Question about Trait Object lifetime

[Yandros]

The short way to express the meaning of T : 'a (such as in T = dyn ... + 'a), is that:

A type T meets the : 'a bound if any instance of type T can be held / owned arbitrarily long within that lifetime 'a.

This comes up in practice with the case 'a = 'static (the infinitely long lifetime):

@Yandros

Such property is expressed as that of having a 'static bound.

For instance:

• &'static _ : 'static
  An infinitely-long borrow of a value can be held infinitely long .
• String : 'static
  Although not all String live infinitely long (quite the opposite, actually), one that you hold will live until you drop it. Thus, a String can be held infinitely long .
• ditto for all non-generic types.

Counter examples:

• a &'a _ reference,
  for some lifetime 'a which is unknown, universal, or referring to a local variable, cannot be held infinitely long .
  Indeed, past the end of the lifetime, the variable the reference refers to may no longer exist, thus leading to a dangling pointer.

  • let s = String::from("Hello!"); // some local
    let at_s: &'_ String = &s; // refers to a local => not 'static
    

See also: Why does this "impl + 'static" code compile? - #4 by cuviper