C gen error when spawning function with mutable type

[CG-SS]

Describe the bug

I wasn't fully able to find exactly the reason the C gen errors, so apologies if the title is incorrect. But basically, the V code on the reproduction steps generated a C gen error.

Reproduction Steps

Try to compile the following code:

module main

import time
import io
import os

interface Message {
	a int
}

struct Impl {
	a int
}

type HookFunc = fn (Message) ?Message

struct TestStruct {
	fns []HookFunc
}

fn (ts TestStruct) run(mut writer io.Writer) {
	spawn handle_fns(ts.fns, mut writer)
}

fn handle_fns(fns []HookFunc, mut writer io.Writer) {
	mut message := Message(Impl{a: 1})
	for f in fns {
		message = f(message) or { continue }
	}
	println(message)
}

fn main() {
	stack := TestStruct{
		fns: [
			fn (m Message) ?Message {
				return Message(Impl{a: m.a + 1})
			},
			fn (m Message) ?Message {
				return none
			}
		]
	}
	stack.run(mut os.stdout())
	time.sleep(2 * time.second)
}

Expected Behavior

Should compile and print:

Message(Impl{
    a: 2
})

Current Behavior

Fails to C gen

Possible Solution

No response

Additional Information/Context

It seems that the presence of a mutable io.Writer influences the C code generation.

V version

V 0.4.1 1c9ab77

Environment details (OS name and version, etc.)

V full version: V 0.4.1 1c9ab77
OS: linux, Linux version 6.4.12-200.fc38.x86_64 (mockbuild@30894952d3244f1ab967aeda9ed417f6) (gcc (GCC) 13.2.1 20230728 (Red Hat 13.2.1-1), GNU ld version 2.39-9.fc38) #1 SMP PREEMPT_DYNAMIC Wed Aug 23 17:46:49 UTC 2023
Processor: 64 cpus, 64bit, little endian, Intel(R) Xeon(R) CPU E5-2682 v4 @ 2.50GHz

getwd: /home/shockwave-1
vexe: /home/shockwave-1/v/v
vexe mtime: 2023-09-23 15:26:12

vroot: OK, value: /home/shockwave-1/v
VMODULES: OK, value: /home/shockwave-1/.vmodules
VTMP: OK, value: /tmp/v_1000

Git version: git version 2.41.0
Git vroot status: weekly.2023.38-5-g77219de1
.git/config present: true

CC version: cc (GCC) 13.2.1 20230728 (Red Hat 13.2.1-1)
thirdparty/tcc status: thirdparty-linux-amd64 12f392c3

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