What does + 'a lifetime syntax do in rust?

Question

Self: Sized + 'a 

or

T: T + 'a

Seen this syntax while following a tutorial which suggests using rust 2018 edition instead of 2021 if it is related to your answer. However, I can not find the meaning of + lifetime parameter syntax.

Answer 1

It means that whatever type you use in the generic must outlive 'a, which means that all references it contains must outlive 'a. Here's some types that will always work.

i32                 // contains no lifetimes, so all zero lifetimes outlive 'a
Vec<u8>             // same as above
&'static str        // 'static outlives all lifetimes
SomeStruct<'static> // same as above

Here's some types that will only work if their lifetimes outlive 'a.

Vec<&'b u8>               // 'b must outlive 'a
&'c str                   // 'c must outlive 'a
SomeStruct<'d>            // 'd must outlive 'a
AnotherStruct<'x, 'y, 'z> // all lifetimes must outlive 'a

This is most often used with + 'static, which means "all contained lifetimes must be 'static". You may also see it on trait objects like Box<dyn Error + 'a> and impl Trait types like impl Iterator<Item = u8> + 'a, where it means "this type only lives for 'a", just like a regular lifetime parameter.

I don't think anything about this has changed since the 2018 edition.