Fundamental Traits

Trait Garbage Bag

  • There are a bunch of traits tied into Rust's internals or standard library with no real organizing principle


  • The Clone trait provides the clone() and clone_into() functions

  • clone_into() is a good idea but little-used

  • A Clone implementation should do a "deep copy"

  • Clone is usually derived, but see e.g.

Marker Traits

  • "Marker traits" are a communication channel between compiled code and the compiler

  • You can use a marker trait like any other trait

  • Marker traits can be ignored, e.g. ?Sized


  • Copy is a marker trait that you implement when you want your values to be automatically copied by the compiler. It has no methods

  • Copy provides Clone for free as a subtrait

  • Use Copy sparingly:

    • Makes the implementation be careful
    • Expensive
    • Semantics sometimes surprising


  • You can implement the Drop trait to get control of a value right before it is freed

  • This is used for e.g. closing files, flushing data, etc

  • A type implementing Copy cannot also implement Drop, because the semantics are too confusing


  • Sized is a marker trait that says that the compiler knows the size of values of the type

  • You cannot implement Sized yourself

  • By default, generic types implicitly require instantiation with something Sized

  • You can turn this off with + ?Sized ("questionably sized") in situations where you don't want it


  • Trait to provide a "default value" for your type

  • Perhaps a bad idea: what does "default value" even mean?

  • … But often convenient

  • Book provides a sketchy use case

  • These days, Clippy tries to insist

Last modified: Monday, 26 April 2021, 11:44 PM