example on String::from_raw_parts is correct in-context, but raise Miri error with seemingly sound modification #106593

Location

String::from_raw_parts example

Summary

The example is technically correct, however adding s.reserve(1) makes Miri complain about an invalid deallocation. From the documentation there is no obvious reason why that is.

use std::mem;

unsafe {
    let mut s = String::from("hello");
    s.reserve(1); // <= not in std example

    // Prevent automatically dropping the String's data
    let mut s = mem::ManuallyDrop::new(s);

    let ptr = s.as_mut_ptr();
    let len = s.len();
    let capacity = s.capacity();

    let s = String::from_raw_parts(ptr, len, capacity);

    assert_eq!(String::from("hello"), s);
}

I opened an issue on rust-lang/miri#2751, where @bjorn3 clarified that there is indeed an issue:

This seems to be an issue in the documentation, albeit a tricky one. The as_mut_ptr method comes from str, not String. The deref from String to str shrinks the region to which the str and thus as_mut_ptr result is valid for to fit exactly the size and not the capacity of the string. I think the only way to do this currently are s.into_bytes().as_mut_ptr() (which uses Vec::as_mut_ptr, which does cover the whole capacity) or s.into_raw_parts(), which is unstable.

They also added:

One way to fix this would be to add an as_mut_ptr method directly to String.

I tagged this A-docs but the solution may be to actually create a String::as_mut_ptr and not update the documentation. An other solution is to describe the issue I experienced and change the ManuallyDrop line to be

    let mut s = mem::ManuallyDrop::new(s.into_bytes());