e019: Let's `Clone` a `Cow`

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The final pieces of the story for (single-threaded) memory management in Rust.

Notes

Sometimes, we actually do need to copy types. Wouldn't it be nice if Rust gave us a convenient way to do that when it's convenient, or when the cost is low enough that the ergonomic tradeoffs are worth it? Well, perhaps unsurprisingly, it does! The Copy and Clone traits, plus the Cow type, give us everything we need!

Links

underhanded.rs
The typess
- std::marker::Copy
  - "Copy types" in the book
  - "Stack-Only Data: Copy" in the new book
  - 7.2.0.2 Moved and copied types:
    
    When a local variable is used as an rvalue, the variable will be copied if its type implements Copy. All others are moved.
  - Extended example in "Traits" section of new book
- std::clone::Clone
- std::borrow::Cow
Default implementations
- discussion in the current book
- discussion in the new book
Supertraits
- from the discussion in the reference (6.1.9 Traits):
  
  Traits may inherit from other traits.... The syntax Circle : Shape means that types that implement Circle must also have an implementation for Shape. Multiple supertraits are separated by +, trait Circle : Shape + PartialEq { }. In an implementation of Circle for a given type T, methods can refer to Shape methods, since the typechecker checks that any type with an implementation of Circle also has an implementation of Shape...
- discussion of trait "inheritance" in the book
- discussion of trait super- and subtyping in the new book (note: still to-be-written at the time this episode was published)
Marker traits
- std::marker
- in the reference: 9 Special Traits
Previous episodes on traits:
- e008: Just like something else
- e009: Composing a Rustic tune

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