Optimistic Peach questions

Optimistic Peach answers

• How can str be copied if it doesn't implement the Copy trait?
• How to call generic trait method in Rust
• Ownership, closures, FnOnce: much confusion
• Why does switching from struct to enum breaks API, exactly?
• When should I use smart pointers?
• Returning a trait with an associated type
• What to use for covariance over fn parameter in Rust?
• How do I annotate rust variables' data type without assignment?
• Understanding `tt` in Rust's `macro_rules!` macros
• Why must be structure initialized in rust?