascii	"called `Option::unwrap()` on a `None` value"

.Lanon.c21f24ad05c622049a6df4006ef31e03.1:
	.ascii	"src/libcore/option.rs"

.Lanon.c21f24ad05c622049a6df4006ef31e03.2:
	.quad	.Lanon.c21f24ad05c622049a6df4006ef31e03.0
	.asciz	"+\000\000\000\000\000\000"
	.quad	.Lanon.c21f24ad05c622049a6df4006ef31e03.1
	.asciz	"\025\000\000\000\000\000\000\000[\001\000\000\025\000\000"

[AndreaCatania]

Hope that this will be improved soon considering the broad use of Option and the related functions unwrap, expect, ...

[klensy]

Slapping -Zmir-opt-level=3 solves issue, at least now https://rust.godbolt.org/z/6d8hW1PqW

Works for String and NonZeroUsize examples, but not for first message (this is -Zmir-enable-passes=+DataflowConstProp)

[saethlin]

The minimized examples have been fixed since 1.77. I'm not completely certain that we have codegen tests for these.

But the original example still has the missing optimizations as described.

[lolbinarycat]

Here's a new MCVE:

pub fn foo() -> usize {
    let r = None..Some(30);
    return r.end.unwrap();
}

[scottmcm]

Hmm, the as_mut() in the OP makes it much harder, because things like #138759 won't be able to help as it's forced through memory.

---

@lolbinarycat Please include a godbolt or playground link with a repro. That works fine when I try it https://rust.godbolt.org/z/cnznoG4K4