Trait associated type bounds are stable

[eduardosm]

Looking at:

x11rb/src/utils.rs

Lines 88 to 98 in efece4d

	/// A wrapper around some piece of raw bytes.
	///
	/// If trait associated type bounds were stable, the Connection trait could just use an associated
	/// type with bound Deref<[u8]>. Since this does not work, we get this enumeration that represents
	/// some owned bytes.
	#[derive(Debug)]
	pub enum Buffer {
	#[cfg(feature = "allow-unsafe-code")]
	CSlice(CSlice),
	Vec(Vec<u8>),
	}

However, the following code is allowed in rust 1.37 (https://godbolt.org/z/Xe9rG9):

pub trait MyTrait {
    type Buf: AsRef<[u8]>;
}

pub fn get<T: MyTrait>(v: &T::Buf) -> &[u8] {
    v.as_ref()
}

[psychon]

One problem that I came across: What to do with stuff like the following in the generated code?

impl TryFrom<&Buffer> for Point {
    type Error = ParseError;
    fn try_from(value: &Buffer) -> Result<Self, Self::Error> {
        Self::try_from(&**value)
    }
}
impl TryFrom<Buffer> for Point {
    type Error = ParseError;
    fn try_from(value: Buffer) -> Result<Self, Self::Error> {
        Self::try_from(&*value)
    }
}

The following is the "correct" replacement, I think:

impl<B: AsRef<[u8]>> TryFrom<B> for Point {
    type Error = ParseError;
    fn try_from(value: B) -> Result<Self, Self::Error> {
        Self::try_from(value.as_ref())
    }
}

However, coherence complains:

error[E0119]: conflicting implementations of trait `std::convert::TryFrom<_>` for type `generated::xproto::Char2b`:
  --> /[snip]/out/generated/xproto.rs:41:1
   |
41 | impl<B: AsRef<[u8]>> TryFrom<B> for Char2b {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: conflicting implementation in crate `core`:
           - impl<T, U> std::convert::TryFrom<U> for T
             where U: std::convert::Into<T>;

[eduardosm]

You caught me looking at it also. As done in this PR, I propose to just use TryFrom<&[u8]>.

[yshui]

Looks like it's just not possible to have a generic TryFrom implementation: rust-lang/rust#50133

[eduardosm]

Closing because #259 has been merged.