fs: avoid contents copy for Vec<u8>, String, Bytes write

[paolobarbolini]

Motivation

As discovered by #7183, tokio::fs::write always copies the whole buffer, even when it's not necessary (the buffer is Send + 'static).

Solution

This issue could have been fixed if we had proper specialization, but without it and with references involved we can't use Any from std directly. However, David made the typeid crate for comparing TypeIds of non-'static types. The bincode folks made their own crate, unty crate, based on this same idea. This gives us poor's man specialization, allowing us to avoid the copy for Vec<u8>, String and Bytes. Box[<u8>], Arc<[u8]> and many other Send + 'static types could also be added in the future. Yesterday Armin also made a crate for doing something similar to this PR https://lucumr.pocoo.org/2025/3/23/from-string/.

Compiler explorer shows good enough output and correct proper functioning https://rust.godbolt.org/z/ef5GaooxW.

Fixes: #7183

[paolobarbolini]

I've removed the dependency on unty. Meanwhile I've discovered that David also has a crate for doing this called typeid, so I guess this is hack is here to stay. It makes sense to me anyway, unless providing a different generic value to a type (PhantomData in this case) doesn't get assigned a different TypeId in the future...

[Darksonn]

I asked about this particular hack on zulip and I am ok with using this hack.