Tracking of possibly useful post-1.56 features

[eduardosm]

This issue is to keep track of post-1.56 Rust features that may be useful for x11rb, but we don't use because our MSRV is 1.56 or they aren't even stable yet.

The main purpose of this issue is to:

1. Determine if it is worth to increase our MSRV
2. If we do it, make sure we don't miss any improvement listed here

Feature list:

• Rust 1.63:
  • Mutex::new is const, possibly allowing us to drop the (optional) dependency on once_cell. (Replacement would be a static LIBXCB_LIBRARY: Mutex<Optional<Result<...>>> = Mutex::new(None);)
    • Is this actually possible? OnceCell allows us to get a &'static out, but a Mutex would not allow this. And we need this so that calls to e.g. xcb_wait_for_event() do not keep the mutex locked.
• Rust 1.64:
  • IntoFuture, see feat: Initial x11rb-async implementation #790 (comment)
  • raw-window-handle which (at the time of writing) has a msrv of 1.64, see Add raw-window-handle feature #772 and Revert adding raw-window-handle #799
  • gethostname >= 0.3
  • criterion >= 0.5
• Rust 1.70
  • Support for abstract unix domain sockets, Tracking Issue for abstract namespaces in Unix domain sockets rust-lang/rust#85410,

    x11rb/src/rust_connection/stream.rs

    Lines 205 to 206 in df1ff30

    	// TODO: Try abstract socket (file name with prepended '\0')
    	// Not supported on Rust right now: https://github.com/rust-lang/rust/issues/42048

  • once_cell, which would allow to remove the once_cell crate dependency. Tracking Issue for once_cell rust-lang/rust#74465
    • Alternatively, it might be possible to build this ourselves with Mutex::new being const, thus Mutex<Option<...>> could be used as a once cell.
  • cairo-rs >= 0.17
• Rust 1.80
  • core::mem::size_of and core::mem::align_of in prelude
• Stabilised in 1.81?
  • IoSlice::advance, which would be useful in Use &mut [IoSlice] for request sending #427
• Unstable:
  • unix_socket_ancillary_data, which allows FD passing. Currently we implement this using the nix crate. Tracking Issue for feature(unix_socket_ancillary_data) rust-lang/rust#76915
  • <T>::split_array_ref (Tracking Issue for split_array rust-lang/rust#90091) would allow to implement parse_u8_array_ref without any unwrapping.