Traits For Sharing

Deref, DerefMut

• Used to transfer a * dereference of a type to some type in the inner structure

• Canonical cases are Box and similar containers

AsRef, AsMut

• Annoying Rust shorthand: "refmut" is usually just spelled "mut"

• Traits for borrowing a reference to your type from a variety of values

• Usually used for generic parameters. Book says

    fn open<P: AsRef<Path>>(path: P) -> ... {
       let path: &Path = path.as_ref()
       ...
  

Borrow, BorrowMut

• Identical to AsRef, AsMut ?!?

• By convention, implement for a type only when references are semi-interchangeable with values

• Mostly for collection type convenience

• This is where the types start to get truly ugly: see the book example

From, Into

• Type conversion traits

• Not really needed, but convenient convention

• Into is just From with the types reversed

• Normally just implement From to get a default implementation of Into

• from() and into() consume their arguments

• from() and into() can only fail by panic()

TryFrom, TryInto

• Recent Rust addition

• Like From and Into but return a Result instead of panicking

• Makes it possible to safely convert types when some of the values may not safely convert

ToOwned

• Trait for "cloning" a thing that implements the Borrow trait

• One standard way to create a String from an &str:

    "hello".to_owned()
  

• Also works for slice to Vec

Cow

• "Copy On Write": keeps a reference until owned

• Glory in the beauty of this: book says

    enum Cow<'a, B: ?Sized + 'a>
         where B: ToOwned
    {
        Borrowed(&'a B),
        Owned(<B as ToOwned>::Owned),
    }
  

Observations

• There's a lot of stuff here

• Read this chapter carefully, then…

• Return to it as you advance in Rust. The details are best appreciated in the context of a problem you are trying to solve

• The result of this mess is a pretty expressive language: much is "hidden under the hood"