Add diagnostics to unknown buffer types

[SUPERCILEX]

Recently discovered this was a thing, pretty cool!

error[E0277]: rustix does not accept `String` buffers
   --> src/buffer.rs:436:34
    |
436 |         let nread = read(&input, String::new()).unwrap();
    |                     ----         ^^^^^^^^^^^^^ Unsupported buffer type
    |                     |
    |                     required by a bound introduced by this call
    |
    = help: the trait `Buffer<u8>` is not implemented for `String`
    = note: only (potentially uninitialized) byte arrays, slices, and Vecs are supported
    = note: please read the docs: https://docs.rs/rustix/latest/rustix/buffer/trait.Buffer.html
    = help: the following other types implement trait `Buffer<T>`:
              &mut [MaybeUninit<T>; N]
              &mut [MaybeUninit<T>]
              &mut [T; N]
              &mut [T]
              &mut std::vec::Vec<MaybeUninit<T>>
              &mut std::vec::Vec<T>
              SpareCapacity<'a, T>
note: required by a bound in `read_write::read`
   --> src/io/read_write.rs:39:28
    |
39  | pub fn read<Fd: AsFd, Buf: Buffer<u8>>(fd: Fd, mut buf: Buf) -> io::Result<Buf::Output> {
    |                            ^^^^^^^^^^ required by this bound in `read`

error[E0277]: the trait bound `String: buffer::private::Sealed<u8>` is not satisfied
   --> src/buffer.rs:436:21
    |
436 |         let nread = read(&input, String::new()).unwrap();
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `buffer::private::Sealed<u8>` is not implemented for `String`
    |
    = help: the following other types implement trait `buffer::private::Sealed<T>`:
              &'a mut [MaybeUninit<T>; N]
              &'a mut [MaybeUninit<T>]
              &'a mut std::vec::Vec<MaybeUninit<T>>
              &mut [T; N]
              &mut [T]
              &mut std::vec::Vec<T>
              SpareCapacity<'a, T>

For more information about this error, try `rustc --explain E0277`.
error: could not compile `rustix` (lib test) due to 2 previous errors

[sunfishcode]

That's a fun feature! We'll need to make it conditional on the compiler though to support our MSRV.

[SUPERCILEX]

Whoops, didn't notice the CI failure. I'll take a look tomorrow.

[SUPERCILEX]

Looks like the freebsd failure is unrelated.

[sunfishcode]

Thanks!