Add Default, IntoIterator and ToOwned trait to the prelude.


Each trait has a distinct motivation:

  • For Default, the ergonomics have vastly improved now that you can write MyType::default() (thanks to UFCS). Thanks to this improvement, it now makes more sense to promote widespread use of the trait.

  • For IntoIterator, promoting to the prelude will make it feasible to deprecate the inherent into_iter methods and directly-exported iterator types, in favor of the trait (which is currently redundant).

  • For ToOwned, promoting to the prelude would add a uniform, idiomatic way to acquire an owned copy of data (including going from str to String, for which Clone does not work).

Detailed design

  • Add Default, IntoIterator and ToOwned trait to the prelude.

  • Deprecate inherent into_iter methods.

  • Ultimately deprecate module-level IntoIter types (e.g. in vec); this may want to wait until you can write Vec<T>::IntoIter rather than <Vec<T> as IntoIterator>::IntoIter.


The main downside is that prelude entries eat up some amount of namespace (particularly, method namespace). However, these are all important, core traits in std, meaning that the method names are already quite unlikely to be used.

Strictly speaking, a prelude addition is a breaking change, but as above, this is highly unlikely to cause actual breakage. In any case, it can be landed prior to 1.0.



Unresolved questions

The exact timeline of deprecation for IntoIter types.

Are there other traits or types that should be promoted before 1.0?