Maybe unsound in StringData

[lwz23]

Hello, thank you for your contribution to this project. I'm currently scanning rust projects for unsound issues, and I notice the following code.

pub struct StringData {
    pub content: *const u8,
    pub len: u32,
}

impl StringData {
    ...................................
    pub fn to_string(&self) -> String {
        unsafe {
            let utf8 = std::slice::from_raw_parts(self.content, self.len as usize);
            String::from_utf8(utf8.to_vec()).unwrap()
        }
    }
}

I am concerned that there may be potential unsound problems here, json_interface is a pub mod, StringData is a pub struct, and content is a pub field. I take this to mean that an external user can call and modify the content field directly, and that to_string calls from_raw_parts without any checking, which I fear could lead to a potential UB (eg.null pointer), So I thought maybe it would be a more appropriate choice to

1. mark to_string as unsafe or
2. make these fields and functions private if there is no external use.

[lwz23]

here is my PoC and result:

use std::ptr::null;

pub struct StringData {
    pub content: *const u8,
    pub len: u32,
}

impl StringData {
    pub fn new(s: &String) -> Self {
        Self {
            content: s.as_ptr(),
            len: s.len() as u32,
        }
    }

    pub fn default() -> Self {
        Self {
            content: null(),
            len: 0,
        }
    }

    pub fn to_string(&self) -> String {
        unsafe {
            let utf8 = std::slice::from_raw_parts(self.content, self.len as usize);
            String::from_utf8(utf8.to_vec()).unwrap()
        }
    }
}
fn main() {
   let a= StringData::default();
   a.to_string();
}

result:

PS E:\Github\lwz> cargo run
   Compiling lwz v0.1.0 (E:\Github\lwz)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.52s
     Running `target\debug\lwz.exe`
thread 'main' panicked at core\src\panicking.rs:223:5:
unsafe precondition(s) violated: slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed `isize::MAX`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread caused non-unwinding panic. aborting.
error: process didn't exit successfully: `target\debug\lwz.exe` (exit code: 0xc0000409, STATUS_STACK_BUFFER_OVERRUN)

It seems that you don't even need to manually modify the content field, just use the provided default function to trigger UB.