I haven't been able to get the most recent rustc to produce that
message anymore.

    #![feature(test)]

    extern crate test;

    use unicode_xid::UnicodeXID;

    #[bench]
    fn bench(b: &mut test::Bencher) {
        b.iter(|| {
            for _ in 0..1000 {
                let ident = test::black_box("is_xid_start");
                test::black_box(xid_ok(ident));
            }
        });
    }

    fn xid_ok(symbol: &str) -> bool {
        let mut chars = symbol.chars();
        let first = chars.next().unwrap();
        if !(first == '_' || UnicodeXID::is_xid_start(first)) {
            return false;
        }
        for ch in chars {
            if !UnicodeXID::is_xid_continue(ch) {
                return false;
            }
        }
        true
    }

Eliminates a clippy lint:

    error: `crate` references the macro call's crate
      --> tests/macros/mod.rs:40:21
       |
    40 |         let $expr = crate::macros::Tokens::parse::<$t>($expr).unwrap();
       |                     ^^^^^ help: to reference the macro definition's crate, use: `$crate`
       |
       = note: `-D clippy::crate-in-macro-def` implied by `-D clippy::all`
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#crate_in_macro_def

The old syntax is now a compile error, and it makes it difficult for users to search when they are looking for the keyword "dyn" on the documentation.

Added `dyn` to Type::TraitObject docs

It looks like the only outstanding issue aliasing issue in this repo
(with raw pointer tagging) was this one move of a Box after saving a
pointer to the allocation that the Box guards. This is UB according to
the Stacked Borrows with raw pointer tagging in combination with the way
rustc applies noalias to Box. As is often the case, the resolution here
is to convert the Box down to a raw pointer before a pointer into the
allocation is created.

Avoid Box invalidation to be compatible with tag-raw-pointers