understanding e0507 error and trying to resolve - rust

Question

I am in a &mut self function, I have a member field that is a Vec<u8>. I want to call a windows xxxA function (via the excellent winapi crate). I have no choice on the fn signature, it is implementing a trait.
I think I need to make a CString. So I tried
fn flush(&mut self) -> std::io::Result<()> {
unsafe {
let str = CString::new(self.buf).unwrap();
OutputDebugStringA(str.as_ptr());
}
Ok(())
}
this doesn't work.
error[E0507]: cannot move out of `self.buf` which is behind a mutable reference
--> src\windbg.rs:51:36
|
51 | let str = CString::new(self.buf).unwrap();
| ^^^^^^^^ move occurs because `self.buf` has type `std::vec::Vec<u8>`, which does not implement the `Copy` trait
I've read the explain of this but none of the 3 suggested solutions seem possible.
Here is the whole struct:
struct WinDbgWriter {
buf: Vec<u8>,
}
use std::io::Write;
impl std::io::Write for WinDbgWriter {
fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
self.buf.extend_from_slice(buf);
Ok(buf.len())
}
fn flush(&mut self) -> std::io::Result<()> {
unsafe {
let str = CString::new(self.buf).unwrap();
OutputDebugStringA(str.as_ptr());
}
Ok(())
}
}

Answer 1

If we take your minimized case and try to borrow self.mut to avoid moving it (transferring its ownership), we get a new error which has a suggestion:
error[E0277]: the trait bound `Vec<u8>: From<&Vec<u8>>` is not satisfied
--> src/lib.rs:9:33
|
9 | let _str = CString::new(&self.buf).unwrap();
| ^^^^^^^^^
| |
| the trait `From<&Vec<u8>>` is not implemented for `Vec<u8>`
| help: consider adding dereference here: `&*self.buf`
|
= help: the following implementations were found:
<Vec<T> as From<&[T]>>
<Vec<T> as From<&mut [T]>>
<Vec<T> as From<BinaryHeap<T>>>
<Vec<T> as From<Box<[T]>>>
and 6 others
= note: required because of the requirements on the impl of `Into<Vec<u8>>` for `&Vec<u8>`
If we follow that suggestion, or explicitly coerce self.buf to a &[u8], then the code compiles.
CString::new takes an argument of some type Into<Vec<u8>>, but self.buf is, at this point of type &Vec<u8> because of it being accessed through a &self borrow, and there's no impl Into<Vec<u8>> for &Vec<u8>.