T: 'a inference in structs

Minimum Rust version: 1.31

An annotation in the form of T: 'a, where T is either a type or anotherlifetime, is called an "outlives" requirement. Note that "outlives" alsoimplies 'a: 'a.

One way in which edition 2018 helps you out in maintaining flow when writingprograms is by removing the need to explicitly annotate these T: 'a outlivesrequirements in struct definitions. Instead, the requirements will beinferred from the fields present in the definitions.

Consider the following struct definitions in Rust 2015:

  2. #![allow(unused_variables)]
  3. fn main() {
  4. // Rust 2015
  6. struct Ref<'a, T: 'a> {
  7.     field: &'a T
  8. }
  10. // or written with a `where` clause:
  12. struct WhereRef<'a, T> where T: 'a {
  13.     data: &'a T
  14. }
  16. // with nested references:
  18. struct RefRef<'a, 'b: 'a, T: 'b> {
  19.     field: &'a &'b T,
  20. }
  22. // using an associated type:
  24. struct ItemRef<'a, T: Iterator>
  25. where
  26.     T::Item: 'a
  27. {
  28.     field: &'a T::Item
  29. }
  30. }

In Rust 2018, since the requirements are inferred, you can instead write:

  1. // Rust 2018
  3. struct Ref<'a, T> {
  4.     field: &'a T
  5. }
  7. struct WhereRef<'a, T> {
  8.     data: &'a T
  9. }
  11. struct RefRef<'a, 'b, T> {
  12.     field: &'a &'b T,
  13. }
  15. struct ItemRef<'a, T: Iterator> {
  16.     field: &'a T::Item
  17. }

If you prefer to be more explicit in some cases, that is still possible.

More details

For more details, see the tracking issueand the RFC.